How does Angular 2 change detection work?

Question

In Angular 1, change detection was by dirty checking the $scope hierarchy. We would implicitly or explicitly create watchers in our templates, controllers or components.

Answer 1

Zone.js

Changes happen as a reaction to something, so in this respect they are asynchronous. They are caused by asynchronous actions, and in the browser world those are Events. To intercept those events angular uses zone.js, which patches JavaScript call stack (I beleive, someone correct me if I'm wrong) and exposes hooks that can be used to take other actions.

function angular() {...}
zone.run(angular);

If you imagine this angular function is the entire Angular, this would be how it is run in zone. By doing so Events can be intercepted and if they are triggered we can assume changes happen, and listen/watch for them.

ApplicationRef

In reality ApplicationRef creates the zone:

/**
 * Create an Angular zone.
 */
export function createNgZone(): NgZone {
  return new NgZone({enableLongStackTrace: assertionsEnabled()});
}

and class NgZone is created with few event emitters:

  this._onTurnStartEvents = new EventEmitter(false);
  this._onTurnDoneEvents = new EventEmitter(false);
  this._onEventDoneEvents = new EventEmitter(false);
  this._onErrorEvents = new EventEmitter(false);

that it exposes to the outside world via getters:

  get onTurnStart(): /* Subject */ any { return this._onTurnStartEvents; }
  get onTurnDone() { return this._onTurnDoneEvents; }
  get onEventDone() { return this._onEventDoneEvents; }
  get onError() { return this._onErrorEvents; }

When ApplicationRef is created it subscribes to the zone's events, specifically onTurnDone():

this.zone.onTurnDone
  .subscribe(() => this.zone.run(() => this.tick());

Changes

When events are triggered tick() function is run which loops through every component:

  this._changeDetectorRefs.forEach((detector) => detector.detectChanges());

and detects changes based on components' ChangeDetectionStrategy. Those changes are collected as an array of SimpleChange objects:

addChange(changes: {[key: string]: any}, oldValue: any, newValue: any): {[key: string]: any} {
  if (isBlank(changes)) {
    changes = {};
  }
  changes[this._currentBinding().name] = ChangeDetectionUtil.simpleChange(oldValue, newValue);
  return changes;
}

witch is available for us through onChanges interface:

export interface OnChanges { 
  ngOnChanges(changes: {[key: string]: SimpleChange}); 
}

Answer 2

Angular creates a change detector object (see ChangeDetectorRef) per component, which tracks the last value of each template binding, such as {{service.a}}. By default, after every asynchronous browser event (such as a response from a server, or a click event, or a timeout event), Angular change detection executes and dirty checks every binding using those change detector objects.

If a change is detected, the change is propagated. E.g.,

• If an input property value changed, the new value is propagated to the component's input property.
• If a {{}} binding value changed, the new value is propagated to DOM property textContent.
• If the value of x changes in a style, attribute, or class binding – i.e., [style.x] or [attr.x] or [class.x] – the new value is propagated to the DOM to update the style, HTML attribute, or class.

Angular uses Zone.js to create its own zone (NgZone), which monkey-patches all asynchronous events (browser DOM events, timeouts, AJAX/XHR). This is how change detection is able to automatically run after each asynchronous event. I.e., after each asynchronous event handler (function) finishes, Angular change detection will execute.

I have a lot more detail and reference links in this answer: What is the Angular2 equivalent to an AngularJS $watch?