Focus——Flutter中的无名英雄

引言

Focus系列的Widget及功能类在Flutter中可以说是无名英雄的存在，默默的付出但却不太为人所知。在日常开发使用中也不太会用到它，这是为什么呢？带着这个问题我们开始今天的内容。

Focus相关Widget及功能类

这里大致介绍一些Focus相关Widget及功能类，便于后面理解Focus Tree部分。

1. FocusNode

FocusNode是Flutter中最重要的Focus相关类，它用于管理焦点。

class FocusNode {
  bool hasFocus;
  bool isInteractive;
  bool consumeKeystrokes;
  bool canRequestFocus;
  bool canClearFocus;
  void requestFocus();
  void unfocus();
}

2. FocusScope

FocusScope是Flutter中另一个重要的Focus相关类，它用于管理FocusNode。

class FocusScope {
  static FocusScopeNode of(BuildContext context);
  static FocusNode getNearest(BuildContext context);
  static FocusNode getPrimary(BuildContext context);
  static FocusNode? getDescendantFocus(BuildContext context, FocusNode? scope);
  static void of(BuildContext context).requestFocus(FocusNode node);
  static void of(BuildContext context).setFirstFocus(FocusNode node);
  static void of(BuildContext context).setPrimaryFocus(FocusNode node);
}

3. RawKeyboardListener

RawKeyboardListener是Flutter中一个用于监听键盘事件的Widget。

class RawKeyboardListener {
  const RawKeyboardListener({
    Key? key,
    required Widget child,
    required RawKeyboardListenerCallback onKey,
    FocusNode? focusNode,
    bool autofocus = true,
  });

  static RawKeyboardListenerCallback? get onKey;
  static FocusNode? get focusNode;
  static bool get autofocus;
  static Widget get child;
}

4. InteractiveViewer

InteractiveViewer是Flutter中一个用于处理手势的Widget。

class InteractiveViewer {
  const InteractiveViewer({
    Key? key,
    required Widget child,
    TransformationController? transformationController,
    bool panEnabled = true,
    bool scaleEnabled = true,
    bool boundaryMargin = true,
    double minScale = 0.1,
    double maxScale = 10.0,
    Axis minScalePointerCount = 2,
    Axis maxScalePointerCount = 2,
    bool constrained = false,
  });

  static Widget get child;
  static bool get panEnabled;
  static bool get scaleEnabled;
  static bool get boundaryMargin;
  static double get minScale;
  static double get maxScale;
  static Axis get minScalePointerCount;
  static Axis get maxScalePointerCount;
  static bool get constrained;
}

5. ActiveWidget

ActiveWidget是Flutter中一个用于指示当前具有焦点的Widget。

class ActiveWidget {
  const ActiveWidget({
    Key? key,
    required Widget child,
  });

  static Widget get child;
}

Focus Tree

Focus Tree是一个包含所有FocusNode的树形结构。

                                FocusScope
                                  /     \
                              FocusNode1   FocusNode2
                          /      |        /      |
                      FocusNode3  FocusNode4  FocusNode5

Focus Tree的构建

Focus Tree的构建是通过FocusScope完成的。FocusScope是一个Widget，它可以将多个FocusNode组织成一个树形结构。

class FocusScope extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Column(
        children: <Widget>[
          FocusNode1(),
          FocusNode2(),
        ],
      ),
    );
  }
}

Focus Tree的遍历

Focus Tree的遍历可以通过FocusScope.of()方法完成。FocusScope.of()方法返回当前FocusScope的根节点。

FocusNode rootNode = FocusScope.of(context);

Focus Tree的管理

Focus Tree的管理可以通过FocusNode.requestFocus()和FocusNode.unfocus()方法完成。FocusNode.requestFocus()方法将焦点请求给指定的FocusNode。FocusNode.unfocus()方法将焦点从指定的FocusNode中移除。

FocusNode node1 = FocusNode();
FocusNode node2 = FocusNode();

node1.requestFocus();
node2.unfocus();

Focus Tree的应用

Focus Tree的应用场景非常广泛。例如，在表单中，可以使用Focus Tree来管理输入框的焦点。在游戏中，可以使用Focus Tree来管理玩家角色的焦点。在聊天应用程序中，可以使用Focus Tree来管理消息输入框的焦点。

结语

Focus系列的Widget及功能类在Flutter中扮演着非常重要的角色。它们可以帮助我们管理焦点，从而实现更好的用户体验。