Triggering change detection manually in Angular

Question

I'm writing an Angular component that has a property Mode(): string.

I would like to be able to set this property programmatically not in response to any event.

The problem is that in the absence of a browser event, a template binding {{Mode}} doesn't update.

Is there a way to trigger this change detection manually?

Rains

Answer 1

Try one of these:

• ApplicationRef.tick() - similar to AngularJS's $rootScope.$digest() -- i.e., check the full component tree
• NgZone.run(callback) - similar to $rootScope.$apply(callback) -- i.e., evaluate the callback function inside the Angular zone. I think, but I'm not sure, that this ends up checking the full component tree after executing the callback function.
• ChangeDetectorRef.detectChanges() - similar to $scope.$digest() -- i.e., check only this component and its children

You can inject ApplicationRef, NgZone, or ChangeDetectorRef into your component.

Pericarp

Thanks, I opted for the 3rd solution so as to not check everything, since the changes are fairly localized. I should investigate the other options when I have more time. Are there any performance implications with each choice? – Rains

+1 for ChangeDetectorRef.detectChanges(). validators were firing before my directive could update an input's value. – Ragi

is ApplicationRef.tick() working nowadays with RC3 ?. – Crambo

I'm on rc4 now and none of these work anymore. The objects do not contain tick(), run() and detectChanges() anymore – Tiltyard

@Guntram, I'm using the first and third in an rc4 app of mine. My guess is you have typings / installation of the libraries messed up? – Rostock

hmmm... I could access these objects, but they did not contain these methods. Maybe there are some libs messed up, as I switched from rc4 to rc5 and rolled back and also from rc4 to rc6 and rolled back - because the updates probably switched too much stuff around. I solved our current problem differently now, but we will need to use "manual" events at some point. (I'll keep this post bookmarked ;) ) – Tiltyard

Thank you! I was able to fix this issue in a select component by using changeDetectionRef.detectChanges(), see commit github.com/tb/angular2-mdl-ext/commit/… – Eringo

ApplicationRef.tick() and ChangeDetectorRef.detectChanges() are still present in 2.0.0 final. – Kowalewski

Just thought I'd mention this. These are not static methods, they're instance methods. You're going to need to inject these classes as services. – Chetnik

@Mark NgZone.run(callback) OR ChangeDetectorRef.detectChanges() which is better in performance? – Interminable

ApplicationRef.tick() helped solve an issue with FireFox v17 (<20) – Hyrcania

Hi Mark , I've asked a similar question and got one more option than those 3. I think that there's one more solution .https://mcmap.net/q/67640/-angular-4-what-is-the-right-way-to-quot-wait-for-operation-quot ( add it so your answer will help others) – Hairpiece

Any way to schedule a function to run on the next digest cycle? Similar to $timeout(fn, 0, true) – Garboard

ApplicationRef.tick() helped me also inside global ErrorHandler. On network errors, UI got stuck. I hide the progress spinner overlay and show a user friendly error message, but the changes were not displayed in UI unless I interacted with the browser window. After adding tick, my error handler started working properly. I guess, after a throw occurs somewhere in the code, Angular aborts processing changes for a moment and needs a nudge to resume. – Spermicide

In Angular 8 you need to create a reference to the ApplicationRef in the constructor for it to work. That being said, neither it nor detectChanges() is helping in my situation where I am changing the active menu item inside a subscription but it doesn't actually goto that menu until you do something else like click a checkbox. Maddening because it works perfectly fine in the same component outside the subscription. – Thrashing

Answer 2

I used accepted answer reference and would like to put an example, since Angular 2 documentation is very very hard to read, I hope this is easier:

1. Import NgZone:

   import { Component, NgZone } from '@angular/core';
   

2. Add it to your class constructor

   constructor(public zone: NgZone, ...args){}
   

3. Run code with zone.run:

   this.zone.run(() => this.donations = donations)
   

Attaint

where should you put the zone.run code and what exactly is donations? – Levitical

@Levitical donations is any property you want to update, so it could be this.foo = bar. zone.run goes wherever you want to update stuff. – Tav

I used this solution to force a view update when using document.addEventListener("resume", callback) – Digressive

As mentioned by @marcovtwout, as of today, this is the only trick I found to work inside chrome.runtime.onMessage.addListener when using Angular to build a Chrome Extension. – Incendiarism

This worked for me where I am creating a component programmatically and showing it in a dialog box. The dialog box would pop-up but was empty until clicked. In my case the displayed component template has an *ngIf on the top-most element and so, for some reason, needed a cd cycle. – Pleiades

Answer 3

I was able to update it with markForCheck()

Import ChangeDetectorRef

import { ChangeDetectorRef } from '@angular/core';

Inject and instantiate it

constructor(private ref: ChangeDetectorRef) { 
}

Finally mark change detection to take place

this.ref.markForCheck();

Here's an example where markForCheck() works and detectChanges() don't.

https://plnkr.co/edit/RfJwHqEVJcMU9ku9XNE7?p=preview

EDIT: This example doesn't portray the problem anymore :( I believe it might be running a newer Angular version where it's fixed.

(Press STOP/RUN to run it again)

Borlase

Goo point for detectChanges() not working. I had noticed the same issue and found that it works if the change detection is no OnPush. Would be good to get an explanation for that... – Benzol

It seems like detectChanges is actually working in this plunker? Am I missing something? – Jesher

You should note that ChangeDetectorRef only works in Components – Bick

(!) This is a bad example, I am sorry. Example does not show anything. First item just get overwritten. Just adjust timers a bit ... – Cohl

This example doesn't portrays the problem anymore :( I believe it might be running a newer Angular version where it's fixed. – Borlase

The distinction does still exist and is probably related to this github.com/juleskremer/angular/commit/… (when using projected content). So it is still possible to demonstrate this but in a different way. – Current

Just my uneducated result: With changeDetection: hangeDetectionStrategy.OnPush , I had to use detectChanges() – Tum

Answer 4

In Angular 2+, try the @Input decorator

It allows for some nice property binding between parent and child components.

First create a global variable in the parent to hold the object/property that will be passed to the child.

Next create a global variable in the child to hold the object/property passed from the parent.

Then in the parent html, where the child template is used, add square brackets notation with the name of the child variable, then set it equal to the name of the parent variable. Example:

<child-component-template [childVariable] = parentVariable>
</child-component-template>

Finally, where the child property is defined in the child component, add the Input decorator:

@Input()
public childVariable: any

When your parent variable is updated, it should pass the updates to the child component, which will update its html.

Also, to trigger a function in the child component, take a look at ngOnChanges.

Halford

NO...thats the problem...if you update in the parent...say for exmaple is updated "by reference" in a static method the child wont pick it up as change detection hasnt occured – Kew

I have got the same problem for days now. I try to retrieve data from PHP backend and data does not Update at all. On Microsoft Edge works fine but on other browser it does not. I keep getting the data i first got on start but if data changes it does not get updated on view – Quodlibet