1. 背景
一个新版本的代码，在4.x版本进入某个页面的时候，必现crash。看到必现，心情就放松了一半。
大致的crash信息如下：
FATAL EXCEPTION: main
java.lang.ClassCastException: android.support.v4.view.ViewPager$LayoutParams cannot be cast to android.widget.AbsListView$LayoutParams
at android.widget.ListView.setupChild(ListView.java:1826)
at android.widget.ListView.makeAndAddView(ListView.java:1793)
at android.widget.ListView.fillDown(ListView.java:691)
at android.widget.ListView.fillSpecific(ListView.java:1349)
at android.widget.ListView.layoutChildren(ListView.java:1608)
at android.widget.AbsListView.onLayout(AbsListView.java:2091)
....
其实还有很多类似的crash堆栈，一google，发现一大片。https://stackoverflow.com/questions/25666274/classcastexception-android-widget-abslistviewlayoutparams-to-android-widget-gri
2. 为什么会出现crash
测试的时候，发现5.x不会crash，4.x必然重现是什么原因呢？
我们发现栈顶setupChild，先找该函数：
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
1887 boolean selected, boolean recycled) {
。。。。。
1898 // Respect layout params that are already in the view. Otherwise make some up...
1899 // noinspection unchecked
1900 AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
1901 if (p == null) {
1902 p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
1903 }
1904 p.viewType = mAdapter.getItemViewType(position);
1905
1906 if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
1907 p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
1908 attachViewToParent(child, flowDown ? -1 : 0, p);
1909 } else {
1910 p.forceAdd = false;
1911 if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
1912 p.recycledHeaderFooter = true;
1913 }
1914 addViewInLayout(child, flowDown ? -1 : 0, p, true);
1915 }
。。。。
1972 }
1973
我们看到 AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams() 一句正是crash的根源，强制转换导致的crash。 这里对比4.x与5.x版本的源码，发现两个版本的这里没有什么区别。 那是什么情况导致的child差异呢？跟踪代码，回到上级调用makeAndAddView里，看下源码两个版本基本一致的。
3. makeAndAddView
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
1847 boolean selected) {
1848 View child;
1849
1850
1851 if (!mDataChanged) {
1852 // Try to use an existing view for this position
1853 child = mRecycler.getActiveView(position);
1854 if (child != null) {
1855 // Found it -- we're using an existing child
1856 // This just needs to be positioned
1857 setupChild(child, position, y, flow, childrenLeft, selected, true);
1858
1859 return child;
1860 }
1861 }
1862
1863 // Make a new view for this position, or convert an unused view if possible
1864 child = obtainView(position, mIsScrap);
1865
1866 // This needs to be positioned and measured
1867 setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
1868
1869 return child;
1870 }
我们看到child = obtainView(position, mIsScrap)来自与listView的缓存相关。所以还是得跟踪obtainView，该函数也很好理解，有缓存的时候，从缓存池中取，否则重新生成。
4. obtainView
先看5.0版本的代码
2304 /**
2305 * Get a view and have it show the data associated with the specified
2306 * position. This is called when we have already discovered that the view is
2307 * not available for reuse in the recycle bin. The only choices left are
2308 * converting an old view or making a new one.
2309 *
2310 * @param position The position to display
2311 * @param isScrap Array of at least 1 boolean, the first entry will become true if
2312 * the returned view was taken from the scrap heap, false if otherwise.
2313 *
2314 * @return A view displaying the data associated with the specified position
2315 */
2316 View obtainView(int position, boolean[] isScrap) {
。。。
2321 // Check whether we have a transient state view. Attempt to re-bind the
2322 // data and discard the view if we fail.
2323 final View transientView = mRecycler.getTransientStateView(position);
2324 if (transientView != null) {
2325 final LayoutParams params = (LayoutParams) transientView.getLayoutParams();
2326
2327 // If the view type hasn't changed, attempt to re-bind the data.
