一、概述
这部分主要是讲解ListView在初始时，即setAdapter之后，如何将Adapter中取得的view添加到ListView中，主要涉及到几个方法：
layoutChildren，fillFromTop，fillDown / fillUp，makeAndAddView，obtainView，setupChild；
二、讲解
layoutChildren是个最重要的方法，也很复杂，本篇略过，会在以后中提到，不过，为了讲解其它方法，还是提一下：
@Override protected void layoutChildren() { ......... boolean dataChanged = mDataChanged; if (dataChanged) { handleDataChanged(); } ......... // Clear out old views detachAllViewsFromParent(); switch (mLayoutMode) { ......... case LAYOUT_FORCE_BOTTOM: sel = fillUp(mItemCount - 1, childrenBottom); adjustViewsUpOrDown(); break; case LAYOUT_FORCE_TOP: mFirstPosition = 0; sel = fillFromTop(childrenTop); adjustViewsUpOrDown(); break; ......... } ......... }setAdapter之后，dataChanged是true，然后走到handleDataChanged方法，在这个方法中会设置mLayoutMode，通常是LAYOUT_FORCE_TOP，即从顶部开始一个一个的往下添加childview：
@Override protected void handleDataChanged() { ......... // Nothing is selected. Give up and reset everything. mLayoutMode = mStackFromBottom ? LAYOUT_FORCE_BOTTOM : LAYOUT_FORCE_TOP; mSelectedPosition = INVALID_POSITION; mSelectedRowId = INVALID_ROW_ID; mNextSelectedPosition = INVALID_POSITION; mNextSelectedRowId = INVALID_ROW_ID; mNeedSync = false; mSelectorPosition = INVALID_POSITION; checkSelectionChanged(); } 这个方法的最后，就会去设置mLayoutMode，并设置其它position为INVALID属性；
回到layoutChildren中，设置mFirstPosition=0后，将childrenTop(=0或padding top 后的值)，进入fillFromTop方法：
/** * Fills the list from top to bottom, starting with mFirstPosition * * @param nextTop The location where the top of the first item should be * drawn * * @return The view that is currently selected */ private View fillFromTop(int nextTop) { mFirstPosition = Math.min(mFirstPosition, mSelectedPosition); mFirstPosition = Math.min(mFirstPosition, mItemCount - 1); if (mFirstPosition < 0) { mFirstPosition = 0; } return fillDown(mFirstPosition, nextTop); }该方法，第一个child的position起始点，然后调用fillDown；
/** * Fills the list from pos down to the end of the list view. * * @param pos The first position to put in the list * * @param nextTop The location where the top of the item associated with pos * should be drawn * * @return The view that is currently selected, if it happens to be in the * range that we draw. */ private View fillDown(int pos, int nextTop) { View selectedView = null; int end = (mBottom - mTop); if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { end -= mListPadding.bottom; } while (nextTop < end && pos < mItemCount) { // is this the selected item? boolean selected = pos == mSelectedPosition; View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected); nextTop = child.getBottom() + mDividerHeight; if (selected) { selectedView = child; } pos++; } return selectedView; }首先，获得可视区域的高度(mBottom - mTop)；
其次，while循环中，判断：累计添加到listview中child的高度，不超过可视区域（添加最后一个child时，有可能只显示部分），且添加的child的下标不超过总的个数（否则系统会报 OutOfBounds 的异常）；
在循环中，会去调用makeAndAddView，这个方法不会真正的去添加child，但会调用之后的setupChild来真正添加到listview中：
/** * Obtain the view and add it to our list of children. The view can be made * fresh, converted from an unused view, or used as is if it was in the * recycle bin. * * @param position Logical position in the list * @param y Top or bottom edge of the view to add * @param flow If flow is true, align top edge to y. If false, align bottom * edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected? * @return View that was added */ 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) { if (ViewDebug.TRACE_RECYCLER) { ViewDebug.trace(child, ViewDebug.RecyclerTraceType.RECYCLE_FROM_ACTIVE_HEAP, position, getChildCount()); } // 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; } 该方法分两种情况：
