Flutter BuildContext 探究

BuildContext 是Flutter的重要部分，但是目前网上讲 BuildContext 的文章太少了，所以本篇文章讲讲 BuildContext 。

0x01 BuildContext 介绍

BuildContext ,顾名思义，Build（构建Widget） Context（应用上下文），就是构建Widget中的应用上下文。

所以 BuildContext 只出现在两个地方：

1. StatelessWidget.build 方法中：创建StatelessWidget的build方法
2. State对象中：一个是创建StatefulWidget的State对象的build方法中，另一个是State的成员变量

BuildContext 实际是 Element , BuildContext 是为了阻止直接对 Element 操作而抽象出来的，所以 BuildContext 是 Element 的抽象类，所有 Element 都继承自 BuildContext 。

每一个Widget都有一个 BuildContext 。

BuildContext 是Widget在Widget树中位置的句柄。

0x02 访问 BuildContext 实例对象

BuildContext 被 WidgetBulder （例如：StatelessWidget.build, State.build）方法传递；

BuildContext 也是 State 的成员变量，在 State 内部，可以通过 context 直接访问

0x03 BuildContext 使用

BuildContext 的作用主要是通过上下文获取指定的数据；

例如： Theme.of(context) 或者 showDialog(context: context,....) 都需要 BuildContext 作为参数，这里的 BuildContext 就是调用这些方法的Widget的代表。

0x05 BuildContext 注意事项

每一个Widget都有一个 BuildContext 。假设有个A Widget，A Widget里肯定会有 StatelessWidget.build 或者 State.build 的build方法，build方法创建了 B Widget并返回，A Widget就是B Widget的父Widget，相应的， A Widget的 BuildContext 也是是B Widget 的 BuildContext 的父节点。

下面给一个例子加深理解：

@override
  Widget build(BuildContext context) {
    // here, Scaffold.of(context) returns null，
    //在Scaffold创建之前，如果在这里调用Scaffold.of(context)会返回null，是因为此时Scaffo//ld还没创建，所以其BuildContext也没有创建，为了在创建Scaffold的时候就取到他的BuildC//ontext，要使用Builder
    return Scaffold(
      appBar: AppBar(title: Text('Demo')),
      body: Builder(
        builder: (BuildContext context) {
          return FlatButton(
            child: Text('BUTTON'),
            onPressed: () {
              // here, Scaffold.of(context) returns the locally created Scaffold
              Scaffold.of(context).showSnackBar(SnackBar(
                content: Text('Hello.')
              ));
            }
          );
        }
      )
    );
  }
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0x04 BuildContext 内部方法解析

为什么 BuildContext 可以用来获取上下文的数据，主要是因为 BuildContext 具有的以下方法:

1. ancestorInheritedElementForWidgetOfExactType(Type targetType) → InheritedElement

Obtains the element corresponding to the nearest widget of the given type, which must be the type of a concrete InheritedWidget subclass. [...]
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2. ancestorRenderObjectOfType(TypeMatcher matcher) → RenderObject

Returns the RenderObject object of the nearest ancestor RenderObjectWidget widget that matches the given TypeMatcher. [...]
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3. ancestorStateOfType(TypeMatcher matcher) → State

Returns the State object of the nearest ancestor StatefulWidget widget that matches the given TypeMatcher. [...]
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4. ancestorWidgetOfExactType(Type targetType) → Widget

Returns the nearest ancestor widget of the given type, which must be the type of a concrete Widget subclass. [...]
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5. findRenderObject() → RenderObject

The current RenderObject for the widget. If the widget is a RenderObjectWidget, this is the render object that the widget created for itself. Otherwise, it is the render object of the first descendant RenderObjectWidget. [...]
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6. inheritFromElement(InheritedElement ancestor, { Object aspect }) → InheritedWidget

Registers this build context with ancestor such that when ancestor's widget changes this build context is rebuilt. [...]
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7. inheritFromWidgetOfExactType(Type targetType, { Object aspect }) → InheritedWidget

Obtains the nearest widget of the given type, which must be the type of a concrete InheritedWidget subclass, and registers this build context with that widget such that when that widget changes (or a new widget of that type is introduced, or the widget goes away), this build context is rebuilt so that it can obtain new values from that widget. [...]
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8. rootAncestorStateOfType(TypeMatcher matcher) → State

Returns the State object of the furthest ancestor StatefulWidget widget that matches the given TypeMatcher. [...]
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9. visitAncestorElements(bool visitor(Element element)) → void

Walks the ancestor chain, starting with the parent of this build context's widget, invoking the argument for each ancestor. The callback is given a reference to the ancestor widget's corresponding Element object. The walk stops when it reaches the root widget or when the callback returns false. The callback must not return null. [...]
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10. visitChildElements(ElementVisitor visitor) → void

Walks the children of this widget. [...]
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