typescript - what is the use of Zone.js in Angular 2

Question

Currently am learning Angular 2.0 and i come accross the file Zone.js, and i would like to know what is the purpose of the file Zone.js and how will it make my application better.

Answer 1

A zone is an execution context that persists across async tasks, and allows the creator of the zone to observe and control execution of the code within the zone.

I think that the main purpose of using zonejs in angular2 is to know when to render.

According to NgZone Primer (Chapter 5: Use Cases/Use Case 3: Framework Auto Render)

Frameworks, such as Angular, need to know when all of the application work has completed and perform DOM update before the host environment performs the pixel rendering. In practice this means that the framework is interested when the main task and the associated micro tasks have executed but before the VM hands over the control to the host.

Angular uses the zone to patch async APIs(addEventListener, setTimeout(), ...) and uses notifications from these patched APIs to run change detection every time some async event happened.

For this Angular zone has onMicrotaskEmpty event

https://github.com/angular/angular/blob/2.2.4/modules/%40angular/core/src/zone/ng_zone.ts#L199

and ApplicationRef subscribes to this event to trigger change detection (Application.tick)

https://github.com/angular/angular/blob/2.2.4/modules/%40angular/core/src/application_ref.ts#L405-L406

Also zonejs is useful for debugging, testing, profiling. It helps you see whole call stack if you face with some error.

Zone patches async APIs like:

Promise
XHR
fetch
Error
addEventListener
removeEventListener
FileReader
WebSocket
MutationObserver
WebKitMutationObserver
document.registerElement
navigator.geolocation.getCurrentPosition
navigator.geolocation.watchPosition

copy cut paste abort blur focus canplay canplaythrough change click contextmenu 
dblclick drag dragend dragenter dragleave dragover dragstart drop 
durationchange emptied ended input invalid keydown keypress keyup 
load loadeddata loadedmetadata loadstart message 
mousedown mouseenter mouseleave mousemove mouseout mouseover mouseup 
pause play playing progress ratechange reset scroll 
seeked seeking select show stalled submit suspend 
timeupdate volumechange waiting 
mozfullscreenchange mozfullscreenerror mozpointerlockchange 
mozpointerlockerror error webglcontextrestored webglcontextlost webglcontextcreationerror

setTimeout/clearTimeout
setInterval/clearInterval
setImmediate/clearImmediate

requestAnimationFrame/cancelAnimationFrame
mozRequestAnimationFrame/mozCancelAnimationFrame
webkitRequestAnimationFrame/webkitCancelAnimationFrame

alert
prompt
confirm

This articles might be usefull to understand how it works in angular2

• http://blog.thoughtram.io/angular/2016/02/01/zones-in-angular-2.html
• http://blog.kwintenp.com/how-the-hell-do-zones-really-work/