204 lines
7.3 KiB
Markdown
204 lines
7.3 KiB
Markdown
[](http://build.chromium.org/p/client.polymer/waterfall) [](https://github.com/igrigorik/ga-beacon)
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## Learn the tech
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### Why observe-js?
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observe-js is a library for observing changes in JavaScript data. It exposes a high-level API and uses Object.observe if available, and otherwise performs dirty-checking. observe-js requires ECMAScript 5.
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### Observable
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observe-js implements a set of observers (PathObserver, ArrayObserver, ObjectObserver, CompoundObserver, ObserverTransform) which all implement the Observable interface:
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```JavaScript
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{
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// Begins observation. Value changes will be reported by invoking |changeFn| with |opt_receiver| as
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// the target, if provided. Returns the initial value of the observation.
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open: function(changeFn, opt_receiver) {},
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// Report any changes now (does nothing if there are no changes to report).
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deliver: function() {},
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// If there are changes to report, ignore them. Returns the current value of the observation.
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discardChanges: function() {},
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// Ends observation. Frees resources and drops references to observed objects.
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close: function() {}
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}
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```
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### PathObserver
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PathObserver observes a "value-at-a-path" from a given object:
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```JavaScript
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var obj = { foo: { bar: 'baz' } };
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var observer = new PathObserver(obj, 'foo.bar');
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observer.open(function(newValue, oldValue) {
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// respond to obj.foo.bar having changed value.
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});
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```
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PathObserver will report a change whenever the value obtained by the corresponding path expression (e.g. `obj.foo.bar`) would return a different value.
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PathObserver also exposes a `setValue` method which attempts to update the underlying value. Setting the value does not affect notification state (in other words, a caller sets the value but does not `discardChanges`, the `changeFn` will be notified of the change).
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```JavaScript
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observer.setValue('boo');
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assert(obj.foo.bar == 'boo');
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```
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Notes:
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* If the path is ever unreachable, the value is considered to be `undefined`.
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* If the path is empty (e.g. `''`), it is said to be the empty path and its value is its root object.
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* PathObservation respects values on the prototype chain
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### ArrayObserver
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ArrayObserver observes the index-positions of an Array and reports changes as the minimal set of "splices" which would have had the same effect.
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```JavaScript
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var arr = [0, 1, 2, 4];
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var observer = new ArrayObserver(arr);
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observer.open(function(splices) {
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// respond to changes to the elements of arr.
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splices.forEach(function(splice) {
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splice.index; // index position that the change occurred.
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splice.removed; // an array of values representing the sequence of elements which were removed
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splice.addedCount; // the number of elements which were inserted.
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});
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});
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```
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ArrayObserver also exposes a utility function: `applySplices`. The purpose of `applySplices` is to transform a copy of an old state of an array into a copy of its current state, given the current state and the splices reported from the ArrayObserver.
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```JavaScript
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AraryObserver.applySplices = function(previous, current, splices) { }
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```
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### ObjectObserver
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ObjectObserver observes the set of own-properties of an object and their values.
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```JavaScript
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var myObj = { id: 1, foo: 'bar' };
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var observer = new ObjectObserver(myObj);
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observer.open(function(added, removed, changed, getOldValueFn) {
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// respond to changes to the obj.
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Object.keys(added).forEach(function(property) {
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property; // a property which has been been added to obj
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added[property]; // its value
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});
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Object.keys(removed).forEach(function(property) {
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property; // a property which has been been removed from obj
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getOldValueFn(property); // its old value
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});
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Object.keys(changed).forEach(function(property) {
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property; // a property on obj which has changed value.
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changed[property]; // its value
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getOldValueFn(property); // its old value
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});
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});
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```
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### CompoundObserver
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CompoundObserver allows simultaneous observation of multiple paths and/or Observables. It reports any and all changes in to the provided `changeFn` callback.
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```JavaScript
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var obj = {
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a: 1,
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b: 2,
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};
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var otherObj = { c: 3 };
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var observer = new CompoundObserver();
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observer.addPath(obj, 'a');
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observer.addObserver(new PathObserver(obj, 'b'));
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observer.addPath(otherObj, 'c');
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observer.open(function(newValues, oldValues) {
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// Use for-in to iterte which values have changed.
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for (var i in oldValues) {
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console.log('The ' + i + 'th value changed from: ' + newValues[i] + ' to: ' + oldValues[i]);
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}
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});
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```
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### ObserverTransform
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ObserverTransform is used to dynamically transform observed value(s).
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```JavaScript
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var obj = { value: 10 };
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var observer = new PathObserver(obj, 'value');
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function getValue(value) { return value * 2 };
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function setValue(value) { return value / 2 };
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var transform = new ObserverTransform(observer, getValue, setValue);
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// returns 20.
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transform.open(function(newValue, oldValue) {
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console.log('new: ' + newValue + ', old: ' + oldValue);
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});
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obj.value = 20;
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transform.deliver(); // 'new: 40, old: 20'
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transform.setValue(4); // obj.value === 2;
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```
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ObserverTransform can also be used to reduce a set of observed values to a single value:
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```JavaScript
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var obj = { a: 1, b: 2, c: 3 };
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var observer = new CompoundObserver();
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observer.addPath(obj, 'a');
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observer.addPath(obj, 'b');
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observer.addPath(obj, 'c');
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var transform = new ObserverTransform(observer, fuction(values) {
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var value = 0;
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for (var i = 0; i < values.length; i++)
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value += values[i]
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return value;
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});
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// returns 6.
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transform.open(function(newValue, oldValue) {
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console.log('new: ' + newValue + ', old: ' + oldValue);
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});
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obj.a = 2;
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obj.c = 10;
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transform.deliver(); // 'new: 14, old: 6'
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```
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### Path objects
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A path is an ECMAScript expression consisting only of identifiers (`myVal`), member accesses (`foo.bar`) and key lookup with literal values (`arr[0]` `obj['str-value'].bar.baz`).
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`Path.get('foo.bar.baz')` returns a Path object which represents the path. Path objects have the following API:
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```JavaScript
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{
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// Returns the current of the path from the provided object. If eval() is available, a compiled getter will be
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// used for better performance.
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getValueFrom: function(obj) { }
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// Attempts to set the value of the path from the provided object. Returns true IFF the path was reachable and
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// set.
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setValueFrom: function(obj, newValue) { }
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}
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```
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Path objects are interned (e.g. `assert(Path.get('foo.bar.baz') === Path.get('foo.bar.baz'));`) and are used internally to avoid excessive parsing of path strings. Observers which take path strings as arguments will also accept Path objects.
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## About delivery of changes
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observe-js is intended for use in environments which implement Object.observe, but it supports use in environments which do not.
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If `Object.observe` is present, and observers have changes to report, their callbacks will be invoked at the end of the current turn (microtask). In a browser environment, this is generally at the end of an event.
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If `Object.observe` is absent, `Platform.performMicrotaskCheckpoint()` must be called to trigger delivery of changes. If `Object.observe` is implemented, `Platform.performMicrotaskCheckpoint()` has no effect.
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