1. adapter中的数据发生了变化，初始setAdapter，或之后我们将adapter中的数据做了新增/删除后，调用Adapter.notifyDataSetChanged；变化了，就会从RecycleBin中的mScrapView中，取之前移出的view，即convertView来复用；
2. 若没有变化，则从RecycleBin的mActiveView中取当前显示的view（为啥会有这种情况？当listview稳定后，我们不滚动它，但有可能点击或长按），这时就会走到这里。
obtainView就是从RecycleBin中，取移出去的View，传给Adapter.getView方法（convertView），我们先来看看这个方法：
/** * 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) { isScrap[0] = false; View scrapView; scrapView = mRecycler.getScrapView(position); View child; if (scrapView != null) { if (ViewDebug.TRACE_RECYCLER) { ViewDebug.trace(scrapView, ViewDebug.RecyclerTraceType.RECYCLE_FROM_SCRAP_HEAP, position, -1); } child = mAdapter.getView(position, scrapView, this); if (ViewDebug.TRACE_RECYCLER) { ViewDebug.trace(child, ViewDebug.RecyclerTraceType.BIND_VIEW, position, getChildCount()); } if (child != scrapView) { mRecycler.addScrapView(scrapView, position); if (mCacheColorHint != 0) { child.setDrawingCacheBackgroundColor(mCacheColorHint); } if (ViewDebug.TRACE_RECYCLER) { ViewDebug.trace(scrapView, ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, position, -1); } } else { isScrap[0] = true; child.dispatchFinishTemporaryDetach(); } } else { child = mAdapter.getView(position, null, this); if (mCacheColorHint != 0) { child.setDrawingCacheBackgroundColor(mCacheColorHint); } if (ViewDebug.TRACE_RECYCLER) { ViewDebug.trace(child, ViewDebug.RecyclerTraceType.NEW_VIEW, position, getChildCount()); } } return child; }这个方法，实际上是在AbsListView类中的，我们知道，ListView, GridView是继承于AbsListView的，而AbsListView是继承于AdapterView<ListAdapter>，而这个是继承于ViewGroup，所以，ListView或AbsListView中，有些变量或方法，都是继承过来的，我们看源码时，需要结合这些类一起看（比较头痛，跳来跳去），但熟悉之后，对我们写出高质量的程序是很有帮助的。
小提示：RecycleBin类，它的作用就是一个View的缓存，将移出屏幕外的view回收，并给新移入到屏幕内的view来复用，这样就能节省大量内存（之后讲解该类）。
回归正题obtainView方法：
1. 先从RecycleBin中获取一个ScrapView，如果缓存中没有，则convertView为null，在Adapter中，需要自己去LayoutInflater一个view；
2. 若有，将这个convertView传给Adapter.getView方法；
3. 判断从getView方法中返回的view是否与scrapview一致，如果一致，表明是复用的，反之，则程序又去创建了一个新的view（浪费了一块内存），且将得到的scrapview重新加入到RecycleBin.mScrapView中；
回到makeAndAddView方法，将child传给setupChild，开始真正的加入到listView中去显示：
/** * Add a view as a child and make sure it is measured (if necessary) and * positioned properly. * * @param child The view to add * @param position The position of this child * @param y The y position relative to which this view will be positioned * @param flowDown If true, align top edge to y. If false, align bottom * edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected? * @param recycled Has this view been pulled from the recycle bin? If so it * does not need to be remeasured. */ private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) { final boolean isSelected = selected && shouldShowSelector(); final boolean updateChildSelected = isSelected != child.isSelected(); final int mode = mTouchMode; final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL && mMotionPosition == position; final boolean updateChildPressed = isPressed != child.isPressed(); final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested(); // Respect layout params that are already in the view. Otherwise make some up... // noinspection unchecked AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams(); if (p == null) { p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT, 0); } p.viewType = mAdapter.getItemViewType(position); if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) { attachViewToParent(child, flowDown ? -1 : 0, p); } else { p.forceAdd = false; if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { