设置的ListView头时抛出ClassCastException不能转换的LinearLayout $的LayoutParams到AbsListView $的LayoutParams

问题描述

首先，我已经想通了，如何解决这个bug，但在这一点上我只是想知道为什么我的修补程序的工作。我跑成​​的情况是，我得到一个 ClassCastException异常，不能转换的LinearLayout $的LayoutParams到AbsListView $的LayoutParams 时，我试着做以下（简化为这些目的）：

  mLayout =（的LinearLayout）getLayoutInflater（）
   .inflate（R.layout.my_header_layout，
   getListView（），FALSE）;
mRootView.addView（mLayout）;
。getListView（）addHeaderView（mLayout）;
getListView（）setAdapter（mAdapter）。
 

我最终破裂下来，并发现，当我删除调用 mRootView.addView（mLayout）问题被固定

我的两方面问题是，为什么会出现这种情况？首先，在概念方面，那为什么当一个标题观点已经在整个事情刚刚去世的布局？是不是因为这种观点有效地试图进行布局两次：一次在头，一次用于实际的布局

？

其次，为什么例外？这似乎非常非描述性的，它看起来并不像它在所有捕获的实际问题？这是不被处理，因为较低的水平功能将只是结果失败过高水平的问题的情况？

解决方案

Your layout params are going crazy here, that's the problem.

• inflate() is called, assigning ListView.LayoutParams to mLayout, since you pass a ListView as the root. From the docs:

• When you use addView(), it checks the type of the child's params. If it doesn't match, it converts it as best it can. It does this by copying the values over in a new constructor. So, your mLayout now has LinearLayout.LayoutParams attached.

• Once you call addHeaderView(), mLayout is now a child of the listview. One of the things done during ListView#setupChild() is this:

  AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();

This is almost definitely where you're getting the ClassCastException. You just can't do a straight cast like that.

---

It's not that bad, but you have to understand how the layout system works. Could they do a simple check, and just throw an error or log message if it's not right? Sure. Could ListView try to gracefully handle the issue the way a LinearLayout does? Sure. But they don't.

It might be worth posting on the Android Developer's forum/group/tracker if you think it could be handled better.

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