在绘制小部件之前如何知道小部件的大小? [英] How to know the size of a widget before painting it?

问题描述

我正尝试将 AnimatedContainer 用于当用户按下扩展效果时的简单动画(类似于 ExpandableTile ).

I’m trying to use an AnimatedContainer for just a simple animation of an expand effect (similar to ExpandableTile) when the user press it.

我的问题是，此容器在展开时可以具有10.0的高度或1000.0的高度(它必须是动态的).好吧，要制作动画，它需要事先知道高度，这是有道理的.但是在渲染之前如何知道某物的最大高度(我将渲染可以有1到N行的Text).

My issue is that this container, when expanded, can have either 10.0 height, or 1000.0 (it must be dynamic). Ok, to animate it, it needs to know the height before-hand, makes sense. But how would I know the max height of something before rendering it (I’m going to render Text which can have 1 to N lines)).

我认为有很多可能性，但似乎没有一个合适的选择.在大多数情况下，我们是从 RenderBox 渲染的东西中获取尺寸的，而不是相反的，所以这很困扰我.

I’ve think in a lot of possibilities, but none seem to fit. Most of the times we get the size from something that is already rendered, from the RenderBox, not the opposite, so this is kind bugging me.

提前谢谢！

推荐答案

我最终使用

I end up finding the solution by using the Offstage widget which fits perfectly in this scenario.

将子树包装在 Offstage 中并为其分配一个键(可以是 GlobalKey )，然后，在实际渲染它之前，我可以通过以下操作获得其高度:

Wrapping the subtree in a Offstage and assigning it a key (can be a GlobalKey), then, before actually rendering it, I can get its height by doing:

  void _getWidgetHeight() {
    if (subtreeHeight == null) {
      RenderBox renderBox = key.currentContext.findRenderObject();
      subtreeHeight = renderBox.size.height;
    }
    setState(() {
      _isOffstage = false;
    });
  }

这样，例如，如果您有一个 AnimatedContainer ，并且需要为其最终大小设置动画，则可以具有以下内容:

this way, if you have an AnimatedContainer, for example, and need to animate to its final size, you can have the following:

AnimatedContainer(
        duration: Duration(milliseconds: 250),
        height: subtreeHeight ?? 100.0 // some default size,
      ...
  )

，它将相应地呈现.

请注意，确保子树中的内容在动画时可以插入 Container 中(例如，将 Offstage 包裹在 Flexible中(如果您在 Column 或 Row )中使用它.

As a note, make sure the content in your subtree can be inserted inside the Container while it is animating (eg. wrapping the Offstage in a Flexible if you´re using it in a Column or Row).

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