Angular2 change detection misunderstanding - With plunker

Question

I'm trying to fully understand change detection with Angular2 final.

This include:

• Dealing with change detection strategies
• Attaching and detaching change detector from a component.

I thought I already got a pretty clear overview of those concepts, but to make sure my assumptions where right, I wrote a small plunkr to test them.

My general understanding about that where globally right, but in some situations, I get a little bit lost.

---

Here is the plunker: Angular2 Change detection playground

Quick explanation of the plunker:

Pretty simple:

• One parent component where you can edit one attribute that will get passed down to two children components:
• On child with change detection strategy set to OnPush
• On child with change detection strategy set to Default

The parent attribute can be passed to children components by either:

• Changing the whole attribute object, and creating a new one (" Change obj " button) (which trigger change detection on the OnPush child)
• Changing the members inside the attribute object ( "Change content" button) (which do not trigger change detection on the OnPush child)

For each child component, ChangeDetector can be attached or detached. ( "detach()" and "reattach()" buttons)

OnPush child have an additional internal property that can be edited, and change detection can be explicitly applied ( "detectChanges()" button)

---

Here are the scenarios where I get behaviours that I cannot explain:

Scenario1:

1. Detach Change detector of OnPush Children and Default Children (click " detach() " on both components)
2. Edit the parent attribute firstname and lastname
3. Click " Change obj " to pass the modified attribute to the children

Expected behavior: I expect BOTH children NOT to be updated, because they both have their change detector detached.

Current behavior: Default child is not updated, but OnPush child is updated .. WHY? It shouldn't because its CD is detached ...

Scenario2:

1. Detach CD for the OnPush component
2. Edit its internal value input and click change internal : Nothing happen, because CD is detached, so change is not detected... OK
3. Click detectChanges() : changes are detected and the view is updated. So far so good.
4. Once again, edit the internal value input and click change internal : Once again, nothing happen, because CD is detached, so change is not detected.. OK
5. Edit the parent attribute firstname and lastname.
6. Click " Change obj " to pass the modified attribute to the children

Expected behavior: OnPush children should NOT be updated at ALL, once again because its CD is detached...CD should not happen at all on this component

Current behavior: Both the value and the internal values are updated, seams like a full CD is applied to this component.

6. For the last time, edit the internal value input and click change internal : Change is detected, and internal value is updated...

Expected behavior: Internal value should NOT be updated because CD is still detached

Current behavior: Internal value change is detected... WHY?

---

Conclusions:

According to those tests I conclude the following, which seams strange to me:

• Component with OnPush strategy get 'changed detected' when their input changes, EVEN IF their change detector is detached.
• Component with OnPush strategy get their change detector re attached each time their input changed...

What do you think about those conclusions?

Can you explain this behavior in a better way?

Is this a bug or the desired behavior?

Answer 1

Update

Component with OnPush strategy get 'changed detected' when their input changes, EVEN IF their change detector is detached.

Since Angular 4.1.1 (2017-05-04) OnPush should respect detach()

https://github.com/angular/angular/commit/acf83b9

Old version

There are a lot of undocumented stuff about how change detection works.

We should be aware about three main changeDetection statuses ( cdMode ):

1) CheckOnce - 0

CheckedOnce means that after calling detectChanges the mode of the change detector will become Checked .

AppView class

detectChanges(throwOnChange: boolean): void {
  ...
  this.detectChangesInternal(throwOnChange);
  if (this.cdMode === ChangeDetectorStatus.CheckOnce) {
    this.cdMode = ChangeDetectorStatus.Checked; // <== this line
  }
  ...
}

2) Checked - 1

Checked means that the change detector should be skipped until its mode changes to CheckOnce .

3) Detached - 3

Detached means that the change detector sub tree is not a part of the main tree and should be skipped.

Here are places where Detached is used

AppView class

Skip content checking

detectContentChildrenChanges(throwOnChange: boolean) {
  for (var i = 0; i < this.contentChildren.length; ++i) {
    var child = this.contentChildren[i];
    if (child.cdMode === ChangeDetectorStatus.Detached) continue; // <== this line
    child.detectChanges(throwOnChange);
  }
}

Skip view checking

detectViewChildrenChanges(throwOnChange: boolean) {
  for (var i = 0; i < this.viewChildren.length; ++i) {
    var child = this.viewChildren[i];
    if (child.cdMode === ChangeDetectorStatus.Detached) continue; // <== this line
    child.detectChanges(throwOnChange);
  }
}

Skip changing cdMode to CheckOnce

markPathToRootAsCheckOnce(): void {
  let c: AppView<any> = this;
  while (isPresent(c) && c.cdMode !== ChangeDetectorStatus.Detached) { // <== this line
    if (c.cdMode === ChangeDetectorStatus.Checked) {
      c.cdMode = ChangeDetectorStatus.CheckOnce;
    }
    let parentEl =
        c.type === ViewType.COMPONENT ? c.declarationAppElement : c.viewContainerElement;
    c = isPresent(parentEl) ? parentEl.parentView : null;
  }
}

Note: markPathToRootAsCheckOnce is running in all event handlers of your view:

So if set status to Detached then your view won't be changed.

Then how works OnPush strategy

OnPush means that the change detector's mode will be set to CheckOnce during hydration.

compiler/src/view_compiler/property_binder.ts

const directiveDetectChangesStmt = isOnPushComp ?
   new o.IfStmt(directiveDetectChangesExpr, [compileElement.appElement.prop('componentView')
           .callMethod('markAsCheckOnce', [])
           .toStmt()]) : directiveDetectChangesExpr.toStmt();

https://github.com/angular/angular/blob/2.1.2/modules/%40angular/compiler/src/view_compiler/property_binder.ts#L193-L197

Let's see how it looks in your example:

Parent factory (AppComponent)

And again back to the AppView class :

markAsCheckOnce(): void { this.cdMode = ChangeDetectorStatus.CheckOnce; }

Scenario 1

1) Detach Change detector of OnPush Children and Default Children (click "detach()" on both components)

OnPush.cdMode - Detached

3) Click "Change obj" to pass the modified attribute to the children

AppComponent.detectChanges
       ||
       \/
//if (self._OnPush_35_4.detectChangesInInputProps(self,self._el_35,throwOnChange)) {
//  self._appEl_35.componentView.markAsCheckOnce();
//}
OnPush.markAsCheckOnce
       ||
       \/
OnPush.cdMode - CheckOnce
       ||
       \/
OnPush.detectChanges
       ||
       \/
OnPush.cdMode - Checked

Therefore OnPush.dectectChanges is firing.

Here is conclusion:

Component with OnPush strategy get 'changed detected' when their input changes, EVEN IF their change detector is detached. Moreover It changes view's status to CheckOnce .

Scenario2

1) Detach CD for the OnPush component

OnPush.cdMode - Detached

6) Click "Change obj" to pass the modified attribute to the children

See 3) from scenario 1 => OnPush.cdMode - Checked

7) For the last time, edit the internal value input and click change internal: Change is detected, and internal value is updated ...

As I mentioned above, all event handlers includes markPathToRootAsCheckOnce . So:

markPathToRootAsCheckOnce
        ||
        \/
OnPush.cdMode - CheckOnce
        ||
        \/
OnPush.detectChanges
        ||
        \/
OnPush.cdMode - Checked

As you can see OnPush strategy and ChangeDetector manage one property - cdMode

Component with OnPush strategy get their change detector re attached each time their input changed ...

In conclusion I want to say that seems you're right.