How setState call build in flutter?

Question

I know build will be called everytime setState called and I want to find the method chain in the flutter framework.

So I check the source code of flutter and find some steps of setState:

_element!.markNeedsBuild() mark state's element dirty.
owner!.scheduleBuildFor(this) call BuildOwner's onBuildScheduled and add element to the dirty elements list.
BuildOwner's onBuildScheduled is assigned in WidgetsBinding's initInstances():
And WidgetsBinding's initInstances() is called in runApp().

I had check carefully but didn't see any code call state's build method.

How setState call build in flutter internal？ Can we find any source code?

score 2 · Accepted Answer · answered May 30 '23 at 07:32

In handleDrawFrame() you see this part:

try {
  // PERSISTENT FRAME CALLBACKS
  _schedulerPhase = SchedulerPhase.persistentCallbacks;
  for (final FrameCallback callback in _persistentCallbacks) {
    _invokeFrameCallback(callback, _currentFrameTimeStamp!);
  }

I don't know exactly where it's added to the persistenCallbacks but looking at that variable in the code shows you

  /// The persistent callbacks (scheduled by
  /// [SchedulerBinding.addPersistentFrameCallback]) are currently executing.
  ///
  /// Typically, this is the build/layout/paint pipeline. See
  /// [WidgetsBinding.drawFrame] and [SchedulerBinding.handleDrawFrame].
  persistentCallbacks,

Meaning it calls drawFrame

Inside drawFrame you'll find this line:

    buildOwner!.buildScope(rootElement!);

And inside this method it looks through all elements in _dirtyElements. Note that in scheduleBuildFor that you posted in this question it wass added to this. So in this buildScope it calls

element.rebuild();

This in turn calls

performRebuild():

which for ComponentElements says

  /// Calls the [StatelessWidget.build] method of the [StatelessWidget] object
  /// (for stateless widgets) or the [State.build] method of the [State] object
  /// (for stateful widgets) and then updates the widget tree.
  ///
  /// Called automatically during [mount] to generate the first build, and by
  /// [rebuild] when the element needs updating.
  @override
  @pragma('vm:notify-debugger-on-exception')
  void performRebuild() {

How setState call build in flutter?

1 Answers1