react-infinite-grid-scroller (RIGS)

Heavy-duty vertical or horizontal infinite scroller

Key Features

• intra-list and inter-list drag and drop (optional)
• vertical or horizontal scrolling
• designed to display both "heavy" and "light" cell content (React components)
• supports both uniform and variable cell lengths (for both vertical and horizontal)
• single or multiple rows or columns
• dynamically variable virtual list size; prepend or append indefinitely
• limited sparse in-memory cache, to preserve content state, with an API
• repositioning mode when rapidly scrolling
• dynamic pivot (horizontal/vertical back and forth) while maintaining position in list
• automatic reconfiguration with viewport resize
• dynamic recalibration with async content refresh
• supports nested RIGS scrollers

Demo Site

See the demo site.

Overview

Key Technologies

RIGS uses these key technologies:

• CSS grid
• React hooks
• React (reverse) portals
• IntersectionObserver
• ResizeObserver
• requestIdleCallback
• react-dnd

Therefore RIGS is best suited for modern browsers.

Architecture

Notes: The Cradle is kept in view of the Viewport, such that the axis is always near the top or left of the Viewport (depending on vertical or horizontal orientation). There are two CSS grids in the Cradle, one on each side of the axis. As CellFrames are added to or removed from the grids, the grid on the top or left expands toward or contracts away from the top or left of the Scrollblock (depending on orientation), and the grid on the bottom or right expands toward or contracts away from the bottom or right of the Scrollblock.

CellFrames display individual user components. CellFrames are created and destroyed on a rolling basis as the Cradle re-configures and moves around the Scrollblock to stay in view, but user components are maintained in the internal cache until they go out of scope. New CellFrames fetch user components from the internal cache (portals in the React virtual DOM) or from the host through the user-supplied getItemPack function, as needed.

Not shown are two triggerlines (0 width or height divs, depending on orientation) which straddle the top or left edge of the Viewport. Whenever one of these triggerlines crosses the Viewport edge (through scrolling), an IntersectionObserver sends an interrupt to the Cradle to update its content and configuration. Triggerlines are located in the last CellFrame of the head grid, unless the scroller is at the very top of its list, in which case the triggerlines are located in the first CellFrame of the tail grid.

Usage

This is the minimum configuration.

import Scroller from 'react-infinite-grid-scroller'

// ...

const lowindex = -50, highindex = 50 // random range values

<div style = { containerstyle }>
  <Scroller 
      cellHeight = { cellHeight }
      cellWidth = { cellWidth }
      startingListRange = { [lowindex, highindex] } // this constitutes the virtual list
      getItemPack = { getItemPack } // a function called by RIGS to obtain a specified user component by index number
  />
</div>

See below for drag and drop configuration.

The scroller's highest level component, the Viewport, is a div with position:absolute, and inset:0, so the host container should be styled accordingly.

Note that scroller-generated elements show a data-type attribute in browser inspectors (eg. 'viewport').

User components loaded to CellFrames are placed in a data-type = 'contentenvelope' div. In 'uniform' layout this has position:absolute and inset:0. In 'variable' layout it has width:100% and max-height = cellHeight for 'vertical' orientation, and height:100% and max-width = cellWidth for 'horizontal' orientation. These elements allow overflow, but the user components should have overflow:hidden as appropriate.

See the "Content Types" section of the demodata.tsx module of the demo site for more code examples.

Compatible browsers

RIGS works on Chrome, Microsoft Edge, Firefox and Safari.

Scroller properties

Table of properties

| property | value | notes | |---|---|---| |[CELL CONFIGURATION]| |cellHeight:integer| number of pixels for cell height|required. Applied to height for 'uniform' layout, 'vertical' orientation. Applied to max-height for 'variable' layout, 'vertical' orientation. Approximate, used for fr (fractional allocation) for 'horizontal' orientation | |cellWidth:integer| number of pixels for cell width|required. Applied to width for 'uniform' layout, 'horizontal' orientation. Applied to max-width for 'variable' layout, 'horizontal' orientation. Approximate, used for fr (fractional allocation) for 'vertical' orientation| |cellMinHeight:integer| default = 25, minimum = 25, maximum = cellHeight|used for 'variable' layout with 'vertical' orientation. Applied to min-height| |cellMinWidth:integer| default = 25, minimum = 25, maximum = cellWidth|used for 'variable' layout with 'horizontal' orientation. Applied to min-width| |gap:integer | integer[]| number of pixels between cells|there is no gap at start or end of rows or columns; default = 0; accepts an array of integers as well as a standalone integer. Values match standard CSS order. Standalone integer = gap (in pixels) for both of column-gap (horizontal) and row-gap (vertical). 1-item array, same as integer. 2-item array = [col-gap, row-gap]| |getItemPack(index:integer, itemID:integer, context:object): object |host-provided function. index signifies position in list; session itemID (integer) is for tracking and matching. context provides an accept property when dnd is installed. Arguments provided by system|required. Must return a simple object with three properties: component - a React component or promise of a component (React.isValidElement), dndOptions (if dnd is enabled; see Drag and Drop section), and profile- a simple host-defined object with reference data about the item, which gets returned to the host for identification in various contexts| |[LIST CONFIGURATION]| |startingListRange:[lowindex, highindex] | []|two part array , or empty array []|lowindex must be <= highindex; both can be positive or negative integers. [] (empty array) creates an empty virtual list. Can be modified at runtime. | |startingIndex:integer| starting index when the scroller first loads|default = 0| |orientation:string| 'vertical' (default) or 'horizontal'|direction of scroll| |layout:string| 'uniform' (default), 'variable', or 'static'|specifies handling of the height or width of cells, depending on orientation. 'uniform' is fixed cellHeight/cellWidth. 'variable' is constrained by cellHeight/cellWidth (maximum) and cellMinHeight/cellMinWidth (minimum). See the staticComponent property regarding the special 'static' option.| |padding:integer | integer[]| number of pixels padding the Scrollblock| default = 0; accepts an array of integers as well as a standalone integer. Values match standard CSS order. Standalone integer = padding (in pixels) for all of top, right, bottom, left. 1-item array, same as integer. 2-item array = [t/b, r/l]. 3-item array = [t, r/l, b]. 4-item array = [t, r, b, l]| |getExpansionCount(position:string, index:ingeger): integer| function optionally provided by host. Called whenever the lowindex or highindex are loaded into the Cradle.| position = "SOL" or "EOL"; index = the lowindex or highindex. Should return the number by which to expand the virtual list| |getDropEffect(sourceScrollerID, targetScrollerID, context): 'move' | 'copy'| 'none' | undefined|function, optional, for RigsDnd component only|called whenever drag isOver and canDrop are true on a list; returns drop constraint. See Drag and Drop below. | |[SYSTEM SETTINGS]| |dndOptions:object|scroller settings for drag and drop|required if drag and drop is enabled. See Drag and Drop section| |runwaySize:integer| number of rows in the Cradle just out of view at head and tail of list|default = 1. minimum = 1. Gives time to assemble cellFrames before display |cache:string| 'cradle' (default), 'keepload', 'preload'|'cradle' matches the cache to the contents of the Cradle. 'keepload' keeps user components in the cache as loaded, up to cacheMax (and always Cradle user components). 'preload' loads user components up to cacheMax, then adjusts cache such that Cradle user components are always in the cache| |cacheMax:integer| at minimum (maintained by system) the number of user components in the Cradle|allows optimization of cache size for memory limits and performance| |useScrollTracker:boolean| default = true|allows suppression of system feedback on position within list while in reposition mode, if the host wants to provide alternative feedback based on data from callbacks | |placeholder:React.FC|a lightweight React component for cellFrames to load while waiting for the intended cellFrame components|optional (replaces default placeholder). parameters are index, listsize, message, error, dndEnabled. Arguments set by system| |usePlaceholder:boolean| default = true|allows suppression of use of default or custom placeholder. Placeholders show messages to the user while user components are fetched, and report errors| |profile:object|a simple object provided by host containing scroller reference data|the scroller profile object is returned to the host by RIGS for reference through getDropEffect, the scrollerProfile property of the getItemPack context argument, the scrollerProfile property of the dragDropTransferCallback context argument, and in the scrollerContext object. See below.| |staticComponent:React.FC|an arbitrary react component inserted as a layer. Ignored unless the layout property is set to 'static'|This allows the root of the Scroller tree to have an arbitrary layout. Specifically the inserted component allows for more than one top-level scroller, with drag and drop between them. If the layout 'static' option is set all other properties (except dndOptions) are ignored| |[OBJECT PROPERTIES]| |styles:object| collection of styles for scroller components|optional. These should be "passive" styles like backgroundColor. See below for details| |placeholderMessages:object| messages presented by the placeholder|optional, to replace default messages. See below for details| |callbacks:object| collection of functions for feedback, and interactions with scroller components|optional. See below for details| |technical:object| collection of values used to control system behaviour|use with caution. optional. See below for details|

Notes: For explicit cache management capability, a unique session itemID (integer) is assigned to a user component as soon as it enters the cache. The itemID is retired as soon as the user component is removed from the cache. If the same component is re-introduced into the cache, it is assigned a new session-unique itemID.

