ValueLink Class

Create link

Link.value( value, nextValue => void ) : Link

Create custom link with the given value and update function.

It may be used for different scenarios. Good example is to use 'edit element' component for adding new element.

Imagine that we have a component <EditUser valueLink={ userLink } /> expecting the link to an object. When editing is finished, EditUser will update the given link with a new values.

Then, following custom link will allow you to add new user with the same form element.

<EditUser valueLink={ Link.value( {}, x => userArrayLink.push( x ) ) } />

Read more about links to objects updates in the next section.

Link.mutable( object ) : Link

Create the link to a mutable object. Useful for testing and linking observable state.

const state = { a : 1 },
    $state = Link.mutable( state );

$state.at( 'a' ).set( 2 );
expect( state.a ).toBe( 2 );

Read more about links to objects updates in the next section.

Link.getValues({ [ name ] : Link }) : { [ name ] : value }

Extracts an object with link values. Leading $ is removed from property names in a result.

export const MyCoolComponent = ( props ) => {
    const $name = useLink( 'a' ),
          $email = useLink( 'b' ),

    ...
    const values = Link.getValues({ $name, $email });
    console.log( values ); // { name : 'a', email : 'b' }
}

Link.getErrors({ [ name ] : Link }) : { [ name ] : value }

Extracts link validation errors. Returns an empty object if there are no errors. Leading $ is removed from property names in a result.

export const MyCoolComponent = ( props ) => {
    const $name = useLink( 'a' ),
          $email = useLink( 'b' ),

    ...
    const values = Link.getErrors({ $name, $email });
    console.log( values ); // { name : 'a', email : 'b' }
}

Link.setValues({ [ name ] : Link }) : void

Bulk set links from the object with values. Values object must not contain the leading $ in names.

export const MyCoolComponent = ( props ) => {
    const $name = useLink( 'a' ),
          $email = useLink( 'b' ),

    ...
    // Somewhere on I/O completion:
    Link.setValues({ $name, $email }, json);
}

Links to object and arrays

$object.at( key ) : Link

Create link to the member of array or object.

If linked value is plain object or array, it's possible to generate links to their members. Whenever this derivative links will be updated, it will lead to proper purely functional update (with shallow copying) of the parent element.

const $name = this.$at( 'array' ).at( 0 ).at( 'name' );
$name.set( 'Joe' ); // Will update component state.array

$object.pick( key1, key2, ... ) : { [ key ] : Link }

Create links to the object's members, and wrap them in an object. When no arguments are provided, it link all object's properties.

const user$ = $user.pick( 'name', 'email' ),
      { name, email } = user$;

$objOrArray.map( ( $item, itemKey ) => any | void ) : any[]

Map and filter through array or object.

var list = $stringArray.map( ( $item, index ) => {
    if( $item.value ){ // Skip empty elements
        return (
            <div key={ index }>
                <Input $value={ $item } />
            </div>
        );
    }
});

Bind to control

$something.props : { value, onChange }

Bind link to the standard form control consuming value and onChange props.

<input {...$something.props} />

Custom data-bound controls

You're encouraged to create your own semantic form controls to take the full advantage of the value links features. An example of the control:

const Input = ({ $value, ...props }) => (
    <div className={`form-control ${ $value.error ? 'error' : '' }`}>
        <input {...props}
            value={ $value.value }
            onChange={ e => $value.set( e.target.value ) }
        />
        <div className="validation-error">{ $value.error || '' }</div>
    </div>
);

Offhand boolean links

$array.contains( element ) : Link

Creates the link to the presence of value in array.

Resulting link value is true whenever element is present in array, and false otherwise. Whenever resulting link is assigned with new value, it will flip element in the array.

Useful for the large checkbox groups.

const optionXBoolLink = arrayLink.contains( 'optionX' );

linkToAny.equals( whenTrue ) : Link

Create boolean link to value equality.

Resulting link value is true whenever parent link value equals to whenTrue, and false otherwise. When resulting link is assigned with true, it sets parent link value with whenTrue, and with null otherwise.

Useful for radio groups.

const optionXLink = stringLink.equals( 'optionX' );

linkToAny.enabled( defaultValue = '' ) : Link

Create boolean link which value is false when parent link is null (or undefined), and true otherwise. Whenever the enabled-link is set to true, it sets parent link to the defaultValue.

