0x00 前言
BuildContext是Flutter的重要部分，但是目前網上講BuildContext的文章太少了，所以本篇文章講講BuildContext。
0x01 BuildContext 介紹
BuildContext,顧名思義，Build（構建Widget） Context（應用上下文），就是構建Widget中的應用上下文。
所以BuildContext只出現在兩個地方：
- StatelessWidget.build 方法中：建立StatelessWidget的build方法
- State物件中：一個是建立StatefulWidget的State物件的build方法中，另一個是State的成員變數
BuildContext實際是Element,BuildContext是為了阻止直接對Element操作而抽象出來的，所以BuildContext是Element的抽象類，所有Element都繼承自BuildContext。
每一個Widget都有一個BuildContext。
BuildContext是Widget在Widget樹中位置的控制程式碼。
0x02 訪問 BuildContext 例項物件
BuildContext被WidgetBulder（例如：StatelessWidget.build, State.build）方法傳遞；
BuildContext也是State的成員變數，在State內部，可以通過context直接訪問
0x03 BuildContext 使用
BuildContext的作用主要是通過上下文獲取指定的資料；
例如：Theme.of(context) 或者 showDialog(context: context,....)都需要BuildContext作為引數，這裡的BuildContext就是呼叫這些方法的Widget的代表。
0x05 BuildContext 注意事項
每一個Widget都有一個BuildContext。假設有個A Widget，A Widget裡肯定會有StatelessWidget.build或者State.build的build方法，build方法建立了 B Widget並返回，A Widget就是B Widget的父Widget，相應的， A Widget的BuildContext也是是B Widget 的BuildContext的父節點。
下面給一個例子加深理解：
@override
Widget build(BuildContext context) {
// here, Scaffold.of(context) returns null，
//在Scaffold建立之前，如果在這裡呼叫Scaffold.of(context)會返回null，是因為此時Scaffo//ld還沒建立，所以其BuildContext也沒有建立，為了在建立Scaffold的時候就取到他的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.')
));
}
);
}
)
);
}
複製程式碼0x04 BuildContext 內部方法解析
為什麼BuildContext可以用來獲取上下文的資料，主要是因為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. [...]
複製程式碼- ancestorRenderObjectOfType(TypeMatcher matcher) → RenderObject
Returns the RenderObject object of the nearest ancestor RenderObjectWidget widget that matches the given TypeMatcher. [...]
複製程式碼- ancestorStateOfType(TypeMatcher matcher) → State
Returns the State object of the nearest ancestor StatefulWidget widget that matches the given TypeMatcher. [...]
複製程式碼- ancestorWidgetOfExactType(Type targetType) → Widget
Returns the nearest ancestor widget of the given type, which must be the type of a concrete Widget subclass. [...]
複製程式碼- 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. [...]
複製程式碼- inheritFromElement(InheritedElement ancestor, { Object aspect }) → InheritedWidget
Registers this build context with ancestor such that when ancestor's widget changes this build context is rebuilt. [...]
複製程式碼- 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. [...]
複製程式碼- rootAncestorStateOfType(TypeMatcher matcher) → State
Returns the State object of the furthest ancestor StatefulWidget widget that matches the given TypeMatcher. [...]
複製程式碼- 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. [...]
複製程式碼- visitChildElements(ElementVisitor visitor) → void
Walks the children of this widget. [...]
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