The itemID for a user component is given to the host with the getItemPack call to obtain the component, so that the host can track the user component in the cache. If the user component is assigned to a new index number (see the returned function object cache management section below) the host will still be able to track the user component with the itemID.

The host can track removal of a user component and its itemID from the cache through tracking its associated index removal through the deleteListCallback return values, and the return values from cache management functions. These feedback mechanisms also return the host's item profile object for further identification. See callback documentation below.

Most of the time the itemID can be ignored.

Also, note that the cache is reloaded with a new getItemPack function.

The styles object

Create a style object for each of the elements you want to modify. The styles are not screened, though the RIGS essential styles pre-empt user styles. Be careful to only include "passive" styles (like color, backgroundColor) so as not to confuse the scroller. Do not add structural items like borders, padding etc.

const styles = {
  viewport: {}, 
  scrollblock: {}, 
  cradle: {},
  scrolltracker: {},
  placeholderframe: {},
  placeholderliner: {},
  placeholdererrorframe: {},
  placeholdererrorliner: {},
  dndDragIcon: {},
  dndHighlights:{ // provide a color string for any you want to change
    source, // default "darkorange"
    target, // default "black"
    dropped, // default "darkorange"
    scroller, // default "red", canDrop but not isOver scroller
    scrolltab, // default "brown", also used for isOver and canDrop scroller
  },

You may want to add overscrollBehavior:'none' to the top level viewport styles to prevent inadvertent reloading of your app in certain browsers when users scroll past the top of a vertical list. Also note that dndDragIcon needs zIndex to be at least 1.

The scrolltracker is the small rectangular component that appears at the top left of the viewport when the list is being rapidly repositioned. The scrolltracker shows the user the current virtual position (index + 1) and total listsize during the repositioning process.

The placeholder styles are applied only to the default placeholder.

The placeholderMessages object

Replace any of the default messages used by the placeholder.

const placeholderMessages = {
    loading:'(loading...)',
    retrieving:'(retrieving from cache)',
    undefined:'host returned "undefined"', // displayed, and returned with itemExceptionCallback
    invalid:'invalid React element', // displayed, and returned with itemExceptionCallback
}

The callbacks object

Callbacks are host-defined closure functions which RIGS calls to provide data back to the host. RIGS returns data by setting the arguments of the callbacks. Include only the callbacks in the callbacks object that you want RIGS to use. The following callbacks are recognized by RIGS:

const callbacks = {

     // called at setup...
     functionsCallback, // (functions) - get an object that has api functions
     
     // index tracking, called when triggered...
     dragDropTransferCallback, // (fromScrollerID, fromIndex, toScrollerID, toIndex, context) - information about successful drop
     referenceIndexCallback, // (index, context) - change of index adjacent to the axis
     repositioningIndexCallback, // (index, context) - current virtual index number during rapid repositioning
     preloadIndexCallback, // (index, context) - current index being preloaded
     itemExceptionCallback, // (index, context) - details about failed getItemPack calls

     // operations tracking, called when triggered
     changeListRangeCallback, // (listrange, context) two part array lowindex, highindex 
     deleteListCallback, // (deleteList, context) - data about which items have been deleted from the cache
     boundaryCallback, // (position, index, context) - position is "SOL" or "EOL", index is the corresponding boundary index
     repositioningFlagCallback, // (flag, context) - notification of start (true) or end (false) of rapid repositioning,
}

An example of a callback closure (functionsCallback):

const scrollerFunctionsRef = useRef(null)

const functionsCallback = (functions) => {

    scrollerFunctionsRef.current = functions // assign the returned functions object to a local Ref

}

//...

scrollerFunctionsRef.current.scrollToIndex(targetIndex)

Details about the callbacks:

