Angular Change Detection
A brief introduction
2 min read
How to explain someone how change detection works in Angular?
Components in Angular are structured as a tree. A lot of abstractization on it
By default, Angular detects if the reference of something that is bound to HTML changes
Every time it changes, it runs the change detection for all the tree nodes. It will start from AppRef which is the root node.
This is quite inefficient because it checks the whole tree, even if many of the components would not need to rerender
If you change from default change detection to OnPush change detection, Angular will trigger change detection for the specific branch only if:
@Inputvalues references changed
Event functions triggered (click), (scroll), timeout, setInterval and AJAX Http Requests
Async pipe used
For the event-bound method, this is possible because Angular overrides the addEventListener function which is browser native to something improved, that calls the markForCheck method
The caveat with the async pipe is that manually under the hood it triggers the change detection for the node and its parents. It calls
Usually, when you use OnPush change detection you use the following two methods
markForCheck()- This will trigger change detection for the current node and its parents
detectChanges()- This will trigger change detection for the current node ONLY
As good practice, don't use OnPush when immutability is not possible because it checks the reference, not the insides of the object.