watch-property

Monitor a property, or multiple properties, on an object and return true when the value has changed between invocations.

What is this useful for?

This module has many use-cases, one of which would be for help with an Immediate-Mode GUI or updating expensive attributes in a render loop.

Install

npm install watch-property --save

Basic Example

var object = {
    string: 'foo'
};
 
var stringChanged = watch(object, 'string');
 
stringChanged(); //false
object.string = 'bar';
stringChanged(); //true
stringChanged(); //false

API

watch(object, key [, predicateFn, retainInitValue])

Provide an object and a key to watch. Optionally you can provide a predicateFn(value, oldValue):Boolean function to determine if changed, or set retainInitValue to true if you would like to consider any value except the first as a changed value.

watch.any(object, keysArray [, predicateFn, retainInitValue])

The same as watchProperty except receives an array of keys to watch and if any have changed values the function returns true. alias: watch.some

watch.all(object, keysArray [, predicateFn, retainInitValue])

The same as watchProperty except receives an array of keys to watch and only returns true if all of the values have changed. alias: watch.every

Web Example

var watch = require('watch-property'),
    eventMap = require('event-map');
    
var byId = document.getElementById.bind(document),
    container = byId('container'),
    pressedDisplay = byId('pressed-display'),
    nameDisplay = byId('name-display'),
    clickDisplay = byId('click-display');
 
 
var parameters = {
    pressed: false,
    numClicks: 0,
    name: 'kyle'
};
 
var clicksChanged = watch(parameters, 'numClicks'),
    btnIsPressed = watch(parameters, 'pressed', true), //retainInitValue is 3rd parameter
    nameChanged = watch(parameters, 'name');
    
function onRender(){
    pressedDisplay.innerHTML = btnIsPressed() ? 'Pressed' : 'Released';
        
    if(clicksChanged()){
        clickDisplay.innerHTML = parameters.numClicks;
    }
    if(nameChanged()){
        nameDisplay.innerHTML = parameters.name;
    }
    
    window.requestAnimationFrame(onRender);
}
 
onRender();
 
 
eventMap({
    'mousedown #button': ()=> parameters.pressed = true,
    'mouseup #button': ()=> parameters.pressed = false,
    'click #button': ()=> parameters.numClicks++,
    'change #name-input': (event)=> parameters.name = event.target.value
});
 

Example using watchProperty.any (alias some)

var watch = require('watch-property');
var el = document.getElementById('moving-element');
 
var position = {
    top: 0,
    left: 0
};
 
var positionChanged = watch.any(position, ['top', 'left']);
 
function updatePosition(){
    el.style.top = position.top + 'px';
    el.style.left = position.left + 'px';
}
 
updatePosition();
 
function onRender(){
    if(positionChanged()){
        updatedPosition();
    }
    
    window.requestAnimationFrame(onRender);
}
 
onRender();
 
//do anything to be modifying the position (a tween or a user action)

License

MIT, see LICENSE.md for details.