ParentDataWidget 的泛型已更改为 ParentData。

Summary
概述
Context
上下文
Description of change
更改描述
Migration guide
迁移指南
Timeline
时间轴
References
参考文献

概述

Summary

ParentDataWidget 的泛型已由 RenderObjectWidget 更改为 ParentData。

The generic type of ParentDataWidget has changed from RenderObjectWidget to ParentData.

上下文

Context

在此次更改之前，ParentDataWidget 是作为祖先节点被绑定至特定的 RenderObjectWidget 类型的。例如：Positioned 部件只能在 Stack 部件中使用。通过此次更改，只要上述 RenderObjectWidget 的 RenderObject 设置了正确的 ParentData 类型， ParentDataWidget 就可以作为任意 RenderObjectWidget 类型的祖先节点和其一起使用。通过这种新的方式，Positioned 部件就可以通过假想的 SuperStack 新部件而被重用了。

Prior to this change, a ParentDataWidget was bound to a specific RenderObjectWidget type as ancestor. For example, a Positioned widget could only be used within a Stack widget. With this change, a ParentDataWidget can be used with any RenderObjectWidget type as ancestor as long as the RenderObject of said RenderObjectWidget sets up the correct ParentData type. In this new world, the Positioned widget can be reused with a hypothetical new SuperStack widget.

更改描述

Description of change

ParentDataWidget 的泛型参数已由 RenderObjectWidget 更改为 ParentData，并添加了一个新的调试属性 debugTypicalAncestorWidgetClass。后者用于展示错误信息，让用户更好地理解 ParentDataWidget 中应该使用的上下文。

The generic type argument of ParentDataWidget has changed from RenderObjectWidget to ParentData, and a new debug property, debugTypicalAncestorWidgetClass, is added to ParentDataWidget. The latter is used for error messages to give users a better idea of the context a given ParentDataWidget is supposed to be used in.

迁移指南

Migration guide

如果您继承或实现了 ParentDataWidget，则必须按照本节所述迁移您的代码。当您升级至包含了此更改的 Flutter 版本时，分析器会展示如下警告信息：

You must migrate your code as described in this section if you’re subclassing or implementing ParentDataWidget. If you do, the analyzer shows the following warnings when you upgrade to the Flutter version that includes this change:

  error • Missing concrete implementation of 'getter ParentDataWidget.debugTypicalAncestorWidgetClass' • lib/main.dart:114:7 • non_abstract_class_inherits_abstract_member
  error • 'FrogJar' doesn't extend 'ParentData' • lib/main.dart:114:41 • type_argument_not_matching_bounds

迁移前代码：

Code before migration:

class FrogSize extends ParentDataWidget<FrogJar> {
  FrogSize({
    Key key,
    @required this.size,
    @required Widget child,
  }) : assert(child != null),
        assert(size != null),
        super(key: key, child: child);

  final Size size;

  @override
  void applyParentData(RenderObject renderObject) {
    final FrogJarParentData parentData = renderObject.parentData;
    if (parentData.size != size) {
      parentData.size = size;
      final RenderFrogJar targetParent = renderObject.parent;
      targetParent.markNeedsLayout();
    }
  }
}

class FrogJarParentData extends ParentData {
  Size size;
}

class FrogJar extends RenderObjectWidget {
  // ...
}

迁移后代码：

Code after migration:

class FrogSize extends ParentDataWidget<FrogJarParentData> { // FrogJar changed to FrogJarParentData
  FrogSize({
    Key key,
    @required this.size,
    @required Widget child,
  }) : assert(child != null),
        assert(size != null),
        super(key: key, child: child);

  final Size size;

  @override
  void applyParentData(RenderObject renderObject) {
    final FrogJarParentData parentData = renderObject.parentData;
    if (parentData.size != size) {
      parentData.size = size;
      final RenderFrogJar targetParent = renderObject.parent;
      targetParent.markNeedsLayout();
    }
  }

  @override
  Type get debugTypicalAncestorWidgetClass => FrogJar; // Newly added
}

父类 ParentDataWidget 的泛型从 FrogJar（一个 RenderObjectWidget）更改为 FrogJarParentData （FrogSize.applyParentData 所操作的 ParentData 类型）。除此之外，子类 ParentDataWidget 实现了新的 debugTypicalAncestorWidgetClass。它给 ParentDataWidget 返回了一个典型的 RenderObjectWidget 类型的祖先节点。大多数时候，您只是想在此处返回旧的泛型（在这个示例中就是 FrogJar）。

The generic type of the ParentDataWidget superclass changes from FrogJar (a RenderObjectWidget) to FrogJarParentData (the ParentData type that FrogSize.applyParentData wants to operate on). Additionally, the new debugTypicalAncestorWidgetClass is implemented for this ParentDataWidget subclass. It returns the type of a typical ancestor RenderObjectWidget for this ParentDataWidget. Most of the time, you just want to return the old generic type here (FrogJar in this example).

时间轴

Timeline

发布于版本：1.16.3
发布于稳定版本：1.17

Landed in version: 1.16.3
In stable release: 1.17

参考文献

References

API 文档：

API documentation:

ParentDataWidget