Flutter 如何正確顯示SnackBar

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簡介

官方API文檔Scaffold的of方法說明有說明調用Scaffold.of方法是在Scallfold的子組件的Build方法中，也就是不能直接在構建Scaffold的build方法裡調用，否則會拋異常。

Typical usage of the Scaffold.of function is to call it from within the build method of a child of a Scaffold.

通常為顯示一個SnackBar需要構建一個Builder，通過Builder的context調用（原因是）

Scallfold.of(context).showSnackBar(SnackBar(contenxt: Text('這是一個SnackBar'));

官方示例

顯示SnackBar，官方典型示例代碼如下所示：

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Code Sample for Scaffold.of.',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: MyScaffoldBody(),
        appBar: AppBar(title: Text('Scaffold.of Example')),
      ),
      color: Colors.white,
    );
  }
}

// 在Scaffold子組件裡的build方法可以調用Scaffold.of方法
class MyScaffoldBody extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: RaisedButton(
        child: Text('SHOW A SNACKBAR'),
        onPressed: () {
          Scaffold.of(context).showSnackBar(
            SnackBar(
              content: Text('Have a snack!'),
            ),
          );
        },
      ),
    );
  }
}

錯誤示例

但是若直接在構建Scallfold的build方法中調用會報異常:

Scaffold.of() called with a context that does not contain a Scaffold.

錯誤代碼如下所示：

import 'package:flutter/material.dart';

class ScaffoldSnackBarDemo extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: RaisedButton(
          child: Text('SHOW A SNACKBAR'),
          onPressed: () {
            ///直接在Scallfold的build方法裡使用會拋異常
            Scaffold.of(context).showSnackBar(
              SnackBar(
                content: Text('Have a snack!'),
              ),
            );
          },
        ),
      ),
      appBar: AppBar(title: Text('Scaffold.of Example')),
    );
  }
}

解決方法一：Scaffold的子組件通過Builder構建

這時候要不就是按官方的，將需要顯示SnackBar的代碼另外抽離一個自定義子組件，在子組件的build方法再顯示SnackBar，要不就是在Scaffold的build方法體對該子組件再包一層Builder，如下所示。

import 'package:flutter/material.dart';

class ScaffoldSnackBarDemo extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return Scaffold(
       ///在子組件外再包一層builder，讓context不共用
      body: Builder(builder: (context) {
        return Center(
          child: RaisedButton(
            child: Text('SHOW A SNACKBAR'),
            onPressed: () {
              Scaffold.of(context).showSnackBar(
                SnackBar(
                  content: Text('Have a snack!'),
                ),
              );
            },
          ),
        );
      }),
      appBar: AppBar(title: Text('Scaffold.of Example')),
    );
  }
}

解決方法二：使用GlobalKey存儲ScaffoldState

import 'package:flutter/material.dart';

class ScaffoldSnackBarDemo extends StatelessWidget {
  final _scallfoldKey = GlobalKey<ScaffoldState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      ///使用GlobalKey解決
      key: _scallfoldKey,
      body: Center(
        child: RaisedButton(
          child: Text('SHOW A SNACKBAR'),
          onPressed: () {
            _scallfoldKey.currentState.showSnackBar(SnackBar(
              content: Text('Have a snack!'),
            ));
          },
        ),
      ),
      appBar: AppBar(title: Text('Scaffold.of Example')),
    );
  }
}

Flutter學習Github代碼倉庫

https://github.com/AiguangLi/Flutter

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