Can addPostFrameCallback get called after the widget is disposed? - flutter

Question

From the docs of addPostFrameCallback, I see that
Post-frame callbacks cannot be unregistered. They are called exactly
once.
and so I wonder if addPostFrameCallback can be called after the widget is disposed since it cannot be unregistered?

Answer 1

I think the _postFrameCallback's callback will be called after the widget is disposed.
View the Flutter source code:
firstly, Post-frame callbacks cannot be unregistered. They are called exactly once. event registered after widget disposed, but still can't unregister.
secondly, when a new frame comes down, Flutter framework just check callback whether null, and then call the callback directly.
void handleBeginFrame(Duration? rawTimeStamp) {
final List<FrameCallback> localPostFrameCallbacks =
List<FrameCallback>.from(_postFrameCallbacks);
_postFrameCallbacks.clear();
for (final FrameCallback callback in localPostFrameCallbacks)
_invokeFrameCallback(callback, _currentFrameTimeStamp!);
}
void _invokeFrameCallback(FrameCallback callback, Duration timeStamp, [ StackTrace? callbackStack ]) {
assert(callback != null);
callback(timeStamp);
}
So, the callback will be called.
But, you can't update the UI (setState) in the callback because widgets have already been disposed.
example:
import 'package:flutter/material.dart';
import 'package:flutter/scheduler.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
home: Test(),
);
}
}
class Test extends StatefulWidget {
#override
_TestState createState() => _TestState();
}
class _TestState extends State<Test> {
bool isEnabled = false;
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child:Listener(
onPointerDown: (event) {
setState(() {
isEnabled = true;
});
},
onPointerUp: (event) {
setState(() {
isEnabled = false;
});
},
child: Container(
height: 200,
width: 300,
color: Colors.red,
child: isEnabled ? Test1() : Text("null"),
),
),
),
);
}
}
class Test1 extends StatefulWidget {
#override
_Test1State createState() => _Test1State();
}
class _Test1State extends State<Test1> {
#override
Widget build(BuildContext context) {
return Text("hahah");
}
_onFrameStart(Duration duration) {
print("in _onFrameStart");
}
#override
void dispose() {
SchedulerBinding.instance.addPostFrameCallback(_onFrameStart);
super.dispose();
}
}