Question

我正在查看setState的文档，并指出

通知框架此对象的内部状态为改变了。

我猜该文档确实意味着强调 this

通知框架此对象的内部状态改变了。

我尝试使用默认生成的代码尝试空 setState，它能够触发通知。

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      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> {
  int _counter = 0;

  void _incrementCounter() {
    _counter++; // moved it out of setState
    setState(() {
//  _counter++; // instead setting it before setState
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

以上代码确实可以正常工作，但是我在互联网上发现的所有示例都明确地更改了值或使用setState进行了一些活动，这真的有必要吗？

Answer 1

是的。

setState中的回调没有source code中所传达的魔力。它并没有被传递下来，而是在一些内部生命周期内被调用。

所提供的回调会立即被同步调用。它一定不能返回future（回调不能是异步的），因为从那时起，不清楚何时实际设置状态。

@protected
void setState(VoidCallback fn) {
  final dynamic result = fn() as dynamic;
  _element.markNeedsBuild();
}

没有断言的源代码。

它只是帮助您立即在闭包内部执行代码，然后将其标记为在下一帧中重建。就这样。尽管它可以与空白回调一起使用，但不建议这样做。

这可能与可维护性有关，因为setState内部的所有事物都应该改变小部件的状态，而毫不奇怪地明确指出。

Answer 2

简短：

You can call setState with an empty body.

（但是它会变得更容易阅读，更容易理解体内是否有东西，对吧？）

长版：

Flutter将检查某些条件，例如回调不为null，声明当前小部件的状态，回调不能为异步... 在需要重新构建标记小部件之前，Flutter会在setState体内执行任何操作。

@protected
  void setState(VoidCallback fn) {
    assert(fn != null);
    assert(() {
      if (_debugLifecycleState == _StateLifecycle.defunct) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() called after dispose(): $this'),
          ErrorDescription(
            'This error happens if you call setState() on a State object for a widget that '
            'no longer appears in the widget tree (e.g., whose parent widget no longer '
            'includes the widget in its build). This error can occur when code calls '
            'setState() from a timer or an animation callback.'
          ),
          ErrorHint(
            'The preferred solution is '
            'to cancel the timer or stop listening to the animation in the dispose() '
            'callback. Another solution is to check the "mounted" property of this '
            'object before calling setState() to ensure the object is still in the '
            'tree.'
          ),
          ErrorHint(
            'This error might indicate a memory leak if setState() is being called '
            'because another object is retaining a reference to this State object '
            'after it has been removed from the tree. To avoid memory leaks, '
            'consider breaking the reference to this object during dispose().'
          ),
        ]);
      }
      if (_debugLifecycleState == _StateLifecycle.created && !mounted) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() called in constructor: $this'),
          ErrorHint(
            'This happens when you call setState() on a State object for a widget that '
            'hasn\'t been inserted into the widget tree yet. It is not necessary to call '
            'setState() in the constructor, since the state is already assumed to be dirty '
            'when it is initially created.'
          ),
        ]);
      }
      return true;
    }());
    final dynamic result = fn() as dynamic;
    assert(() {
      if (result is Future) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() callback argument returned a Future.'),
          ErrorDescription(
            'The setState() method on $this was called with a closure or method that '
            'returned a Future. Maybe it is marked as "async".'
          ),
          ErrorHint(
            'Instead of performing asynchronous work inside a call to setState(), first '
            'execute the work (without updating the widget state), and then synchronously '
           'update the state inside a call to setState().'
          ),
        ]);
      }
      // We ignore other types of return values so that you can do things like:
      //   setState(() => x = 3);
      return true;
    }());
    _element.markNeedsBuild();
  }

setState应该在flutter小部件中具有任何主体

2 个答案: