Life cycle in flutter
Does flutter have a method like Activity.resume() which can tell developer the user has gone back to the activity.
When I pick the data from internet in Page-B and go back to Page-A, how can I let Page-A know that the data is prepared.
Solution 1:
-
createState(): When the Framework is instructed to build a StatefulWidget, it immediately callscreateState() -
mountedis true: WhencreateStatecreates your state class, abuildContextis assigned to that state.buildContextis, overly simplified, the place in the widget tree in which this widget is placed. Here's a longer explanation. All widgets have abool this.mountedproperty. It is turned true when thebuildContextis assigned. It is an error to callsetStatewhen a widget is unmounted. -
initState(): This is the first method called when the widget is created (after the class constructor, of course.)initStateis called once and only once. It must callsuper.initState(). -
didChangeDependencies(): This method is called immediately afterinitStateon the first time the widget is built. -
build(): This method is called often. It is required, and it must return a Widget. -
didUpdateWidget(Widget oldWidget): If the parent widget changes and has to rebuild this widget (because it needs to give it different data), but it's being rebuilt with the sameruntimeType, then this method is called. This is because Flutter is re-using the state, which is long lived. In this case, you may want to initialize some data again, as you would ininitState. -
setState(): This method is called often from the framework itself and from the developer. Its used to notify the framework that data has changed -
deactivate(): Deactivate is called when State is removed from the tree, but it might be reinserted before the current frame change is finished. This method exists basically because State objects can be moved from one point in a tree to another. -
dispose():dispose()is called when the State object is removed, which is permanent. This method is where you should unsubscribe and cancel all animations, streams, etc. -
mountedis false: The state object can never remount, and error will be thrown ifsetStateis called.
Solution 2:
Constructor
This function is not part of the life cycle, because this time the State of the widget property is empty, if you want to access the widget properties in the constructor will not work. But the constructor must be to the first call.
createState
When Flutter is instructed to build a StatefulWidget, it immediately calls createState()
Init State
Called when this object is inserted into the tree.
When inserting the render tree when invoked, this function is called only once in the life cycle. Here you can do some initialization, such as initialization State variables.
setState
The setState() method is called often from the Flutter framework itself and from the developer.
didChangeDependencies
Called when a dependency of this [State] object changes.
didUpdateWidget
Called whenever the widget configuration changes.
deactivate
Called when this object is removed from the tree. Before dispose, we will call this function.
dispose
Called when this object is removed from the tree permanently.
didChangeAppLifecycleState
Called when the system puts the app in the background or returns the app to the foreground.
Here is a good detail document: https://www.bookstack.cn/read/flutterbyexample/aebe8dda4df3319f.md
import 'package:flutter/material.dart';
class ScreenLifecyle extends StatefulWidget {
ScreenLifecyleState state;
//createState(): When the Framework is instructed to build a StatefulWidget, it immediately calls createState()
@override
State<StatefulWidget> createState() {
// TODO: implement createState
return ScreenLifecyleState();
}
}
class ScreenLifecyleState extends State<ScreenLifecyle> {
/*
mounted is true: When createState creates your state class, a buildContext is assigned to that state.
BuildContext is, overly simplified, the place in the widget tree in which this widget is placed.
Here's a longer explanation. All widgets have a bool this.mounted property.
It is turned true when the buildContext is assigned. It is an error to call setState when a widget is unmounted.
mounted is false: The state object can never remount, and an error is thrown is setState is called.
*/
/*
This is the first method called when the widget is created (after the class constructor, of course.)
initState is called once and only once. It must called super.initState().
*/
@override
void initState() {
// TODO: implement initState
super.initState();
print("initState");
}
/*
This method is called immediately after initState on the first time the widget is built.
*/
@override
void didChangeDependencies() {
// TODO: implement didChangeDependencies
super.didChangeDependencies();
print("didChangeDependencies");
}
/*
build(): This method is called often. It is required, and it must return a Widget.
*/
@override
Widget build(BuildContext context) {
print("build");
// TODO: implement build
return Container();
}
/*
If the parent widget changes and has to rebuild this widget (because it needs to give it different data),
but it's being rebuilt with the same runtimeType, then this method is called.
This is because Flutter is re-using the state, which is long lived.
In this case, you may want to initialize some data again, as you would in initState.
*/
@override
void didUpdateWidget(ScreenLifecyle oldWidget) {
print("didUpdateWidget");
// TODO: implement didUpdateWidget
super.didUpdateWidget(oldWidget);
}
@override
void setState(fn) {
print("setState");
// TODO: implement setState
super.setState(fn);
}
/*
Deactivate is called when State is removed from the tree,
but it might be reinserted before the current frame change is finished.
This method exists basically because State objects can be moved from one point in a tree to another.
*/
@override
void deactivate() {
// TODO: implement deactivate
print("deactivate");
super.deactivate();
}
/*
Dispose is called when the State object is removed, which is permanent.
This method is where you should unsubscribe and cancel all animations, streams, etc.
*/
@override
void dispose() {
// TODO: implement dispose
super.dispose();
}
@override
void didChangeAppLifecycleState(AppLifecycleState state) {
super.didChangeAppLifecycleState(state);
switch (state) {
case AppLifecycleState.inactive:
print('appLifeCycleState inactive');
break;
case AppLifecycleState.resumed:
print('appLifeCycleState resumed');
break;
case AppLifecycleState.paused:
print('appLifeCycleState paused');
break;
case AppLifecycleState.suspending:
print('appLifeCycleState suspending');
break;
}
}
}
Solution 3:
There's an example here: https://github.com/flutter/flutter/blob/master/examples/layers/services/lifecycle.dart
You need to use WidgetsBindingObserver
Solution 4:
Only StatefulWidget hold state . Lifecyle of it is as follow
- createState () : When we build a new StatefulWidget, this one calls createState() right away and this override method must exist
- initState() :it is the first method called after the Widget is created.This is our equivalent to onCreate() and viewDidLoad()
- didChangeDependencies() : This method is called immediately after initState() on the first time the widget is built
- build() : called right after didChangeDependencies(). All the GUI is render here and will be called every single time the UI needs to be render
- didUpdateWidget() : it’ll be called once the parent Widget did a change and needs to redraw the UI
- deactivate() : framework calls this method whenever it removes this State object from the tree
- dispose() :is called when this object and its State is removed from the tree permanently and will never build again.
AppLifecycleState are as follows
inactive - The application is in an inactive state and is not receiving user input. iOS only
paused - The application is not currently visible to the user, not responding to user input, and running in the background.
resumed - The application is visible and responding to user input.
suspending - The application will be suspended momentarily. Android only