Flutter: Is it possible to know if you're currently off stage? - flutter

Question

I have a number of pages in my app wrapped in Offstage widgets. Each page makes use of the provider package to render based on state updates (e.g. the user does something, we make a network call and display the result).
As the pages are wrapped in Offstage widgets, the build() methods (and subsequent network calls) are called even if it's not the current page.
Is there a way inside the build() method to know if the widget is currently off stage (and if so, skip any expensive logic)?
I'm assuming I can work something with global state etc, but I was wondering if there was anything built-in in relation to the Offstage widget itself, similar to mounted

Answer 1

You can try finding the parent OffStage widget and see if the offstage property is true or false
#override
Widget build(BuildContext context) {
final offstageParent = context.findAncestorWidgetOfExactType<Offstage>();
if (offstageParent != null && offstageParent.offstage == false) {
// widget is currently offstage.
print('offstaged child');
} else {
// widget is not offstage
print('non-offstaged child');
}
return const Text('Example Widget');
}

Answer 2

I made a custom-made mechanism for the goal you wanna achieve:
First, I am declaring a new Map<String, bool> in a separate file alone that will hold the offStage bool value with the key of each class widget.
Map<String, bool> offStageMap = {};
then in the implementation of the StatefulWidget where the offstage widget is in:
class ExampleWidget extends StatefulWidget {
ExampleWidget({super.key}) {
widgetMapKey = runtimeType.toString();
}
late final String widgetMapKey;
#override
State<ExampleWidget> createState() => _ExampleWidgetState();
}
class _ExampleWidgetState extends State<ExampleWidget> {
final bool defaultIsOffStaged = false;
bool? localStateIsOffStages;
#override
void initState() {
offStageMap[widget.widgetMapKey] ??= defaultIsOffStaged;
super.initState();
}
#override
Widget build(BuildContext context) {
return GestureDetector(
onTap: () {
bool previousIsOffStaged = offStageMap[widget.widgetMapKey]!;
setState(() {
localStateIsOffStages =
offStageMap[widget.widgetMapKey] = !previousIsOffStaged;
});
},
child: Offstage(
offstage: localStateIsOffStages ?? offStageMap[widget.widgetMapKey]!,
child: Container(),
),
);
}
} },
child: Offstage(
offstage: localStateIsOffStages ?? offStageMap[widget.widgetMapKey]!,
child: Container(),
),
);
}
}
let me explain what this is about.
first I declared a defaultIsOffStaged where it should be the initial offStage value when nothing is saved in that map.
when that widget is inserted in the widget tree (initState() called), the widget.widgetMapKey of the ExampleWidget widget will be saved in that map with the value of the default one which is defaultIsOffStaged.
offStageMap[widget.widgetMapKey] ??= defaultIsOffStaged;
in the offstage property o the OffStage widget, in this line:
offstage: localStateIsOffStages ?? offStageMap[widget.widgetMapKey]!,
the nullable localStateIsOffStages will be null for the first time since it has no value yet, so offStageMap[widget.widgetMapKey]! which equals to defaultIsOffStaged will be the bool value of offstage.
until now what we have, is a map containing the key that belongs only to the ExampleWidget which is its widget.widgetMapKey with its offStage value, right?
now from all places in your app, you can get the offStage value of that widget with its widgetMapKey like this:
print(offStageMap[ExampleWidget().widgetMapKey]); // true
now let's say you want to change the offstage property of that widget, in my code I used a simple example of GestureDetector, so when we tap in the Text("toggle offstage") area, it toggles offStage, here is what happens:
we got the existing value in the map:
bool previousIsOffStaged = offStageMap[widget.widgetMapKey]!;
then assign the opposite of it, to that widget key in the map, and the localStateIsOffStages bool variable which was nullable, now it has a value.
and as normal so the state updates I wrapped it in a SetState(() {})
now the widget's offstage will be toggled, and every time the widget key in the map will be updated with that new value.
the localStateIsOffStages I declared just to hold the local state when this is happening while the StatefulWidget state updates.
after the StatefulWidget is disposed of (when you pop the route as an example) and open that route again, the initState() will execute but since we have now an entry in the map, it's not null so nothing will happen inside initState().
the localStateIsOffStages will be null, so the offStage property of the Offstage widget will be the value from the map, which is the previous value before the widget is disposed.
that's it, from other places you can check for the offstage value of that specific widget like this:
print(offStageMap[ExampleWidget().widgetMapKey])
you can do it for all your widget pages, so you will have a map containing the offStage values of them all.
I take it one step up, and made those methods that I guess they will help:
this will return a List with the pages where the value is true.
List<String> offstagedPages() {
List<String> isOffStagedPages = [];
offStageMap.forEach((runtimeType, isOffStaged) {
if (isOffStaged) {
isOffStagedPages.add(runtimeType);
}
});
return isOffStagedPages;
}
this will return a true if a page is off staged and false if not:
bool isPageWidgetOffStaged(String runtimeType) {
if (offStageMap.containsKey(runtimeType)) {
return offStageMap[runtimeType]!;
}
return false;
}
Hope this helps a little.

Answer 3

Maybe it's not applicable to you, but you might be able to solve it by simply not using Offstage. Consider this app:
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatefulWidget {
MyApp({super.key});
#override
State<MyApp> createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
bool showFirst = true;
void switchPage() {
setState(() {
showFirst = !showFirst;
});
}
#override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
body: Center(
child: Stack(children: [
Offstage(offstage: !showFirst,child: A("first", switchPage)),
Offstage(offstage: showFirst,child: A("second", switchPage)),
]))));
}
}
class A extends StatelessWidget {
final String t;
final Function onTap;
const A(this.t, this.onTap, {Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
print('$t is building');
return TextButton(onPressed: ()=> onTap(), child: Text(t));
}
}
You will notice by the prints that both pages are build. But if you rewrite it like this without Offstage, only the visible one is build:
#override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
body: Center(
child: Stack(children: [
if (showFirst) A("first", switchPage),
if (!showFirst) A("second", switchPage),
]))));
}

Answer 4

If you want to just keep state alive your pages , you can use https://api.flutter.dev/flutter/widgets/AutomaticKeepAliveClientMixin-mixin.html , you may check this blog for example usage, https://medium.com/manabie/flutter-simple-cheatsheet-4370a68f98b3

Answer 5

If you are using Navigator, you can just extends NavigatorObserver. Then you will get didpush and didpop, use state to manage elementlifecycle, you will get page onPause and onResume fun.