Life cycle in flutter

Flutter Problem Overview

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.

Flutter Solutions

Solution 1 - Flutter

createState(): When the Framework is instructed to build a StatefulWidget, it immediately calls createState()
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.
initState(): 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 call super.initState().
didChangeDependencies(): This method is called immediately after initState on 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 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.
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.
mounted is false: The state object can never remount, and error will be thrown if setState is called.

Solution 2 - Flutter

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 - Flutter

There's an example here: https://github.com/flutter/flutter/blob/master/examples/layers/services/lifecycle.dart

You need to use WidgetsBindingObserver

Solution 4 - Flutter

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

Solution 5 - Flutter

App Lifecycle

For LifeCycle, you need to use WidgetsBindingObserver it works when the app goes on foreground and background.

import 'package:flutter/widgets.dart';
  class YourWidgetState extends State<YourWidget> with WidgetsBindingObserver {

       @override
      void initState() {
        super.initState();
        WidgetsBinding.instance.addObserver(this);
      }


      @override
      void dispose() {
        WidgetsBinding.instance.removeObserver(this);
        super.dispose();
      }


       @override
      void didChangeAppLifecycleState(AppLifecycleState state) {
        if (state == AppLifecycleState.resumed) {
           //do your stuff
        }
      }
    }

But in my case, i was unable to catch the situation of OnResume when I move from one screen to another. so below code working similarly as startActivityForResult.

use this code when your reaching to another activity

Navigator.push(context,
              MaterialPageRoute(builder: (context) => ProductDetails(pid: productList[index]["pid"],),
                  settings: RouteSettings(name: '/productdetail')),).then((value){
                    setState(() {
                      length=value;
                    });
                    debugPrint('CHECK BACK FROM DETAIL $length');
            });

when you press back

onPressed: (){Navigator.pop(context,length);}

Solution 6 - Flutter

I don't think flutter app lifecycle callbacks are going to help you here. You can try this logic.

In 1st page (when navigating to 2nd page)

Navigator.push(context, MaterialPageRoute(builder: (context) => Page2())).then((value) {
  print("Value returned form Page 2 = $value");
};

In 2nd page (when navigating back to 1st page)

Navigator.pop(context, returnedValue);

Lifecycle callback

void main() => runApp(HomePage());

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    super.didChangeAppLifecycleState(state);
    print("Current state = $state");
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text("Lifecycle")),
        body: Center(child: Text("Center"),),
      ),
    );
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }
}

Solution 7 - Flutter

createState(): When we create a stateful widget, the Flutter framework instruct to createState() method.

@override _DeveloperLibsWidgetState createState() => _DeveloperLibsWidgetState();

2.mounted(true/false): Once we create a State object, the framework mounted the State object by associating it with a BuildContext before calling initState() method. All widgets have a bool mounted property. It is turned true when the buildContext is assigned.

bool get mounted => _element != null;

3. initState(): This is the first method called when a stateful widget is created after the class constructor. The initState() is called only once. It must called super.initState().

@override
initState() {
  super.initState();
    // TO DO
}

4. didChangeDependencies(): This method is called immediately after initState() method on the first time the widget is built.

@protected
@mustCallSuper
void didChangeDependencies() { 

}

5. build(): it shows the part of the user interface represented by the widget. The framework calls this method in several different situations: After calling initState() method. The framework always calls build() method after calling didUpdateWidget After receiving a call for setState to update the screen.

@override
Widget build(BuildContext context, MyButtonState state) {
      return Container(color: const Color(0xFF2DBD3A));     
}

6. didUpdateWidget(Widget oldWidget): If the parent widget change configuration and has to rebuild this widget. But it's being rebuilt with the same runtimeType, then didUpdateWidget() method is called.

@mustCallSuper
@protected
void didUpdateWidget(covariant T oldWidget) {

}

7. setState(): This method is called from the framework and the developer. We can change the internal state of a State object and make the change in a function that you pass to setState().

@override
_DeveloperLibsWidgetState createState() => _DeveloperLibsWidgetState();

8. deactivate(): This is called when State is removed from the widgets tree, but it might be reinserted before the current frame change is finished.

@protected
@mustCallSuper
void deactivate() { }

9. dispose(): This is called when the State object is removed permanently. Here you can unsubscribe and cancel all animations, streams, etc.

