To Make the Asynchronous operation will use Future
Uses
future.then get future value and catch future exception Combined async,await
future.whenComplete
future.timeout
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import 'dart:async'; Future<String> testFuture() { // throw new Error(); return Future.value('success'); // return Future.error('error'); } main() { testFuture().then((s) { print(s); }, onError: (e) { print('onError:'); print(e); }).catchError((e) { print('catchError:'); print(e); }); } |
Sometimes we need to Futuredo something at the end, and we know then().catchError()the pattern is similar try-catch, try-catch there is a finally code block, and future.whenCompletethat Future Is finally
FutureBuilder combines asynchronous operations and asynchronous UI updates. Through it we can update the results of network requests, database reads, etc.
future: Future object represents the asynchronous calculation currently connected to this builder;
initialData: Indicates the initialization data of a non-empty Future before completion;
builderA return function of type AsyncWidgetBuilder is a function that builds a widget based on asynchronous interaction
Properties
connectionState -Enumeration of ConnectionState, which indicates the connection status with asynchronous calculation. ConnectionState has four values: none, waiting, active, and done;
data -Asynchronous calculation of the latest data received;
error -Asynchronously calculate the latest error object received;
Example
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class FutureWidget extends StatefulWidget { String computeListOfTimestamps(int count) { StringBuffer sb = new StringBuffer(); var random = Random(); for (int i = 0; i < 2000; i++) { sb.writeln("Random Number : ${random.nextInt(2000)}"); sb.writeln(""); } return sb.toString(); } Future<String> createFutureCalculation(int count) { return new Future(() { return computeListOfTimestamps(count); },); } @override FutureWidgetState createState() => new FutureWidgetState(); } class FutureWidgetState extends State<FutureWidget> { bool _showCalculation = false; bool isLoading = false; GlobalKey<ScaffoldState>_scaffoldKey=GlobalKey(); void _onInvokeFuturePressed() { setState(() { _showCalculation = !_showCalculation; isLoading=true; }); } @override Widget build(BuildContext context) { Widget child = _showCalculation ? FutureBuilder( future: widget.createFutureCalculation(1000), builder: (BuildContext context, AsyncSnapshot snapshot) { return Expanded( child: SingleChildScrollView( child: (snapshot.data == null)?Center(child: CircularProgressIndicator()):Text( '${snapshot.data == null ? "" : snapshot.data}', style: TextStyle(fontSize: 20.0)))); }) : Text('Press Refresh to load Data'); return new Scaffold( key: _scaffoldKey, appBar: new AppBar( backgroundColor: Colors.pink, title: new Text("Future Builder "), ), body: new Center( child: new Column( mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[child])), floatingActionButton: new FloatingActionButton( backgroundColor: Colors.pink, onPressed: _onInvokeFuturePressed, tooltip: 'Call Future', child: new Icon(Icons.refresh), ), // This trailing comma makes auto-formatting nicer for build ); } } |
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