|callback function (parameters:datatypes)|context object parameter|notes| |---|---|---| |[GET FUNCTIONS]| |functionsCallback(functions: object)||the object returned contains Cradle functions that the host can call directly. This is the API. functionsCallback is called once at startup. See below for details| |[TRACK INDEXES]| |dragDropTransferCallback(fromScrollerID:number, fromIndex:number, toScrollerID:number, toIndex:number, content:object)|contextType:'dragDropTransfer', scrollerID, scrollerProfile, item |item contains item source data: dndOptions (type, dragText), dropEffect, index, itemID, profile, scrollerID| |referenceIndexCallback(index: integer, context:object)|contextType: 'referenceIndex', action, cradleState, scrollerID|action can be 'setCradleContent' or 'updateCradleContent'. cradleState is the state change that triggered the action. Keeps the host up to date on the index number adjacent to the Cradle axis| |repositioningIndexCallback(index: integer, context:object)|contextType: 'repositioningIndex', scrollerID|the current index during repositioning. Useful for feedback to user when host sets useScrollTracker property to false| |preloadIndexCallback(index: integer, conext:object)|contextType: 'preloadIndex', scrollerID|during a preload operation, this stream gives the index number being preloaded| |deleteListCallback(deleteList: array, context:object)|contextType: 'deleteList', scrollerID, message |gives an array of objects with index numbers that have been deleted from the cache, together with itemID and profile.message gives the reason for the deletion(s)| |itemExceptionCallback(index: integer, context:object)|contextType: 'itemException', itemID, scrollerID, profile, dndOptions, component, action, error |action can be 'preload' or 'fetch'. Triggered whenever getItemPack does not return a valid React component| |[TRACK OPERATIONS]| |changeListRangeCallback(listrange:array, context:object) |contextType: 'changeListRange', scrollerID| notification of a change of list range. listrange is a two part array = lowindex, highindex | |boundaryCallback(position:string, index:integer, context:object) |contextType: 'boundary', scrollerID| called whenever the lowindex or highindex are loaded into the Cradle. position is "SOL" or "EOL", index is the corresponding boundary index| |repositioningFlagCallback(flag: boolean, context:object)|contextType: 'repositioningIndex', scrollerID | called with true when repositioning starts, and false when repositioning ends. Useful for feedback to user when host sets useScrollTracker property to false|

The returned API functions object

Details about the functions returned in an object by functionsCallback.

Functions with no return values:

|function(parameters: datatypes):return value: datatype|notes| |---|---| |[OPERATIONS]| |scrollToIndex(index:integer): void|places the requested index item at the top visible row or left visible column of the scroller, depending on orientation| |scrollToPixel(pixel:integer[,behavior:string = 'smooth']): void|scrolls the scroller to the provided pixel, along the current orientation. behavior = 'smooth' | 'instant' | 'auto'; default = 'smooth'. pixel must be >=0 | |scrollByPixel(pixel:integer[,behavior:string = 'smooth']): void|scrolls the scroller up or down by the number of provided pixels, along the current orientation. behavior = 'smooth' | 'instant' | 'auto'; default = 'smooth'. pixel can be positive (scroll down) or negative (scroll up) | |setListRange(array [lowindex, highindex] | []): void|lowindex must be <= highindex; lowindex and highindex can be positive or negative integers. [] (empty array) creates an empty virtual list| |prependIndexCount(count:integer): void|the number of indexes to expand the start of the virtual list| |appendIndexCount(count:integer): void|the number of indexes to expand the end of the virtual list| |[CACHE MANAGEMENT]| |reload(): void|clears the cache and reloads the Cradle at its current position in the virtual list| |clearCache(): void|clears the cache and the Cradle (leaving nothing to display)|

Functions with return values

|function(parameters: datatypes):return value: datatype|context object properties|notes| |---|---|---| |[SNAPSHOTS]| |getCacheIndexMap(): [Map, context]|contextType: 'cacheIndexMap', scrollerID|snapshot of cache index (=key) to itemID (=value) map for the subject scroller| |getCacheItemMap(): [Map, context]|contextType: 'cacheItemMap', scrollerID|snapshot of cache itemID (=key) to object (=value) map for the subject scroller. Object = {index, component} where component = user component| |getCradleIndexMap(): [Map, context]|contextType: 'cradleIndexMap', scrollerID|snapshot of Cradle index (=key) to itemID (=value) map| |getPropertiesSnapshot():[object, context]|contextType: 'propertiesSnapshot', scrollerID|object is a copy of scroller.current from scrollerContext object. See below.| |[CACHE MANAGEMENT]| |insertIndex(index:integer, rangehighindex: integer | null):array[[shiftedList:array, replacedList:array, removedList:array, deletedList:array], context:object]|contextType: 'insertIndex', scrollerID|can insert a range of indexes. Displaced indexes, and higher indexes, are renumbered; virtual list lowindex remains the same. Changes the list size by increasing virtual list highindex; synchronizes the Cradle. deletedList contains an object for each item, with index, itemID, and profile| |removeIndex(index:integer, rangehighindex:integer | null):array[[shiftedList:array, replacedList:array, removedList:array, deletedList:array], context:object]|contextType: 'removeIndex', scrollerID|a range of indexes can be removed. Higher indexes are renumbered; virtual list lowindex remains the same. Changes the list size by decreasing virtual list highindex; synchronizes to the Cradle| |moveIndex(toindex:integer, fromindex:integer, fromhighrange: integer | null): array[[processedIndexList:array, movedList:array], context:object]|contextType: 'moveIndex', scrollerID|a range of indexes can be moved. Displaced and higher indexes are renumbered. Synchronizes to the Cradle. movedList contains an object for each item moved with fromIndex, toIndex, itemID, and profile. Returns false for error or undefined for nothing to do|

Notes: cache management functions are provided to support drag-n-drop, sorting, and filtering operations.

