A recent global key duplication detection refactor now requires `Element` subclasses that override the `forgetChild()` to call `super()`.
Original behavior
When encountering a global key duplication that will be cleaned up by an element rebuild later, we must not report global key duplication. Our previous implementation threw an error as soon as duplication was detected, and didn't wait for the rebuild if the element with the duplicated global key would have rebuilt.
New behavior
The `forgetChild` of abstract class `Element` has a base implementation to remove global key reservation, and it is enforced by the `@mustCallSuper` meta tag. All subclasses that override the method have to call `super`; otherwise, the analyzer shows a linting error and global key duplication detection might throw an unexpected error.
- Who is affected
- Applications and packages relying on the previous behaviour or signature. Code that uses Element.
- Practical impact
- Behaviour or API differences that can break builds or change runtime behaviour.
- What action should I take?
- Follow the official migration guide below, update affected call sites, run `dart fix --apply` where a fix is available, then run your tests.
Before and after (from the official guide)
| 1 | class CustomElement extends Element { | |
| 2 | ||
| 3 | @override | |
| 4 | void forgetChild(Element child) { | |
| 5 | ... | |
| 6 | } | |
| 7 | } |
| 1 | class CustomElement extends Element { | |
| 2 | ||
| 3 | @override | |
| 4 | void forgetChild(Element child) { | |
| 5 | ... | |
| 6 | + | super.forgetChild(child); |
| 7 | } | |
| 8 | } |
Raw before / after
class CustomElement extends Element {
@override
void forgetChild(Element child) {
...
}
}class CustomElement extends Element {
@override
void forgetChild(Element child) {
...
super.forgetChild(child);
}
}Official migration guide (excerpt, © The Flutter Authors, CC BY 3.0)
In the following example, an app's Element
subclass overrides the forgetChild method.
Code before migration:
class CustomElement extends Element {
@override
void forgetChild(Element child) {
...
}
}Code after migration:
class CustomElement extends Element {
@override
void forgetChild(Element child) {
...
super.forgetChild(child);
}
}AI_PROVIDER (for example to openai-compatible with a local Ollama or LM Studio endpoint) to enable grounded explanations. Everything else on this page works without an AI provider.Listed in the official breaking-changes index (docs.flutter.dev/release/breaking-changes).
An official announcement or migration guide documents an intended behaviour or API change. A verified source change (git diff) shows code changed but does not by itself prove a public API is breaking. AI interpretations are labelled and never used to classify a change.
- Source commit
- b3045439b7c0
- Source last updated
- Oct 21, 2026
- Last synchronized
- Oct 21, 2026
- Publication status
- published
Publication history
- Oct 21, 2026, 8:31 PM: new → published (ingestion) — Backed by an official breaking-change/migration guide.
AI_PROVIDER (for example to openai-compatible with a local Ollama or LM Studio endpoint) to enable grounded explanations. Everything else on this page works without an AI provider.