2328 if (params.viewType == mAdapter.getItemViewType(position)) {
2329 final View updatedView = mAdapter.getView(position, transientView, this);
2330
2331 // If we failed to re-bind the data, scrap the obtained view.
2332 if (updatedView != transientView) {
2333 setItemViewLayoutParams(updatedView, position);
2334 mRecycler.addScrapView(updatedView, position);
2335 }
2336 }
2337
2338 // Scrap view implies temporary detachment.
2339 isScrap[0] = true;
2340 return transientView;
2341 }
2342
2343 final View scrapView = mRecycler.getScrapView(position);
2344 final View child = mAdapter.getView(position, scrapView, this);
。。。。。
2364 setItemViewLayoutParams(child, position);
。。。。
2376
2377 return child;
2378 }
private void setItemViewLayoutParams(View child, int position) {
2381 final ViewGroup.LayoutParams vlp = child.getLayoutParams();
2382 LayoutParams lp;
2383 if (vlp == null) {
2384 lp = (LayoutParams) generateDefaultLayoutParams();
2385 } else if (!checkLayoutParams(vlp)) {
2386 lp = (LayoutParams) generateLayoutParams(vlp);
2387 } else {
2388 lp = (LayoutParams) vlp;
2389 }
2390
2391 if (mAdapterHasStableIds) {
2392 lp.itemId = mAdapter.getItemId(position);
2393 }
2394 lp.viewType = mAdapter.getItemViewType(position);
2395 child.setLayoutParams(lp);
2396 }
可以看到在5.0版本，参数先校验时，通不过!checkLayoutParams(vlp)) 重新设置了LayoutParams lp。
然后我们看4.4版本的代码 http://androidxref.com/4.4.2_r2/xref/frameworks/base/core/java/android/widget/AbsListView.java#95
View obtainView(int position, boolean[] isScrap) {
2228 Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");
2229
2230 isScrap[0] = false;
2231 View scrapView;
2232
2233 scrapView = mRecycler.getTransientStateView(position);
2234 if (scrapView == null) {
2235 scrapView = mRecycler.getScrapView(position);
2236 }
2237
2238 View child;
2239 if (scrapView != null) {
2240 child = mAdapter.getView(position, scrapView, this);
2241
。。。。
2262 } else {
2263 child = mAdapter.getView(position, null, this);
。。。。
2272 }
2273
2274 if (mAdapterHasStableIds) {
2275 final ViewGroup.LayoutParams vlp = child.getLayoutParams();
2276 LayoutParams lp;
2277 if (vlp == null) {
2278 lp = (LayoutParams) generateDefaultLayoutParams();
2279 } else if (!checkLayoutParams(vlp)) {
2280 lp = (LayoutParams) generateLayoutParams(vlp);
2281 } else {
2282 lp = (LayoutParams) vlp;
2283 }
2284 lp.itemId = mAdapter.getItemId(position);
2285 child.setLayoutParams(lp);
2286 }
2287
。。。。
2299 return child;
2300 }
mAdapterHasStableIds 为true时才检验参数(http://androidxref.com/4.4.2_r2/xref/frameworks/base/core/java/android/widget/AbsListView.java#mAdapterHasStableIds))
那么我们的child的getLayoutParams来自于哪儿呢？
5. inflater.inflate
childView = (ViewGroup) inflater.inflate(R.layout.xxx, container, false); 两个版本代码基本一致。
最终调用LayoutInflater的方法
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)
。。。
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
由于我们的布局attachToRoot为false，调用 setLayoutParams方法时，将container的参数被设置给了child View 。所以结论就是container的参数被塞给了child View ， 在obtainView的时候因为版本差异导致异化处理， 而在setupchild设置的时候4.x版crash了。根本原因在于parent View的设置不正确。 那么正确姿势就很简单了， 设置正确的parent的就行，那么是谁呢？ 当然是child view添加进的listView了。 为什么不设置null，也很简单， null就导致父类的参数没法设置进child View 了。