Cache management functions operate on indexes and itemIDs in the cache, and generally ignore indexes and itemIDs that are not in the cache. They synchronize Cradle cell content as appropriate.

This is a sparse in-memory cache, and indexes in the cache are not guaranteed to be contiguous.

The technical object

These properties would rarely be changed.

|property:datatype = default|notes| |---|---| |showAxis:boolean = false |axis can be made visible for debug| |triggerlineOffset:integer = 10| distance from cell head or tail for content shifts above/below axis| |VIEWPORT_RESIZE_TIMEOUT:integer = 250|milliseconds before the Viewport resizing state is cleared| |ONAFTERSCROLL_TIMEOUT:integer = 100|milliseconds after last scroll event before onAfter scroll event is fired| |IDLECALLBACK_TIMEOUT:integer = 175|milliseconds timeout for requestIdleCallback| |VARIABLE_MEASUREMENTS_TIMEOUT:integer = 250|milliseconds to allow setCradleContent changes to render before being measured for 'variable' layout| |CACHE_PARTITION_SIZE:integer = 30|the cache is partitioned for performance reasons| |MAX_CACHE_OVER_RUN:number = 1.5|max streaming cache size over-run (while scrolling) as ratio to cacheMax| |SCROLLTAB_INTERVAL_MILLISECONDS:number = 100|SetInterval milliseconds for ScrollTab| |SCROLLTAB_INTERVAL_PIXELS:number = 100|SetInterval scrollBy pixels for dnd ScrollTab|

The cell component's acquired scrollerContext property

Cell components can get access to dynamically updated parent RIGS properties, by requesting the scrollerContext object.

The scrollerContext object is requested by host components by initializing a scrollerContext component property to null. The property is then recognized by RIGS and set to the scrollerContext object by the system on loading of the component to a CellFrame. Here is the internal code that does this:

if (usercontent.props.hasOwnProperty('scrollerContext')) {
  component = React.cloneElement(usercontent, {scrollerContext})
} else {
  component = usercontent
}

The scrollerContext object contains two properties:

{
  cell,
  scroller,
}

Each of these is a reference object, with values found in propertyRef.current.

The cell.current object is instantiated only when a component is instantiated in the Cradle. It contains two properties:

{
  itemID, // session cache itemID
  index, // place in virtual list
}

The scroller.current object contains the following properties, which are identical to the properties set for the scroller (they are passed through):

orientation, cellHeight, cellWidth, cellMinHeight, cellMinWidth, layout, cache, cacheMax, startingIndex

It also contains scrollerID, the internal session id (integer) of the current scroller, as well as dndInstalled, and dndEnabled.

Finally, it contains four objects with bundled properties: virtualListProps, cradleContentProps, gapProps, and paddingProps.

virtualListProps is an object with the following properties:

{
   size, // the length (number of virtual cells) of the virtual list
   range, // a two-part array [lowindex,highindex]
   lowindex, // the virtual list low index
   highindex, // the virtual list high index
   baserowblanks, // cell offset count in the first row
   endrowblanks, // blank cells at the end of the last row
   crosscount, // number of cells perpendicular to the orientation
   rowcount, // number of rows in virtual list
   rowshift, // row shift from zero to accommodate lowindex
}

cradleContentProps is an object with the following properties:

{
   cradleRowcount, // number of rows in the cradle (including any blank cells)
   viewportRowcount, // number of full rows that can be shown in the viewport
   runwayRowcount, // calculated current extra leading and trailing cell rows beyond the viewport boundary
   SOL, // true or false, at start of virtual list in the cradle
   EOL, // true or false, at end of virtual list in the cradle
   lowindex, // of cells in the cradle
   highindex, // of cells in the cradle
   axisReferenceIndex, // the first index of the tail grid
   size, // count of cells in the cradle
}

gapProps is an object with the following properties:

{
  CSS, // the CSS to be applied to the grid
  column, // column gap
  row, // row gap
  list, // [column, row]
  original, // normalized parameter value
  source, // original parameter value
}

paddingProps is an object with the following properties:

{
  CSS, the CSS to be applied to the Scrollblock
  top, // top padding
  right, // right padding
  bottom, // bottom padding
  left, // left padding
  list, // [top, right, bottom, left]
  original, // normalized parameter value
  source, // original parameter value
}

Drag and drop

Overview

The following are the basic steps to implement drag and drop on RIGS. Note that RIGS drags and drops its CellFrame components. The user content (React components) come along for the ride. The following assumes multiple sub-scrollers, although using just the main scroller is fine.

