ListView加载不同布局时的复用及原理分析

当加载不同布局时，需要使用到getViewTypeCount和getItemViewType。首先来看看如何来实现加载不同而已时的复用

步骤：
重(@Override)写 getViewTypeCount() – 返回你有多少个不同的布局
重写 getItemViewType(int) – 由position返回view type id
根据view item的类型，在getView中创建正确的convertView

代码如下：

private class MyAdapter extends BaseAdapter {

@Override
public int getCount() {
return list.size();
        }

@Override
public Object getItem(int position) {
return position;
        }

@Override
public long getItemId(int position) {
return 0;
        }

/**
 * 视图类型的个数
* @return
*/
@Override
public int getViewTypeCount() {
return 2;
        }

/**
 * 返回不同位置的视图的类型
* @param position
* @return
*/
@Override
public int getItemViewType(int position) {

if (position % 2 == 0) {
return 1;
            } else {
return 2;
            }

        }

@Override
public View getView(int position, View convertView, ViewGroup parent) {
            ViewHolder holder;
if (convertView == null) {
                holder = new ViewHolder();
if (getItemViewType(position) == 1) {
                    convertView = View.inflate(MutipleItemListActivity.this, R.layout.item_listview_left, null);
                } else {
                    convertView = View.inflate(MutipleItemListActivity.this, R.layout.item_listview_right, null);
                }
                holder.textView = (TextView) convertView.findViewById(R.id.textView);
                convertView.setTag(holder);

            } else {
                holder = (ViewHolder) convertView.getTag();
            }


            String text = list.get(position);
            holder.textView.setText(text);

return convertView;
        }
    }


static class ViewHolder {
        TextView textView;
    }
}

其实很简单，无非就是在getView方法中调用调用getItemViewType方法去判断类型，从而填充不同的布局。

下面来分析一下利用的原理
既然有复用，就有缓存，那么ListView的item是在哪里缓存的呢？
分析原码要有一个入口，ListView的入口无疑是setAdapter方法，因为我们第一个调用的就是它。
源码的中有一名
mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());
getViewTypeCount方法设置的视图类型数量被mRecycler获取。继续追踪，可以发现， mRecycler为AbListView的一个内部类，而它就是我们要找的缓存所在RecycleBin 。RecycleBin 中有两个成员变量数组， mActiveViews与mScrapView 。从字面上就可以理解，mActiveViews为活动中的view,即显示在界面上的item, 而mScrapView则缓存着用来利用的item.
而外还有一个LongSparseArray mTransientStateViewsById， 保存着mScrapView中空闲view的引用 。
继续看setViewTypeCount

public void setViewTypeCount(int viewTypeCount) {
if (viewTypeCount < 1) {
throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
    }
//noinspection unchecked
    ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];
for (int i = 0; i < viewTypeCount; i++) {
        scrapViews[i] = new ArrayList<View>();
    }
    mViewTypeCount = viewTypeCount;
    mCurrentScrap = scrapViews[0];
    mScrapViews = scrapViews;
}

这个方法只是对mScrapView作的一个初始化。而mScrapView的大小 就是viewTypeCount. 接下来要通过搜索mRecycler.addScrapView发现第一次调用在obtainView方法中，这个方法会调用getView完成视图的初始化工作.

final View transientView = mRecycler.getTransientStateView(position);
if (transientView != null) {
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. 这里只是不使用复用时才会执行下去
if (updatedView != transientView) {
        setItemViewLayoutParams(updatedView, position);
        mRecycler.addScrapView(updatedView, position);
    }
}
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();
    }
}

代码分两部分，一部分是从空闲的集合mTransientStateViewsById中到取view时行利用，
如果没取到，则直接从mScrapViews中取。可以发现，这里并没有填充mScrapViews. 还有一个地方执行addScrapView方法的则是在
trackMotionScroll方法中，这个方法会在界面滑动时会被调用。

for (int i = childCount - 1; i >= 0; i--) {
final View child = getChildAt(i);
if (child.getTop() <= bottom) {
break;
    } else {
        start = i;
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
// The view will be rebound to new data, clear any
// system-managed transient state.
            child.clearAccessibilityFocus();
            mRecycler.addScrapView(child, position);
        }
    }
}

mScrapViews会把所有滑过的item的条目的引用都保存起来，以便以后的复用，

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;
}

getScrapView先判断类型，然后调用retrieveFromScrap取出数组中相应位置的view, 同时删除当前位置的引用。

fillActivityViews则会填充所有活动中item

void fillActiveViews(int childCount, int firstActivePosition) {
if (mActiveViews.length < childCount) {
        mActiveViews = new View[childCount];
    }
    mFirstActivePosition = firstActivePosition;

//noinspection MismatchedReadAndWriteOfArray
final View[] activeViews = mActiveViews;
for (int i = 0; i < childCount; i++) {
        View child = getChildAt(i);
        AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();
// Don't put header or footer views into the scrap heap
if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
// Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in active views.
// However, we will NOT place them into scrap views.
            activeViews[i] = child;
// Remember the position so that setupChild() doesn't reset state.
            lp.scrappedFromPosition = firstActivePosition + i;
        }
    }
}

那么 mActiveViews与mScrapView的添加还是要回到ListView中

  protected void layoutChildren()   
    {  
if (dataChanged)   
        {  
for (int i = 0; i < childCount; i++)   
            {  
                recycleBin.addScrapView(getChildAt(i));     
            }  
        } else   
        {  
            recycleBin.fillActiveViews(childCount, firstPosition);  
        }  
        ....  
    }  

当ListView执行OnLayout时，会调用layoutChildren方法，并填充 mActiveViews与mScrapView。 当数据项发生变化时,调用addScrapView。

再看一个重要的方法makeAndAddView，这个方法将决定从哪里取取出child,并绘制出来。

private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
    View child;


if (!mDataChanged) {
// Try to use an existing view for this position
        child = mRecycler.getActiveView(position);
if (child != null) {
// Found it -- we're using an existing child
// This just needs to be positioned
            setupChild(child, position, y, flow, childrenLeft, selected, true);

return child;
        }
    }
// Make a new view for this position, or convert an unused view if possible
    child = obtainView(position, mIsScrap);

// This needs to be positioned and measured
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

return child;
}

分析的有点乱。总结下，也就是mActiveViews保存活动中的item, mScrapView 保存着未被显示出来的位置的item.当数据项发生变化，或者滑动时，会取出 mScrapView中的item进行复用，并对缓存进行重新的设置。