使用导航器显示页面/路线时，从最近的MaterialApp父级创建一个新分支。表示两个页面（Main和New）都将在内存中，并且如果它们正在侦听相同的ChangeNotifier，则会重建。
我无法确定用户当前在屏幕上看到哪个窗口小部件。 我需要用它来处理一个场景，该场景从可能在窗口小部件树中但当前不可见的窗口小部件中跳过执行异步或冗长的过程并产生一些副作用。
注意：此处提供的示例代码代表了我当前正在使用的应用的基本架构，但重现了确切的问题。
我的应用程序中有一个非常不同且复杂的窗口小部件树，从屏幕上不可见的窗口小部件执行doLongProcess()时遇到了这个问题。另外，doLongProcess()会更改我的应用程序中的一些常见属性，这会导致问题，因为任何后台小部件都可以修改其他小部件上可见的详细信息。
我正在寻找解决这个问题的方法，如果除了找到屏幕上的哪个小部件，还有其他方法可以实现该目标，请也告诉我。
我的最终目标是允许仅从可见的窗口小部件执行较长的过程。
请运行一次该应用程序，以正确理解以下详细信息。
注释2 ：
我尝试使用状态的mounted确定是否可以使用它，但对于两个小部件（MainPage TextDisplay和NewPage TextDisplay）都显示为真
在评论中让我知道是否有更多详细信息，或者我错过了必需的内容。
使用以下示例代码包含provider依赖项来重现该问题：
// add in pubspec.yaml: provider: ^4.3.2+1
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
@override
Widget build(BuildContext context) {
print('MainPage: build');
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
TextDisplay(
name: 'MainPage TextDisplay',
),
SizedBox(
height: 20,
),
RaisedButton(
child: Text('Open New Page'),
onPressed: () => Navigator.of(context).push(MaterialPageRoute(
builder: (context) => NewPage(),
)),
),
],
),
),
);
}
}
class TextDisplay extends StatefulWidget {
final String name;
const TextDisplay({Key key, @required this.name}) : super(key: key);
@override
_TextDisplayState createState() => _TextDisplayState();
}
class _TextDisplayState extends State<TextDisplay> {
@override
Widget build(BuildContext context) {
return Container(
child: ChangeNotifierProvider.value(
value: dataHolder,
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Center(child: Text(widget.name)),
SizedBox(
height: 20,
),
Consumer<DataHolder>(
builder: (context, holder, child) {
// need to detect if this widget is on the screen,
// only then we should go ahead with this long process
// otherwise we should skip this long process
doLongProcess(widget.name);
return Text(holder.data);
},
),
RaisedButton(
child: Text('Randomize'),
onPressed: () => randomizeData(),
),
],
),
),
);
}
void doLongProcess(String name) {
print('$name: '
'Doing a long process using the new data, isMounted: $mounted');
}
}
class NewPage extends StatelessWidget {
@override
Widget build(BuildContext context) {
print('NewPage: build');
return Scaffold(
appBar: AppBar(
automaticallyImplyLeading: true,
title: Text('New Page'),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
TextDisplay(
name: 'NewPage TextDisplay',
),
],
),
),
);
}
}
/////////////////// Data Holder Class and methods ///////////////////
class DataHolder extends ChangeNotifier {
String _data;
String get data => _data ?? 'Nothing to show, Yet!';
setData(String newData) {
print('\n new data found: $newData');
_data = newData;
notifyListeners();
}
}
final dataHolder = DataHolder();
randomizeData() {
int mills = DateTime.now().millisecondsSinceEpoch;
dataHolder.setData(mills.toString());
}
答案 0 :(得分：0)
发布解决方案供其他人参考。
请参阅以下Flutter插件/软件包： https://pub.dev/packages/visibility_detector
// add in pubspec.yaml: provider: ^4.3.2+1
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:visibility_detector/visibility_detector.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
@override
Widget build(BuildContext context) {
print('MainPage: build');
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
TextDisplay(
name: 'MainPage TextDisplay',
),
SizedBox(
height: 20,
),
RaisedButton(
child: Text('Open New Page'),
onPressed: () => Navigator.of(context).push(MaterialPageRoute(
builder: (context) => NewPage(),
)),
),
],
),
),
);
}
}
class TextDisplay extends StatefulWidget {
final String name;
const TextDisplay({Key key, @required this.name}) : super(key: key);
@override
_TextDisplayState createState() => _TextDisplayState();
}
class _TextDisplayState extends State<TextDisplay> {
/// this holds the latest known status of the widget's visibility
/// if [true] then the widget is fully visible, otherwise it is false.
///
/// Note: it is also [false] if the widget is partially visible since we are
/// only checking if the widget is fully visible or not
bool _isVisible = true;
@override
Widget build(BuildContext context) {
return Container(
child: ChangeNotifierProvider.value(
value: dataHolder,
/// This is the widget which identifies if the widget is visible or not
/// To my suprise this is an external plugin which is developed by Google devs
/// for the exact same purpose
child: VisibilityDetector(
key: ValueKey<String>(widget.name),
onVisibilityChanged: (info) {
// print('\n ------> Visibility info:'
// '\n name: ${widget.name}'
// '\n visibleBounds: ${info.visibleBounds}'
// '\n visibleFraction: ${info.visibleFraction}'
// '\n size: ${info.size}');
/// We use this fraction value to determine if the TextDisplay widget is
/// fully visible or not
/// range for fractional value is: 0 <= visibleFraction <= 1
///
/// Meaning we can also use fractional values like, 0.25, 0.3 or 0.5 to
/// find if the widget is 25%, 30% or 50% visible on screen
_isVisible = info.visibleFraction == 1;
},
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Center(child: Text(widget.name)),
SizedBox(
height: 20,
),
Consumer<DataHolder>(
builder: (context, holder, child) {
/// now that we have the status of the widget's visiblity
/// we can skip the long process when the widget is not visible.
if (_isVisible) {
doLongProcess(widget.name);
}
return Text(holder.data);
},
),
RaisedButton(
child: Text('Randomize'),
onPressed: () => randomizeData(),
),
],
),
),
),
);
}
void doLongProcess(String name) {
print('\n ============================ \n');
print('$name: '
'Doing a long process using the new data, isMounted: $mounted');
final element = widget.createElement();
print('\n name: ${widget.name}'
'\n element: $element'
'\n owner: ${element.state.context.owner}');
print('\n ============================ \n');
}
}
class NewPage extends StatelessWidget {
@override
Widget build(BuildContext context) {
print('NewPage: build');
return Scaffold(
appBar: AppBar(
automaticallyImplyLeading: true,
title: Text('New Page'),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
TextDisplay(
name: 'NewPage TextDisplay',
),
],
),
),
);
}
}
/////////////////// Data Holder Class and methods ///////////////////
class DataHolder extends ChangeNotifier {
String _data;
String get data => _data ?? 'Nothing to show, Yet!';
setData(String newData) {
print('\n new data found: $newData');
_data = newData;
notifyListeners();
}
}
final dataHolder = DataHolder();
randomizeData() {
int mills = DateTime.now().millisecondsSinceEpoch;
dataHolder.setData(mills.toString());
}