Flutter-爲何包裝一層Builder控件以後，路由或點擊彈框事件正常使用了？

事故回放

一朋友面試，被問到在Flutter中一些因 context 引發的路由異常的問題，爲何包裝一層 Builder 控件以後，路由或點擊彈框事件正常使用了？而後就沒而後了。。。相信不少人都會用，至於爲何，也沒深究。

相信不少剛開始玩Flutter的同窗都會在學習過程當中都會寫到相似下面的這種代碼：

import 'package:flutter/material.dart';

class BuilderA extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: GestureDetector(
          onTap: () {
            Scaffold.of(context).showSnackBar(SnackBar(
              content: Text('666666'),
            ));
          },
          child: Center(
            child: Container(
              width: 100,
              height: 100,
              color: Colors.red,
            ),
          ),
        ),
      ),
    );
  }
}
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開開心心寫完，而後一頓運行：

void main() => runApp(BuilderA());
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點擊，發現 SnackBar 並無正常彈出，而是出現了下面這種異常：

════════ Exception caught by gesture
═══════════════════════════════════════════════════════════════
The following assertion was thrown while handling a gesture:
Scaffold.of() called with a context that does not contain a Scaffold.
...

網上不少資料都說須要外包一層 Builder 能夠解決這種問題，可是基本上沒說緣由，至於爲何說能夠外包一層 Builder 就能夠解決，我想大部分只是看了 Scaffold 的源碼中的註釋瞭解到的：

scaffold.dart 第1209行到1234行：
...
/// {@tool snippet --template=stateless_widget_material}
/// When the [Scaffold] is actually created in the same `build` function, the
/// `context` argument to the `build` function can't be used to find the
/// [Scaffold] (since it's "above" the widget being returned in the widget
/// tree). In such cases, the following technique with a [Builder] can be used
/// to provide a new scope with a [BuildContext] that is "under" the
/// [Scaffold]:
///
/// ```dart
/// Widget build(BuildContext context) {
/// return Scaffold(
/// appBar: AppBar(
/// title: Text('Demo')
/// ),
/// body: Builder(
/// // Create an inner BuildContext so that the onPressed methods
/// // can refer to the Scaffold with Scaffold.of().
/// builder: (BuildContext context) {
/// return Center(
/// child: RaisedButton(
/// child: Text('SHOW A SNACKBAR'),
/// onPressed: () {
/// Scaffold.of(context).showSnackBar(SnackBar(
/// content: Text('Have a snack!'),
/// ));
/// },
...
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那究竟是什麼緣由外包一層 Builder 控件就能夠了呢？

---

緣由分析

異常緣由

上面那種寫法爲何會異常？要想知道這個問題，咱們首先看這句描述：

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

意思是說在不包含Scaffold的上下文中調用了Scaffold.of()。
咱們仔細看看這個代碼，會發現，此處調用的 context 是 BuilderA 的，而在BuilderA中的 build 方法中咱們才指定了 Scaffold ，所以確實是不存的。

爲何包一層Builder就沒問題了？

咱們把代碼改爲下面這種：

class BuilderB extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Builder(
          builder: (context) => GestureDetector(
            onTap: () {
              Scaffold.of(context).showSnackBar(SnackBar(
                content: Text('666666'),
              ));
            },
            child: Center(
              child: Container(
                width: 100,
                height: 100,
                color: Colors.red,
              ),
            ),
          ),
        ),
      ),
    );
  }
}
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運行以後發現確實沒問題了？爲何呢？咱們先來看看 Builder 源碼：

// ##### framework.dart文件下
typedef WidgetBuilder = Widget Function(BuildContext context);


// ##### basic.dart文件下
class Builder extends StatelessWidget {
  /// Creates a widget that delegates its build to a callback.
  ///
  /// The [builder] argument must not be null.
  const Builder({
    Key key,
    @required this.builder,
  }) : assert(builder != null),
       super(key: key);

  /// Called to obtain the child widget.
  ///
  /// This function is called whenever this widget is included in its parent's
  /// build and the old widget (if any) that it synchronizes with has a distinct
  /// object identity. Typically the parent's build method will construct
  /// a new tree of widgets and so a new Builder child will not be [identical]
  /// to the corresponding old one.
  final WidgetBuilder builder;

  @override
  Widget build(BuildContext context) => builder(context);
}
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代碼很簡單，Builder 類繼承 StatelessWidget ，而後經過一個接口回調將本身對應的 context 回調出來，供外部使用。沒了~ 可是！外部調用：

onTap: () {
  Scaffold.of(context).showSnackBar(SnackBar(
    content: Text('666666'),
  ));
}
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此時的 context 將再也不是 BuilderB 的 context 了，而是 Builder 本身的了！！！

那麼問題又來了~~~憑什麼改爲 Builder 中的 context 就能夠了？我能這個時候就不得不去看看 Scaffold.of(context) 的源碼了：

...

static ScaffoldState of(BuildContext context, { bool nullOk = false }) {
    assert(nullOk != null);
    assert(context != null);
    final ScaffoldState result = context.ancestorStateOfType(const TypeMatcher<ScaffoldState>());
    if (nullOk || result != null)
      return result;
    throw FlutterError(
      
...省略不重要的
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@override
  State ancestorStateOfType(TypeMatcher matcher) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    Element ancestor = _parent;
    while (ancestor != null) {
      if (ancestor is StatefulElement && matcher.check(ancestor.state))
        break;
      ancestor = ancestor._parent;
    }
    final StatefulElement statefulAncestor = ancestor;
    return statefulAncestor?.state;
  } 
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上面的核心部分揭露了緣由： of() 方法中會根據傳入的 context 去尋找最近的相匹配的祖先 widget，若是尋找到返回結果，不然拋出異常，拋出的異常就是上面出現的異常！

此處，Builder 就在 Scafflod 節點下，因在 Builder 中調用 Scafflod.of(context) 恰好是根據 Builder 中的 context 向上尋找最近的祖先，而後就找到了對應的 Scafflod，所以這也就是爲何包裝了一層 Builder 後就能正常的緣由！

總結時刻

• Builder 控件的做用，個人理解是在於從新提供一個新的子 context ，經過新的 context 關聯到相關祖先從而達到正常操做的目的。
• 一樣的對於路由跳轉 Navigator.of(context)【注：Navigator 是由 MaterialApp 提供的】 等相似的問題，採用的都是相似的原理，只要搞懂了其中一個，其餘的都不在話下！

固然，處理這類問題不單單這一種思路，道路千萬條，找到符合本身的那一條纔是關鍵！