1. 背景

一個新版本的程式碼，在4.x版本進入某個頁面的時候，必現crash。看到必現，心情就放鬆了一半。
大致的crash資訊如下：

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)
 ....

2. 為什麼會出現crash

測試的時候，發現5.x不會crash，4.x必然重現是什麼原因呢？
我們發現棧頂setupChild，先找該函式：

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

我們看到 AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams() 一句正是crash的根源，強制轉換導致的crash。 這裡對比4.x與5.x版本的原始碼，發現兩個版本的這裡沒有什麼區別。 那是什麼情況導致的child差異呢？跟蹤程式碼，回到上級呼叫makeAndAddView裡，看下原始碼兩個版本基本一致的。

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    }

我們看到child = obtainView(position, mIsScrap)來自與listView的快取相關。所以還是得跟蹤obtainView，該函式也很好理解，有快取的時候，從快取池中取，否則重新生成。

4. obtainView

先看5.0版本的程式碼

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，引數先校驗時，通不過!checkLayoutParams(vlp)) 重新設定了LayoutParams lp。

 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    }

5. inflater.inflate

childView = (ViewGroup) inflater.inflate(R.layout.xxx, container, false); 兩個版本程式碼基本一致。
最終呼叫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);
                        }
                    }

由於我們的佈局attachToRoot為false，呼叫 setLayoutParams方法時，將container的引數被設定給了child View 。所以結論就是container的引數被塞給了child View ， 在obtainView的時候因為版本差異導致異化處理， 而在setupchild設定的時候4.x版crash了。根本原因在於parent View的設定不正確。 那麼正確姿勢就很簡單了， 設定正確的parent的就行，那麼是誰呢？ 當然是child view新增進的listView了。 為什麼不設定null，也很簡單， null就導致父類的引數沒法設定進child View 了。