p.recycledHeaderFooter = true; } addViewInLayout(child, flowDown ? -1 : 0, p, true); } if (updateChildSelected) { child.setSelected(isSelected); } if (updateChildPressed) { child.setPressed(isPressed); } if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) { if (child instanceof Checkable) { ((Checkable) child).setChecked(mCheckStates.get(position)); } else if (getContext().getApplicationInfo().targetSdkVersion >= android.os.Build.VERSION_CODES.HONEYCOMB) { child.setActivated(mCheckStates.get(position)); } } if (needToMeasure) { int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec, mListPadding.left + mListPadding.right, p.width); int lpHeight = p.height; int childHeightSpec; if (lpHeight > 0) { childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY); } else { childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED); } child.measure(childWidthSpec, childHeightSpec); } else { cleanupLayoutState(child); } final int w = child.getMeasuredWidth(); final int h = child.getMeasuredHeight(); final int childTop = flowDown ? y : y - h; if (needToMeasure) { final int childRight = childrenLeft + w; final int childBottom = childTop + h; child.layout(childrenLeft, childTop, childRight, childBottom); } else { child.offsetLeftAndRight(childrenLeft - child.getLeft()); child.offsetTopAndBottom(childTop - child.getTop()); } if (mCachingStarted && !child.isDrawingCacheEnabled()) { child.setDrawingCacheEnabled(true); } if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition) != position) { child.jumpDrawablesToCurrentState(); } }attachViewToParent 和 addViewInLayout两者大致差不多，都是将view添加到parent view的array中，区别在于，attachView是不用去requestLayout的，而addViewInLayout的最后一个参数指明（true不用requestLayout，false则需要requestLayout），这两个方法都在ViewGroup中。
flowDown ? -1 : 0 ， 不知道大家注意没有， -1 和 0 的区别？我们看下attachViewToParent代码吧：
/** * Attaches a view to this view group. Attaching a view assigns this group as the parent, * sets the layout parameters and puts the view in the list of children so it can be retrieved * by calling [email protected] #getChildAt(int)}. * * This method should be called only for view which were detached from their parent. * * @param child the child to attach * @param index the index at which the child should be attached * @param params the layout parameters of the child * * @see #removeDetachedView(View, boolean) * @see #detachAllViewsFromParent() * @see #detachViewFromParent(View) * @see #detachViewFromParent(int) */ protected void attachViewToParent(View child, int index, LayoutParams params) { child.mLayoutParams = params; if (index < 0) { index = mChildrenCount; } addInArray(child, index); child.mParent = this; child.mPrivateFlags = (child.mPrivateFlags & ~DIRTY_MASK & ~DRAWING_CACHE_VALID) | DRAWN | INVALIDATED; this.mPrivateFlags |= INVALIDATED; if (child.hasFocus()) { requestChildFocus(child, child.findFocus()); } }
如果是-1，则将index = mChildrenCount，mChildrenCount就是当前parent view已经有多少个child view，这里将index设置为当前child view个数，意在表明如果是 -1 则将该view加入到parent view中的child view最后，如果是0， 则加入到child view 数组中的第一个；即：该view是插入到顶部，还是添加到底部。
默认情况下，添加到listview中的item，即child都会measure一次高度和宽度，然后，调用child.layout，通知新添加的child，layout一下它里面的children。
然后，然后就没有了然后，整个流程走完，回到ListView.layoutChildren中，adjustViewsUpOrDown将所有child调整对齐，刷新一下RecycleBin的Active和Scrap缓存，调用updateScrollIndicators更新一下滚动条的值，若有注意OnScrollListener，也通知一下invokeOnItemScrollListener。
三、总结
虽然，只讲了一个fillDown，其实fillUp以及ListView中的其它fillXXX方法，是差不多的，本篇的目的，也只是希望大家了解ListView / GridView继承于AbsListView时，如何将child添加进去。
之后，会涉及ListView / GridView， AbsListView 以及其内部类 RecycleBin的一些讲解，也欢迎大家留言，一起讨论。