• design a type system for scroller items and scrollers. Scroller item types must be included in scroller accept type lists
• install the dnd system by invoking the specialized RigsDnd root component
• provide every scroller and subscroller with a scroller dndOptions object as a scroller parameter. Also providing a host-defined scroller profile object is highly recommended, for use during interaction with the system
• provide every item fetched using getItemPack with a cell dndOptions property through the returned object. Also providing items with a host-defined cell profile object is highly recommended, for use during interaction with the system. (Remember that subscrollers require both a direct scroller dndOptions property, and a cell dndOptions object returned by getItemPack. Sub-scrollers are both item repositories, and part of scroller items themselves).
• prepare to respond to specialized 'dndFetchRequest' getItemPack requests, which are for dropped copies of existing items, or replacements for dropped moved items that have gone out of scope as the result of scrolling during the drag activity
• request a scrollerContext object from the host scroller for each of your content items so that the items will be aware of dnd status in their host scrollers for layout purposes (see scrollerContext section)
• design and implement accommodating layout features on your cell components
• design and implement dnd configuration options as required
• create a getDropEffect function if needed, and pass this to the RigsDnd root component
• creat a nativeTypeCallback function and provide it to scroller dndOptions for handling of native type (file, url or text) drag and drop
• track drag and drop transfers with dragDropTransferCallback if desired

See below for details.

Installation

To install drag and drop ('dnd') capability,

1. first invoke the RigsDnd component (instead of the default component) for the root of the RIGS tree to install the dnd Provider. This can only be done once per environment. For embedded lists (sub-scrollers), use the default RIGS component.

import  { RigsDnd as Scroller } from 'react-infinite-grid-scroller'
...
<div style = { containerstyle }>
  <Scroller 
      cellHeight = { cellHeight }
      cellWidth = { cellWidth }
      startingListRange = { [lowindex, highindex] } // this constitutes the virtual list
      getItemPack = { getItemPack } // a function called by RIGS to obtain a specified user component by index number
      dndOptions = { dndOptions } // required for both the root instance and all child RIGS instances
  />
</div>

2. dndOptions is a required property for all scrollers when dnd is enabled. It must include an accept property, with an array of accepted dnd content types (strings or Symbols). For the root scroller it may also include a master property. A complete list here:

const dndOptions = {
  accept:['type1','type2','type3',...], // required for all participating RIGS scrollers - any number of string (or Symbol) identifiers
  master:{enabled}, // optional, default true, for root `RigsDnd` component only. Serves as default for scroller enabled setting.
  enabled, // optional for all participating scrollers, default set by master.enabled
  dropEffect, // optional. the prescribed value ('move' or 'copy') for dragged scroller items; can be overridden by getDropEffect.
    // undefined dropEffect means default = 'move', posibly modified to 'copy' by pressing the altKey on desktop systems
  showScrollTabs, // default = true. When set to false, suppresses the scroll tabs on the scroller during drag.
  nativeTypeCallback, // a host provided function to return the result of a native type drag and drop. See below.
}

3. When dnd is enabled, all data packages returned to RIGS with getItemPack must include a dndOptions object (together with the component and profile properties). The dndOptions object must contain a type property with a string that matches one of the accept array strings of its containing scroller, and a dragText property with text that will be shown in the drag image for the item.

// in host getItemPack function
...
return {
  component, // a valid React component, or promise of a component
  dndOptions:{
    type, // string (or Symbol) required to match one of the list accept values
    dragText, // required. Displayed to user in the drag image for the component
  }
  profile, // recommended. Simple object created by host, returned to host for item identification in various contexts
}

RIGS does not check for matches of type values returned with getItemPack, with accept values sent to scrollers via the dndOptions scroller property.

With dnd enabled, the context parameter of the getItemPack function sent to the host will include the accept list of the enclosing scroller, for convenience.

The canDrop and dropEffect properties, and the getDropEffect function

canDrop is an internal property controlled by react-dnd, by comparing the dragged item's type property (from the cell dndOptions property) to the list of accept values (from the scroller dndOptions property) of the target scroller. If a match is found, then react-dnd signals that the item can be dropped on the scroller.

canDrop can be further constrained by the host-provided getDropEffect function, by returning 'none'.