This type of links is used to support enabling/disabling of individual form controls with a dedicated checkbox. <Input> control and the rest of form controls must be modified to disable themselves when its valueLink.value === null.

const textLink = this.linkAt( 'text' );

return (
    <Checkbox checkedLink={ textLink.enabled() } />
    <Input valueLink={ textLink } /> 
);

Custom links

link.onChange( callback : any => void ) : Link

Create the wrapper for existing link which will invoke callback whenever new value is set. Similar to:

Link.value( link.value, x => {
    callback( x );
    link.set( x );
});

link.pipe( transform : ( next, prev ) => any ) : Link

Create the wrapper for existing link which will invoke given transform function before new value is set. Returned value will be used as new link value, and if it's undefined update will be rejected. Similar to:

Link.value( link.value, x => {
    const y = callback( x, link.value );
    if( y !== undefined ){
        link.set( y );
    }
});

Usage example:

<Input valueLink={ strLink.pipe( x => x && x.toUpperCase() ) }/>

Note for TypeScript users

Link is the parametric type Link< T >, where T is the type of the enclosed value.

TypeScript properly infers type of the link and perform static checks failing on missing state members.

interface MyState {
    name : string
}
...
const nameLink = this.linkAt( 'name' ); // Link< string >
const missingLink = this.linkAt( 'age' ); // Compile-time error - no such a member in state.

Link updates

Simple value updates

link.set( x ) : void

link.requestChange( x ) : void

Set link to the given value.

<button onClick={ () => boolLink.set( !boolLink.value ) } />

link.update( prevValue => any ) : void

Update link value using the given value transform function.

<button onClick={ () => boolLink.update( x => !x ) } />

link.action( ( prevValue, event ) => any ) : ( event => void )

Create UI event handler which will transform the link.

link.action takes transform function, and produce a new function which takes single event argument. When it's called, event and link value are passes as transform parameters, and link will be updated with returned value.

This is particularly useful in (but not restricted to) UI event handlers.

// simple click event handler...
<button onClick={ boolLink.action( x => !x ) } />

// manual binding to input control:
const setValue = ( x, e ) => e.target.value;
...
<input  value={ link.value }
        onChange={ link.action( setValue ) } />

Link to objects and arrays updates

Plain objects and arrays are shallow copied by link.update() and within link.action() handlers, thus it's safe just to update the value in place.

$object.update( clonedObject => Object ) : void

Update enclosed object or array.

$object.action( ( clonedObject, event ) => Object ) : ( event => void )

Creates action to update enclosed object or array. Object is shallow copied before the update and it's safe to

<button onClick={ () => $object.update( obj => {
                                obj.a = 1;
                                return obj;
                            }) } />

$object.removeAt( key ) : void

$object.at( key ).remove() : void

Remove element with a given key from the enclosed object ar array.

Link to arrays updates

Link to arrays proxies some important Array methods.

$array.splice( ... ) : void

$array.push( ... ) : void

$array.unshift( ... ) : void

Works in the same way and accepts the same parameters as corresponding Array method, but returns undefined and leads to the proper purely functional update of the parent object chain.

Links validation

It's highly recommended to read tutorial on validation with value links.

$link.check( value => boolean, error = 'Invalid value' ) : Link

Evaluate given condition for the current link value, and assign given error object to the link.error when it fails. There are no restriction on the error object shape and type.

It's possible to assign default error message to the validator function. linked-controls package provides isRequired and isEmail generic validator functions as an examples:

export const isRequired = x => x != null && x !== '';
isRequired.error = 'Required';

Checks can be chained. In this case, the first check which fails will leave its error in the link.

$link.error : any | void

This field is populated by the link.check method and must not be assigned manually. It should be used by a custom <Input /> control to display an error (see linked-controls and examples).

// Simple check
const $num = this.$at( 'num' )
                .check( x => x >= 0 && x <=5 );

console.log( $num.error );

// Check with error message
const $num = this.$at( 'num' )
                .check( x => x >= 0 && x <=5, 'Number must be between 0 and 5' );

console.log( $num.error );

// Chained checks
const $num = this.$at( 'num' )
                .check( x => x >= 0, 'Negative numbers are not allowed' )
                .check( x => x <= 5, 'Number should be not greater than 5' );

console.log( $num.error );