watchable

The module implements 2 functions: watch() and unwatch() to create and remove watchable fields in an existing object.

These functions can be used through a base-class Watchable(), a mixin, or as static functions of the base-class. The use of the static functions of the base-class is expected to be the most commen.

The base-class Watchable() is written as an ES5 class, but can also be used as an ES6 base-class.

Usage

The call Watchable.watch(obj, prop, set_handler, get_handler) to create the watch has the following parameters:

• obj - the object on which to create the watch

• prop - the name of the watched field
  If the field prop already exists on the object obj, its value is kept.

• set_handler - a function taking 3 parameters:

  1. the name of the property
  2. the current value, and
  3. the intended next value.

  This handler must return the next value, which may be different from the intended next value, but typically isn't.

• get_handler - an optional function taking 2 parameters:

  1. the name of the property, and
  2. the current value.

  This handler must return a value that is returned to the caller, which may be different from the current/actual value, but typically isn't.

The call Watchable.unwatch(obj, prop) to remove the watch only takes the first 2 parameters: obj and prop.

NOTE: when the object, on which the watch is defined, also supports an event-emitter or publish/subscribe protocol, simple, but very powerful, handler functions can be created. This is not shown in the examples.

Static Example

import * as Watchable from 'watchable';

let obj = { a: 1, b: 2, c: 3};

Watchable.watch(obj, 'b', (prop, old, val) => {
    console.log(prop+':', old, '=>', val);
    return val;
});
obj.b = 5;
// Expect output:
// b: 2 => 5

Using the get_handler is rare, and is probably only useful to debug, unknown or unexpected access to a field.

Watchable.watch(obj, 'c',
        (prop, old, val) => {
            console.log(prop+':', old, '=>', val);
            return val;
        },
        (prop, val) => {
            // optional log
            console.log('access property:', prop, ', value:', val);
            // optional breakpoint
            return val;
        },
    );
obj.c += 4
// Expect output:
// access property: c, value: 4
// c: 3 => 7

Mixin Example

/**
 * Adds the functions `watch()` and `unwatch()` to the prototype
 * of the function `cls()`.
 * @param {function} cls 
 */
function mixinWatchable(cls) {

    Object.defineProperties(cls.prototype, {
        watch: {
            enumerable: false,
            configurable: true,
            writable: false,
            value: function (...args) {return Watchable.watch(this, ...args)},
        },
        unwatch: {
            enumerable: false,
            configurable: true,
            writable: false,
            value: function (...args) {return Watchable.unwatch(this, ...args)},
        },
    });
    
}

function Target() {
    this.a = 1;
    this.b = 2;
    this.c = 3;
}
mixinWatchable(Target);

obj = new Target();
obj.watch('a', (prop, old, val) => {
    console.log(prop+':', old, '=>', val);
    return val;
});
obj.a = 7;
// Expect output:
// a: 1 => 7

Credits and Changes

The original code was written by Eli Grey, in april 2012, as an Object.watch() polyfill.

The changes made to the original code are:

1. the polyfill was changed to the base-class Watchable(),
2. the static functions were added, and
3. and the get_handler parameter was added to the watch() function.

License

ISC