The internal dropEffect property for a dragged item can be set by the dropEffect property of the dndOptions of the source scroller. If this is undefined, then it becomes the default 'move' operation, or the 'copy' operation when the altKey is pressed by the user on most browsers.

dropEffect can be further constrained by the return value of the host-provided function getDropEffect (see Scroller Properties above). getDropEffect (if provided by the host) is called by RIGS whenever the drag location crosses into a scroller for which the internal isOver and canDrop properties are both true. This function has three parameters: sourceScrollerID (the scrollerID from the source of the drag), targetScrollerID (the scrollerID from the target of the drag), and context. Here are the properties of the context parameter:

const context = {
  sourceDndOptions:{ // taken from the source scroller
    accept, // the array of strings signifying the types of content items accepted by drop
    enabled, // the current state of dnd for the scroller
    dropEffect // an optional prescribed value for use by contained list items
  },
  sourceProfile, // a host-provided simple object to assist with identification of the scroller
  targetDndOptions:{
    ... // same as sourceDndOptions, but taken from the target scroller
  },
  targetProfile, // similar to sourceProfile
  itemData:{ // identifying information of the current item being dragged
    itemID, // the itemID assigned to the content item by RIGA
    index, // the item's logical position in the source scroller
    profile, // host provided simple object to assist with item identification
    dndOptions: {, // taken from getItemPack function
      type, // the type of the item, matches one of the accept items in both the source and target scroller
      dragText, // the text shown in the dragBar when the item is being dragged
    } 
    dropEffect, // the current internally prescribed dropEffect taken from scroller settings and altKey setting if any
  }
}

The host can return 'move', 'copy', 'none', or undefined from the getDropEffect function. 'none' prevents a drop, 'move' and 'copy' override the item's dropEffect value, and undefined yields to the value of the scroller's calculated dropEffect property.

The 'dndFetchRequest' specialized getItemPack call

When the user drags and drops an item which exists in the RIGS cache, and with a 'move' dropEffect, then RIGS takes care of the data transfer itself.

However, if the drop involves a 'copy', then the host has to supply what it considers to be a copy of the item (a RIGS cache item can only be used for one display instance). Also, if the item has been moved, but has gone out of scope as the result of scrolling during the drag operation and therefore removed from the cache, then a fresh instance of the moved item has to be obtained from the host. In those cases, RIGS sends out a specialized getItemPack(index, itemID, context) call with sufficient information for the host to respond appropriately. In this case, the context parameter contains the following:

const context = {
  contextType:'dndFetchRequest', // identifiaction of the specialized request
  accept, // the accept list of the target scroller
  scrollerID, // the target scroller ID
  scrollerProfile, // the target scroller profile, as provided by the host as the scroller property
  item:{
    scrollerID, // the source scrollerID
    index, // the item index at source
    itemID, // the itemID at the start of drag (now to be replaced)
    profile, // the item profile as passed to RIGS through the original instantiating getItemPack response (could include host item ID)
    dndOptions, // the original item dndOptions from getItemPack
    dropEffect, // the final dropEffect at the point of drop
  },
}

The native type drag and drop (file, url, and text)

On desktop systems, RIGS scrollers can accept native types (file, url, and text).

To accomplish this, first add the following to the scroller dndOptions.accept array:['__NATIVE_FILE__', '__NATIVE_URL__', '__NATIVE_TEXT__']

Then provide a scroller dndOptions.nativeTypeCallback function, which RIGS uses to return the result of the drag and drop for further processing. The nativeTypeCallback has two parameters, item and context. item is data returned by react-dnd for further processing, and context is provided by RIGS for more information, as follows:

item: { // provided by system for further processing
  dataTransfer,
  files,
  items,
}

context: { // provided by RIGS for further information
  contextType:'nativeType',
  scrollerID, // current scrollerID
  internalDropResult: {
    dataType, // location type of the drop - 'cellframe' or 'viewport'
    dropEffect, // the computed drop effect at the time of drop
    target, // target data - scrollerID, and itemID and index if dataType is 'cellframe'
  },
  itemType, // the native type
  whitespaceposition, // 'head' or 'tail' for internaleDropResult.dataType == 'viewport' (ie. whitespace)
  listrange, // the current scroller listrange [lowindex, highindex]
}

Layout

RIGS shows a drag icon for draggable items at the top left of each CellFrame. Since this icon can overlay some of the content of the host provided content, the component may want to move this content out of the way of the drag icon when it is present. Here is how that can be done.

First, obtain a scrollerContext object from the host scroller. See the scrollerContext section for details.

Then code something like this.

const isDnd = scrollerContext?.scroller.current.dndEnabled

const float = useMemo(() => {
  return <div 
    style = {{float:'left', height: '28px', width:'34px'}} 
    data-type = 'dnd-float'
  />
},[])

return <div 
  ... // properties
>
  {isDnd && float}
  ... // content
</div>

Configuration

Drag and drop can be enabled or disabled for individual scrollers, by setting the scroller's dndOptions.enabled property. If the scroller's enabled property is undefined, then it is set to the RigsDnd component's dndOptions.master.enabled property (which is itself set to default true if undefined. The enabled property can be set at runtime.

