深入瞭解ListView工作原理(二) -- 之convertView的ViewType篇

現在繼續探討《深入瞭解ListView工作原理(一) – 之viewHolder優化篇》 中的第二個問題，這個問題我也有過類似困擾，當我有多種佈局給給不同的item的時候，adapter裏傳給getView裏的convertview是此position的item想要的那種viewType嗎？因爲這涉及到我是否需要在getView裏對此convertview做進一步的處理，比如判斷是否爲viewType下的不同處理方式。那麼，我們下面就從源碼的角度來尋找答案，首先我們知道getView()是被ListView父類AbsListView裏的obtainView()調用的，接下來我們先看看obtainView()是如何對convertview賦值的。

/**
     * Get a view and have it show the data associated with the specified
     * position. This is called when we have already discovered that the view is
     * not available for reuse in the recycle bin. The only choices left are
     * converting an old view or making a new one.
     *
     * @param position The position to display
     * @param isScrap Array of at least 1 boolean, the first entry will become true if
     *                the returned view was taken from the scrap heap, false if otherwise.
     *
     * @return A view displaying the data associated with the specified position
     */
    View obtainView(int position, boolean[] isScrap) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");

        isScrap[0] = false;

        // Check whether we have a transient state view. Attempt to re-bind the
        // data and discard the view if we fail.
        final View transientView = mRecycler.getTransientStateView(position);
        if (transientView != null) {
            final LayoutParams params = (LayoutParams) transientView.getLayoutParams();

            // If the view type hasn't changed, attempt to re-bind the data.
            if (params.viewType == mAdapter.getItemViewType(position)) {
                final View updatedView = mAdapter.getView(position, transientView, this);

                // If we failed to re-bind the data, scrap the obtained view.
                if (updatedView != transientView) {
                    setItemViewLayoutParams(updatedView, position);
                    mRecycler.addScrapView(updatedView, position);
                }
            }

            isScrap[0] = true;

            // Finish the temporary detach started in addScrapView().
            transientView.dispatchFinishTemporaryDetach();
            return transientView;
        }

        final View scrapView = mRecycler.getScrapView(position);
        final View child = mAdapter.getView(position, scrapView, this);
        if (scrapView != null) {
            if (child != scrapView) {
                // Failed to re-bind the data, return scrap to the heap.
                mRecycler.addScrapView(scrapView, position);
            } else {
                isScrap[0] = true;

                // Finish the temporary detach started in addScrapView().
                child.dispatchFinishTemporaryDetach();
            }
        }

        if (mCacheColorHint != 0) {
            child.setDrawingCacheBackgroundColor(mCacheColorHint);
        }

        if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
            child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
        }

        setItemViewLayoutParams(child, position);

        if (AccessibilityManager.getInstance(mContext).isEnabled()) {
            if (mAccessibilityDelegate == null) {
                mAccessibilityDelegate = new ListItemAccessibilityDelegate();
            }
            if (child.getAccessibilityDelegate() == null) {
                child.setAccessibilityDelegate(mAccessibilityDelegate);
            }
        }

        Trace.traceEnd(Trace.TRACE_TAG_VIEW);

        return child;
    }

我們這裏先不考慮transient state view的情況。那麼，從代碼可以看出，getView中的convertview參數是scrapView傳值的，所以我們接着看 mRecycler.getScrapView(position)是如何獲取convertview的。

      /**
         * @return A view from the ScrapViews collection. These are unordered.
         */
        View getScrapView(int position) {
            final int whichScrap = mAdapter.getItemViewType(position);
            if (whichScrap < 0) {
                return null;
            }
            if (mViewTypeCount == 1) {
                return retrieveFromScrap(mCurrentScrap, position);
            } else if (whichScrap < mScrapViews.length) {
                return retrieveFromScrap(mScrapViews[whichScrap], position);
            }
            return null;
        }

代碼很簡單，從代碼中我們可以看出，mAdapter.getItemViewType(position)就是根據postion來獲取此位置上的item是哪種viewType，而getItemViewType()就是我們自己override的函數，一般情況下如果不override BaseAdapter的getItemViewType()和 getViewTypeCount()的情況下，默認getViewTypeCount() == 1， getItemViewType(int position) == 0。

// BaseAdapter.java
public int getItemViewType(int position) {
        return 0;
    }

    public int getViewTypeCount() {
        return 1;
    }

所以默認情況下就進入getScrapView（）的第一個分支，但是如果我們定義了多種viewType的話，就進入第二個分支，所以接下來我們來看看mScrapViews[whichScrap]是什麼。