@protected
@mustCallSuper
void dispose() {
  assert(_debugLifecycleState == _StateLifecycle.ready);
  assert(() { _debugLifecycleState = _StateLifecycle.defunct; return true; }());
}

Solution 8 - Flutter

Since everyone here is talking about app life cycle and not addressing the question which OP is asking. here is the answer to the question.

The requirement is he want to open page B from page A, let say to choose file from page B, and once file selected he want to go back to page A and need to process that selected file in page A. Like in android we can do it in onActivityResult() method. Below is the way we can achieve in flutter.

You can open the page B from page A as below

Map results =  await Navigator.of(context).push(MaterialPageRoute(
      builder: (BuildContext context) {
        return new PageB(title: "Choose File");
        },
      ));

    if (results != null && results.containsKey('selection')) {
      setState(() {
        _selection = results['selection'];
      });

    **//here you can do whatever you want to do with selection variable.**

    }

In Page B you can select the file or what ever things you need to return to page A as below (return file or any other variable after selection.

Navigator.of(context).pop({'selection':file});

Solution 9 - Flutter

You can simulate onResume and onPause states in your Flutter extending LifecycleState class. Make sure to push new routes using push() or pushNamed() method.

/// Inherit this State to be notified of lifecycle events, including popping and pushing routes.
///
/// Use `pushNamed()` or `push()` method to track lifecycle events when navigating to another route.
abstract class LifecycleState <T extends StatefulWidget> extends State<T>
    with WidgetsBindingObserver {
  ResumeResult resumeResult = new ResumeResult();
  bool _isPaused = false;

  AppLifecycleState lastAppState = AppLifecycleState.resumed;

  void onResume() {}

  void onPause() {}

  /// Use instead of Navigator.push(), it fires onResume() after route popped
Future<T> push<T extends Object>(BuildContext context, Route<T> route, [String source]) {
    _isPaused = true;
    onPause();

    return Navigator.of(context).push(route).then((value) {
        _isPaused = false;

        resumeResult.data = value;
        resumeResult.source = source;

        onResume();
        return value;
    });
}

/// Use instead of Navigator.pushNamed(), it fires onResume() after route popped
Future<T> pushNamed<T extends Object>(BuildContext context, String routeName, {Object arguments}) {
    _isPaused = true;
    onPause();

    return Navigator.of(context).pushNamed<T>(routeName, arguments: arguments).then((value) {
        _isPaused = false;

        resumeResult.data = value;
        resumeResult.source = routeName;

        onResume();
        return value;
    });
}

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    if (state == AppLifecycleState.paused) {
      if (!_isPaused) {
        onPause();
      }
    } else if (state == AppLifecycleState.resumed &&
        lastAppState == AppLifecycleState.paused) {
      if (!_isPaused) {
        onResume();
      }
    }
    lastAppState = state;
  }
}

class ResumeResult {
  dynamic data;
  String source;
}

Solution 10 - Flutter

I know I might be a little bit late but will answer this question in case someone else needs an answer, please refer to this solution https://stackoverflow.com/a/58504433/3037840 but I want to clarify something, in your stateful widget after implementing the RouteAware mixin, please note that:

@override
  void didPushNext() { //similar to onPause
    // will be called when a new route has been pushed, and the current route is no longer visible. acts similar to onPause
  }

  @override
  void didPopNext() { //similar to onResume
     // will be called when the top route has been popped off, and the current route shows up. acts similar to onResume when you are navigated back to your activity/fragment
  }

Content Type	Original Author	Original Content on Stackoverflow
Question	zhang qinglian	View Question on Stackoverflow
Solution 1 - Flutter	Shelly Pritchard	View Answer on Stackoverflow
Solution 2 - Flutter	Amit Prajapati	View Answer on Stackoverflow
Solution 3 - Flutter	Ian Hickson	View Answer on Stackoverflow
Solution 4 - Flutter	akshay_shahane	View Answer on Stackoverflow
Solution 5 - Flutter	Farhana Naaz Ansari	View Answer on Stackoverflow
Solution 6 - Flutter	CopsOnRoad	View Answer on Stackoverflow
Solution 7 - Flutter	mewadaarvind	View Answer on Stackoverflow
Solution 8 - Flutter	Riya Parmar	View Answer on Stackoverflow
Solution 9 - Flutter	BigSt	View Answer on Stackoverflow
Solution 10 - Flutter	Randa Omar Fahmy	View Answer on Stackoverflow