Flutter BuildContext 探究
0x00 前言
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：
StatelessWidget.build 方法中：创建StatelessWidget的build方法
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.')
));
}
);
}
)
);}
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:
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. [...]
ancestorRenderObjectOfType(TypeMatcher matcher) → RenderObject
Returns the RenderObject object of the nearest ancestor RenderObjectWidget widget that matches the given TypeMatcher. [...]
ancestorStateOfType(TypeMatcher matcher) → State
Returns the State object of the nearest ancestor StatefulWidget widget that matches the given TypeMatcher. [...]
ancestorWidgetOfExactType(Type targetType) → Widget
Returns the nearest ancestor widget of the given type, which must be the type of a concrete Widget subclass. [...]
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. [...]
inheritFromElement(InheritedElement ancestor, { Object aspect }) → InheritedWidget
Registers this build context with ancestor such that when ancestor's widget changes this build context is rebuilt. [...]
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. [...]
rootAncestorStateOfType(TypeMatcher matcher) → State
Returns the State object of the furthest ancestor StatefulWidget widget that matches the given TypeMatcher. [...]
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. [...]
visitChildElements(ElementVisitor visitor) → void
Walks the children of this widget. [...]
