InheritedWidget

概述

在Flutter進(jìn)行界面開(kāi)發(fā)時(shí)，我們經(jīng)常會(huì)遇到數(shù)據(jù)傳遞的問(wèn)題惩阶。由于Flutter采用節(jié)點(diǎn)樹(shù)的方式組織頁(yè)面，以致于一個(gè)普通頁(yè)面的節(jié)點(diǎn)層級(jí)會(huì)很深扣汪。此時(shí)断楷，我們?nèi)绻€是一層層傳遞數(shù)據(jù)，當(dāng)需要修改數(shù)據(jù)時(shí)崭别，就會(huì)比較麻煩冬筒。

此時(shí)，我們需要一種機(jī)制茅主，能夠讓某一個(gè)節(jié)點(diǎn)下的所有子節(jié)點(diǎn)舞痰，訪問(wèn)該節(jié)點(diǎn)下的數(shù)據(jù)。

InheritedWidget就滿足了我們這一需求诀姚。

使用方法

我們創(chuàng)建一個(gè)繼承InheritedWidget的Widget响牛。

class MyInheritedWidget extends InheritedWidget {
  final int data;

  MyInheritedWidget(@required this.data, Widget child) : super(child: child);

  static MyInheritedWidget getData(BuildContext context) {
    return context.inheritFromWidgetOfExactType(MyInheritedWidget);
  }


  @override
  bool updateShouldNotify(MyInheritedWidget oldWidget) {
    return oldWidget.data != data;
  }


}

可以看到，我們提供了三個(gè)方法赫段。

構(gòu)造方法

InheritedWidget本身是一個(gè)沒(méi)有界面的Widget娃善，或者說(shuō)是一個(gè)代理Widget，我們需要傳入我們實(shí)際的Widget瑞佩。

所以聚磺，在構(gòu)造方法中，我們需要傳入兩個(gè)參數(shù)炬丸，一個(gè)是我們希望共享的數(shù)據(jù)（在本例中數(shù)據(jù)是int型瘫寝，實(shí)際業(yè)務(wù)中共享的通常是一個(gè)相對(duì)復(fù)雜的數(shù)據(jù)）蜒蕾，另一個(gè)就是我們帶界面的Widget。

數(shù)據(jù)獲取方法

  static MyInheritedWidget getData(BuildContext context) {
    return context.inheritFromWidgetOfExactType(MyInheritedWidget);
  }

我們看一下inheritFromWidgetOfExactType方法的源碼：

  /// 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.
  
  /// The [aspect] parameter is only used when [targetType] is an
  /// [InheritedWidget] subclasses that supports partial updates, like
  /// [InheritedModel]. It specifies what "aspect" of the inherited
  /// widget this context depends on.
  InheritedWidget inheritFromWidgetOfExactType(Type targetType, { Object aspect });
  

注釋說(shuō)得很清楚焕阿，inheritFromWidgetOfExactType是獲取指定context下的咪啡，最近的指定類(lèi)型的Widget，并且此Widget必須是InheritedWidget的子類(lèi)暮屡。

值得注意的是撤摸，它是一個(gè)靜態(tài)方法。所以褒纲，只要是相同BuildContext我們即可以在任意子節(jié)點(diǎn)上准夷，通過(guò)這個(gè)靜態(tài)方法，獲取到我們的InheritedWidget莺掠，然后讀取或修改它的共享數(shù)據(jù)衫嵌。

更新通知

這個(gè)方法，決定了是否通知子節(jié)點(diǎn)中StatefulWidget的didChangeDependencies方法是否調(diào)用彻秆。

  /// Called when a dependency of this [State] object changes.
  ///
  /// For example, if the previous call to [build] referenced an
  /// [InheritedWidget] that later changed, the framework would call this
  /// method to notify this object about the change.
  ///
  /// This method is also called immediately after [initState]. It is safe to
  /// call [BuildContext.inheritFromWidgetOfExactType] from this method.
  ///
  /// Subclasses rarely override this method because the framework always
  /// calls [build] after a dependency changes. Some subclasses do override
  /// this method because they need to do some expensive work (e.g., network
  /// fetches) when their dependencies change, and that work would be too
  /// expensive to do for every build.
  @protected
  @mustCallSuper
  void didChangeDependencies() { }

StatefulWidget的didChangeDependencies方法就是與InheritedWidget配合使用的楔绞。只有當(dāng)InheritedWidget發(fā)生更新并且決定通知時(shí)，didChangeDependencies才會(huì)調(diào)用唇兑。

使用場(chǎng)景

通常酒朵，當(dāng)我們需要在Widget樹(shù)，由父節(jié)點(diǎn)向子節(jié)點(diǎn)傳遞數(shù)據(jù)時(shí)扎附，會(huì)使用InheritedWidget耻讽。一旦某個(gè)節(jié)點(diǎn)的Widget繼承了InheritedWidget，那么它的子節(jié)點(diǎn)帕棉，不論深度是多少针肥，都可以獲取到繼承了InheritedWidget的Widget，并取得其中的數(shù)據(jù)成員香伴。

class TextWidget extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return TextWidgetState();
  }

}

class TextWidgetState extends State<TextWidget> {
  @override
  Widget build(BuildContext context) {
    print("refresh text");
    return Text(
        "count : ${MyInheritedWidget.getData(context).data}"
    );
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    //父或祖先widget中的InheritedWidget改變(updateShouldNotify返回true)時(shí)會(huì)被調(diào)用慰枕。
    //如果build中沒(méi)有依賴(lài)InheritedWidget，則此回調(diào)不會(huì)被調(diào)用即纲。
    print("Dependencies change");
  }

}

所以具帮，InheritedWidget適合做某個(gè)子樹(shù)的數(shù)據(jù)管理Widget。當(dāng)某個(gè)頁(yè)面低斋、模塊或控件依賴(lài)一個(gè)或一組數(shù)據(jù)時(shí)蜂厅。我們可以在其上層，創(chuàng)建一個(gè)繼承了InheritedWidget的數(shù)據(jù)管理Widget膊畴。

通過(guò)這樣的方式掘猿，至少我們不用對(duì)數(shù)據(jù)進(jìn)行層層傳遞。