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Angular can skip an OnPush component subtree when nothing has marked it for checking. It checks that subtree when a template binding supplies a changed input, an event is handled within it, or another Angular mechanism—such as a changed signal read by its template—marks it for a check. Skipping is conditional traversal, not detachment: the exact result depends on where an update originates and how it reaches the component.
What “skipping a subtree” means
During change detection, Angular traverses the component tree. With OnPush, Angular can skip checking a component and its descendants when the subtree has no relevant trigger. The official guide describes this as skipping descendant subtrees whose OnPush roots have not received new inputs. Angular’s subtree-skipping guide explains the traversal cases.
This does not detach the component or make its view permanently inert. A later input, event, or other mechanism that marks it for checking can bring it back into the traversal.
When Angular checks or skips an OnPush subtree
A template binding supplies a changed input
When a parent passes an input to an OnPush child through a template binding, Angular checks whether the bound value changed using Object.is. If it did, Angular checks the receiving child. An OnPush descendant that receives no changed input of its own may still be skipped.
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For object and array inputs, this makes reference identity important. Mutating an object in place leaves the reference unchanged, so that mutation alone does not satisfy the changed-input condition. Creating a new object or array reference does. This is an input-trigger rule, not a guarantee that Angular enforces deep immutability.
An event is handled in the subtree
Where the event is handled affects which parts of the tree Angular checks:
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- Event in an eager component: Angular traverses the component tree, but can skip descendant subtrees rooted in
OnPushcomponents that have not received new inputs. - Event in an OnPush component: Angular checks the relevant tree for the event. Unrelated
OnPushbranches outside that component’s scope can still be skipped if they have no new inputs. - Event in a descendant: Angular checks the event component and its ancestors, including an
OnPushancestor, as well as the relevant subtree.
Thus, an event does not mean every OnPush component is necessarily checked; unrelated branches can remain eligible to skip.
Another Angular mechanism marks it for checking
An OnPush component is also checked when an Angular mechanism marks it for a future change-detection run. Signals are one example: when a template reads a signal, Angular tracks that component as a dependency. A change to the signal marks the component for checking. See the Angular signals guide.
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How to diagnose a stale OnPush view
If the rendered view does not reflect an update, trace how the update reaches the component rather than assuming that OnPush disables change detection:
- Check the input reference. If a parent mutates an object or array but passes the same reference, the template-bound input has not changed under
Object.is. Update the state in a way that provides a new reference when the input needs to trigger the child. - Check how the input was assigned. A change made through a component reference obtained with
ViewChildorContentChilddoes not automatically check anOnPushcomponent. When that imperative change needs a future check, useChangeDetectorRef.markForCheck(). - Check where the event was handled. An event handled inside the relevant subtree can trigger checking there and in its ancestors. An unrelated branch may remain skipped.
- Check signal use. Confirm that the component’s template reads the signal whose value changed. A template-read signal is tracked as a dependency and marks the component when its value changes.
Signals do not make contained objects deeply immutable: Angular’s signals guidance notes that readonly signals do not prevent deep mutation of their values. Prefer state updates that Angular can observe through the relevant input or signal mechanism.
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OnPush and Eager in current Angular
The current Angular API documents two strategies, OnPush and Eager. The comparison is about when a component is checked as Angular traverses the tree:
| Strategy | Checking behavior | What to know |
|---|---|---|
OnPush |
Angular can skip the subtree unless a relevant update marks it for checking. | Relevant triggers include changed template-bound inputs, events in the subtree, and mechanisms such as template-read signals. |
Eager |
Angular checks the component eagerly when change-detection traversal reaches it. | Descendant OnPush subtrees can still be skipped if they have not received new inputs. |
In Angular v22 and later, OnPush is the default. The API lists Default as a deprecated alias equivalent to Eager, with removal planned. Older Angular versions have different defaults and API details; check the documentation for the version used by your application. See the ChangeDetectionStrategy API reference.
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What performance benefit to expect
Subtree skipping is an option for controlling change-detection work, not a guaranteed speedup. Angular’s performance guide recommends profiling to find actual bottlenecks, such as slow computations or interactions; Angular DevTools can inspect the component tree and profile performance. The official guidance does not give a universal percentage improvement for OnPush, so measure your application rather than assuming a particular gain.
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