Want run a task in the background in flutter? You’d create an Isolate, make the call to Kotlin in its run method, and your other dart code could asynchronously wait for it to be finished on the UI thread while still running the event queue. The Flutter team has an example of how that might work.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 | import 'dart:convert'; import 'dart:isolate'; import 'package:flutter/foundation.dart'; import 'package:flutter/material.dart'; import 'package:flutter/services.dart'; typedef OnProgressListener = void Function(double completed, double total); typedef OnResultListener = void Function(String result); // An encapsulation of a large amount of synchronous processing. // // The choice of JSON parsing here is meant as an example that might surface // in real-world applications. class Calculator { Calculator({ @required this.onProgressListener, @required this.onResultListener, String data }) : assert(onProgressListener != null), assert(onResultListener != null), // In order to keep the example files smaller, we "cheat" a little and // replicate our small json string into a 10,000-element array. _data = _replicateJson(data, 10000); final OnProgressListener onProgressListener; final OnResultListener onResultListener; final String _data; // This example assumes that the number of objects to parse is known in // advance. In a real-world situation, this might not be true; in that case, // the app might choose to display an indeterminate progress indicator. static const int _NUM_ITEMS = 110000; static const int _NOTIFY_INTERVAL = 1000; // Run the computation associated with this Calculator. void run() { int i = 0; final JsonDecoder decoder = JsonDecoder( (dynamic key, dynamic value) { if (key is int && i++ % _NOTIFY_INTERVAL == 0) onProgressListener(i.toDouble(), _NUM_ITEMS.toDouble()); return value; } ); try { final List<dynamic> result = decoder.convert(_data) as List<dynamic>; final int n = result.length; onResultListener('Decoded $n results'); } catch (e, stack) { print('Invalid JSON file: $e'); print(stack); } } static String _replicateJson(String data, int count) { final StringBuffer buffer = StringBuffer()..write('['); for (int i = 0; i < count; i++) { buffer.write(data); if (i < count - 1) buffer.write(','); } buffer.write(']'); return buffer.toString(); } } // The current state of the calculation. enum CalculationState { idle, loading, calculating } // Structured message to initialize the spawned isolate. class CalculationMessage { CalculationMessage(this.data, this.sendPort); String data; SendPort sendPort; } // A manager for the connection to a spawned isolate. // // Isolates communicate with each other via ReceivePorts and SendPorts. // This class manages these ports and maintains state related to the // progress of the background computation. class CalculationManager { CalculationManager({ @required this.onProgressListener, @required this.onResultListener }) : assert(onProgressListener != null), assert(onResultListener != null), _receivePort = ReceivePort() { _receivePort.listen(_handleMessage); } CalculationState _state = CalculationState.idle; CalculationState get state => _state; bool get isRunning => _state != CalculationState.idle; double _completed = 0.0; double _total = 1.0; final OnProgressListener onProgressListener; final OnResultListener onResultListener; // Start the background computation. // // Does nothing if the computation is already running. void start() { if (!isRunning) { _state = CalculationState.loading; _runCalculation(); } } // Stop the background computation. // // Kills the isolate immediately, if spawned. Does nothing if the // computation is not running. void stop() { if (isRunning) { _state = CalculationState.idle; if (_isolate != null) { _isolate.kill(priority: Isolate.immediate); _isolate = null; _completed = 0.0; _total = 1.0; } } } final ReceivePort _receivePort; Isolate _isolate; void _runCalculation() { // Load the JSON string. This is done in the main isolate because spawned // isolates do not have access to the root bundle. However, the loading // process is asynchronous, so the UI will not block while the file is // loaded. rootBundle.loadString('services/data.json').then<void>((String data) { if (isRunning) { final CalculationMessage message = CalculationMessage(data, _receivePort.sendPort); // Spawn an isolate to JSON-parse the file contents. The JSON parsing // is synchronous, so if done in the main isolate, the UI would block. Isolate.spawn<CalculationMessage>(_calculate, message).then<void>((Isolate isolate) { if (!isRunning) { isolate.kill(priority: Isolate.immediate); } else { _state = CalculationState.calculating; _isolate = isolate; } }); } }); } void _handleMessage(dynamic message) { if (message is List<double>) { _completed = message[0]; _total = message[1]; onProgressListener(_completed, _total); } else if (message is String) { _completed = 0.0; _total = 1.0; _isolate = null; _state = CalculationState.idle; onResultListener(message); } } // Main entry point for the spawned isolate. // // This entry point must be static, and its (single) argument must match // the message passed in Isolate.spawn above. Typically, some part of the // message will contain a SendPort so that the spawned isolate can // communicate back to the main isolate. // // Static and global variables are initialized anew in the spawned isolate, // in a separate memory space. static void _calculate(CalculationMessage message) { final SendPort sender = message.sendPort; final Calculator calculator = Calculator( onProgressListener: (double completed, double total) { sender.send(<double>[ completed, total ]); }, onResultListener: sender.send, data: message.data, ); calculator.run(); } } // Main app widget. // // The app shows a simple UI that allows control of the background computation, // as well as an animation to illustrate that the UI does not block while this // computation is performed. // // This is a StatefulWidget in order to hold the CalculationManager and // the AnimationController for the running animation. class IsolateExampleWidget extends StatefulWidget { @override IsolateExampleState createState() => IsolateExampleState(); } // Main application state. class IsolateExampleState extends State<StatefulWidget> with SingleTickerProviderStateMixin { String _status = 'Idle'; String _label = 'Start'; String _result = ' '; double _progress = 0.0; AnimationController _animation; CalculationManager _calculationManager; @override void initState() { super.initState(); _animation = AnimationController( duration: const Duration(milliseconds: 3600), vsync: this, )..repeat(); _calculationManager = CalculationManager( onProgressListener: _handleProgressUpdate, onResultListener: _handleResult, ); } @override void dispose() { _animation.dispose(); super.dispose(); } @override Widget build(BuildContext context) { return Material( child: Column( mainAxisAlignment: MainAxisAlignment.spaceAround, children: <Widget>[ RotationTransition( turns: _animation, child: Container( width: 120.0, height: 120.0, color: const Color(0xFF882222), ), ), Opacity( opacity: _calculationManager.isRunning ? 1.0 : 0.0, child: CircularProgressIndicator( value: _progress ), ), Text(_status), Center( child: RaisedButton( child: Text(_label), onPressed: _handleButtonPressed, ), ), Text(_result), ], ), ); } void _handleProgressUpdate(double completed, double total) { _updateState(' ', completed / total); } void _handleResult(String result) { _updateState(result, 0.0); } void _handleButtonPressed() { if (_calculationManager.isRunning) _calculationManager.stop(); else _calculationManager.start(); _updateState(' ', 0.0); } String _getStatus(CalculationState state) { switch (state) { case CalculationState.loading: return 'Loading...'; case CalculationState.calculating: return 'In Progress'; case CalculationState.idle: default: return 'Idle'; } } void _updateState(String result, double progress) { setState(() { _result = result; _progress = progress; _label = _calculationManager.isRunning ? 'Stop' : 'Start'; _status = _getStatus(_calculationManager.state); }); } } void main() { runApp(MaterialApp(home: IsolateExampleWidget())); } |
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