This presents a number of options. For example the user can set the dnd capability of a scroller themselves, something like this:

const dndOptionsRef = useRef(dndOptions) // for use elsewhere, below
dndOptionsRef.current = dndOptions
...
const checkdnd = (event:React.ChangeEvent) => {
  const
    target = event.target as HTMLInputElement,
    isChecked = target.checked
  dndOptionsRef.current.enabled = isChecked
  setSubscrollerState('revised')
}

return ...
<FormControl borderTop = '1px' style = {{clear:'left'}}>
  <Checkbox 
    isChecked = {dndOptions.current.enabled} 
    size = 'sm'
    mt = {2}
    onChange = {checkdnd}
  >
    Drag and drop
  </Checkbox>
</FormControl>}
...
<Scroller
  ...
  dndOptions = {dndOptionsRef.current}
  ...
/>

Or you can create a global React context, say const DndEnabledContext = React.createContext(true). Then control that value through a global checkbox control. Finally, in your scroller component:

const dndEnabledContext = useContext( DndEnabledContext )
...
useEffect(()=>{

  dndOptionsRef.current.enabled = dndEnabledContext
  setSubscrollerState('revised')

},[dndEnabledContext])
...

The combination of the local and global settings would allow the user much flexibility in terms of how to use drag and drop. This could be particularly useful in settings involving dozens of subscrollers.

Performance

If there are dozens of subscrollers on screen (say by zooming out to 50%), then performance of drag and drop (notably for the drag icon) can degrade owing to the pub/sub system of react-dnd. One way of dealing with this is to disable all drag and drop (see Configuration above), and let the user selectively enable drag and drop for the scrollers that they want to work with. That way, performance is quite good.

Restoring scroll positions coming out of cache

This is only of concern if your cell components support scrolling.

RIGS loads components into a cache (React portals), and into Cradle cells from there. Moreover RIGS moves components from one side of the (hidden) axis to the other through the cache during scrolling. Plus caching can be extended (by RIGS property settings) beyond the Cradle. So going into and out of cache happens a lot for components. While in cache, the component elements have their scrollTop, scrollLeft, width, and height values set to 0 by browsers. width and height values are restored by browsers when moved back into the visible DOM area, but scroll positions have to be manually restored.

Here is one way of restoring scroll positions. Basically, save scroll positions on an ongoing basis, detect going into cache when width and height values are both zero, and detect coming out of cache when width and height are no longer zero. When coming out of cache, restore the saved scroll positions.

This code is Typescript, in a function component.

    // ------------------------[ handle scroll position recovery ]---------------------

    // define required data repo
    const scrollerElementRef = useRef<any>(null),
        scrollPositionsRef = useRef({scrollTop:0, scrollLeft:0}),
        wasCachedRef = useRef(false)

    // define the scroll event handler - save scroll positions
    const scrollerEventHandler = (event:React.UIEvent<HTMLElement>) => {

        const scrollerElement = event.currentTarget

        // save scroll positions if the scroller element is not cached
        if (!(!scrollerElement.offsetHeight && !scrollerElement.offsetWidth)) {

            const scrollPositions = scrollPositionsRef.current

            scrollPositions.scrollTop = scrollerElement.scrollTop
            scrollPositions.scrollLeft = scrollerElement.scrollLeft

        }

    }

    // register the scroll event handler
    useEffect( () => {

        const scrollerElement = scrollerElementRef.current

        scrollerElement.addEventListener('scroll', scrollerEventHandler)

        // unmount
        return () => {
            scrollerElement.removeEventListener('scroll', scrollerEventHandler)
        }

    },[])

    // define the cache sentinel - restore scroll positions
    const cacheSentinel = () => {
        const scrollerElement = scrollerElementRef.current

        if (!scrollerElement) return // first iteration

        const isCached = (!scrollerElement.offsetWidth && !scrollerElement.offsetHeight) // zero values == cached

        if (isCached != wasCachedRef.current) { // there's been a change

            wasCachedRef.current = isCached

            if (!isCached) { // restore scroll positions

                const {scrollTop, scrollLeft} = scrollPositionsRef.current

                scrollerElement.scrollTop = scrollTop
                scrollerElement.scrollLeft = scrollLeft

            }

        }

    }

    // run the cache sentinel on every iteration
    cacheSentinel()

    // register the scroller element through the ref attribute
    return <div ref = {scrollerElementRef} style = {scrollerstyles}>
        {scrollercontent}
    </div>

Licence

MIT © 2020-present Henrik Bechmann