/**
         * Unsorted views that can be used by the adapter as a convert view.
         */
        private ArrayList<View>[] mScrapViews;

mScrapViews就是用來存儲RecycleBin裏用來複用的convertview的，它是一個數組，那麼它是怎麼存儲的呢，我們重點來看addScrapView：

/**
         * Puts a view into the list of scrap views.
         * <p>
         * If the list data hasn't changed or the adapter has stable IDs, views
         * with transient state will be preserved for later retrieval.
         *
         * @param scrap The view to add
         * @param position The view's position within its parent
         */
        void addScrapView(View scrap, int position) {
            final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
            if (lp == null) {
                // Can't recycle, but we don't know anything about the view.
                // Ignore it completely.
                return;
            }

            lp.scrappedFromPosition = position;

            // Remove but don't scrap header or footer views, or views that
            // should otherwise not be recycled.
            final int viewType = lp.viewType;
            if (!shouldRecycleViewType(viewType)) {
                // Can't recycle. If it's not a header or footer, which have
                // special handling and should be ignored, then skip the scrap
                // heap and we'll fully detach the view later.
                if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                    getSkippedScrap().add(scrap);
                }
                return;
            }

            scrap.dispatchStartTemporaryDetach();

            // The the accessibility state of the view may change while temporary
            // detached and we do not allow detached views to fire accessibility
            // events. So we are announcing that the subtree changed giving a chance
            // to clients holding on to a view in this subtree to refresh it.
            notifyViewAccessibilityStateChangedIfNeeded(
                    AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE);

            // Don't scrap views that have transient state.
            final boolean scrapHasTransientState = scrap.hasTransientState();
            if (scrapHasTransientState) {
                if (mAdapter != null && mAdapterHasStableIds) {
                    // If the adapter has stable IDs, we can reuse the view for
                    // the same data.
                    if (mTransientStateViewsById == null) {
                        mTransientStateViewsById = new LongSparseArray<>();
                    }
                    mTransientStateViewsById.put(lp.itemId, scrap);
                } else if (!mDataChanged) {
                    // If the data hasn't changed, we can reuse the views at
                    // their old positions.
                    if (mTransientStateViews == null) {
                        mTransientStateViews = new SparseArray<>();
                    }
                    mTransientStateViews.put(position, scrap);
                } else {
                    // Otherwise, we'll have to remove the view and start over.
                    getSkippedScrap().add(scrap);
                }
            } else {
                if (mViewTypeCount == 1) {
                    mCurrentScrap.add(scrap);
                } else {
                    mScrapViews[viewType].add(scrap);
                }

                if (mRecyclerListener != null) {
                    mRecyclerListener.onMovedToScrapHeap(scrap);
                }
            }
        }

可以看出mScrapViews數組是的每一項是以viewType分類的，也就是說相同viewType的convertview放在數組的相同下標裏的ArrayList < View >裏，如下圖所示。

接下來我們繼續看mRecycler.getScrapView 裏的 retrieveFromScrap（）：

//AbsListView.java
private View retrieveFromScrap(ArrayList<View> scrapViews, int position) {
            final int size = scrapViews.size();
            if (size > 0) {
                // See if we still have a view for this position or ID.
                for (int i = 0; i < size; i++) {
                    final View view = scrapViews.get(i);
                    final AbsListView.LayoutParams params =
                            (AbsListView.LayoutParams) view.getLayoutParams();

                    if (mAdapterHasStableIds) {
                        final long id = mAdapter.getItemId(position);
                        if (id == params.itemId) {
                            return scrapViews.remove(i);
                        }
                    } else if (params.scrappedFromPosition == position) {
                        final View scrap = scrapViews.remove(i);
                        clearAccessibilityFromScrap(scrap);
                        return scrap;
                    }
                }
                final View scrap = scrapViews.remove(size - 1);
                clearAccessibilityFromScrap(scrap);
                return scrap;
            } else {
                return null;
            }
        }

可以看出，由於ArrayList< View > scrapViews 已經是根據position來找到對應viewType的convertview List，因此retrieveFromScrap只需要從中選擇一個合適即可傳遞給getView中的convertview參數。

因此，當我們override getView()時，系統已經幫我們做了預先處理，即convertview！= null 時，此被用來複用的convertview 一定是我們將要顯示的item的viewType，因此顯示的item layout 風格就是正確的，我們只需要填充對應的數據，比如圖片，文本等即可。