先来看ListView类中的makeAndAddView方法：
1 /**
2 * 获取视图填充到列表的item中去，视图可以是从未使用过的视图转换过来，也可以是从回收站复用的视图。
3 * 在该方法中，先查找是否有可重用视图，如果有，使用可重用视图。
4 * 然后通过obtainView方法获取一个view（有可能是从未使用视图转换过来
5 * （obtainView方法是在AbsListView方法中定义）），再重新测量和定位View。
6 * Obtain the view and add it to our list of children. The view can be made
7 * fresh, converted from an unused view, or used as is if it was in the
8 * recycle bin.
9 *
10 * @param position Logical position in the list
11 * @param y Top or bottom edge of the view to add
12 * @param flow If flow is true, align top edge to y. If false, align bottom
13 * edge to y.
14 * @param childrenLeft Left edge where children should be positioned
15 * @param selected Is this position selected?
16 * @return View that was added
17 */
18 private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
19 boolean selected) {
20 View child;
21
22
23 if (!mDataChanged) {
24 // Try to use an existing view for this position
25 child = mRecycler.getActiveView(position);
26 if (child != null) {
27 if (ViewDebug.TRACE_RECYCLER) {
28 ViewDebug.trace(child, ViewDebug.RecyclerTraceType.RECYCLE_FROM_ACTIVE_HEAP,
29 position, getChildCount());
30 }
31
32 // Found it -- we‘re using an existing child
33 // This just needs to be positioned
34 setupChild(child, position, y, flow, childrenLeft, selected, true);
35
36 return child;
37 }
38 }
39
40 // Make a new view for this position, or convert an unused view if possible
41 child = obtainView(position, mIsScrap);
42
43 // This needs to be positioned and measured
44 setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
45
46 return child;
47 }
第41行调用了obtainView方法，该方法的实现是在package android.widget;的AbsListView类中
1 /**
2 * Get a view and have it show the data associated with the specified
3 * position. This is called when we have already discovered that the view is
4 * not available for reuse in the recycle bin. The only choices left are
5 * converting an old view or making a new one.
6 *
7 * @param position The position to display
8 * @param isScrap Array of at least 1 boolean, the first entry will become true if
9 * the returned view was taken from the scrap heap, false if otherwise.
10 *
11 * @return A view displaying the data associated with the specified position
12 */
13 View obtainView(int position, boolean[] isScrap) {
14 Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");
15
16 isScrap[0] = false;
17 View scrapView;
18
19 scrapView = mRecycler.getTransientStateView(position);
20 if (scrapView == null) {
21 // 从回收站回收一个View
22 scrapView = mRecycler.getScrapView(position);
23 }
24
25 View child;
26 if (scrapView != null) {
27 // 这里调用了getView!注意，第二个参数也就是convertView传 入的是刚才从回收站中回收的View(如果有的话)
28 child = mAdapter.getView(position, scrapView, this);
29
30 if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
31 child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
32 }
33
34 if (child != scrapView) {
35 mRecycler.addScrapView(scrapView, position);
36 if (mCacheColorHint != 0) {
37 child.setDrawingCacheBackgroundColor(mCacheColorHint);
38 }
39 } else {
40 isScrap[0] = true;
41
42 // Clear any system-managed transient state so that we can
43 // recycle this view and bind it to different data.
44 if (child.isAccessibilityFocused()) {
45 child.clearAccessibilityFocus();
46 }
47
48 child.dispatchFinishTemporaryDetach();
49 }
50 } else {
51 child = mAdapter.getView(position, null, this);
52
53 if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
54 child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
55 }
56
57 if (mCacheColorHint != 0) {
58 child.setDrawingCacheBackgroundColor(mCacheColorHint);
59 }
60 }
61
62 if (mAdapterHasStableIds) {
63 final ViewGroup.LayoutParams vlp = child.getLayoutParams();
64 LayoutParams lp;
65 if (vlp == null) {
66 lp = (LayoutParams) generateDefaultLayoutParams();
67 } else if (!checkLayoutParams(vlp)) {
68 lp = (LayoutParams) generateLayoutParams(vlp);
69 } else {
70 lp = (LayoutParams) vlp;
71 }
72 lp.itemId = mAdapter.getItemId(position);
73 child.setLayoutParams(lp);
74 }
75
76 if (AccessibilityManager.getInstance(mContext).isEnabled()) {
77 if (mAccessibilityDelegate == null) {
78 mAccessibilityDelegate = new ListItemAccessibilityDelegate();
79 }
80 if (child.getAccessibilityDelegate() == null) {
81 child.setAccessibilityDelegate(mAccessibilityDelegate);
82 }
83 }
84
85 Trace.traceEnd(Trace.TRACE_TAG_VIEW);
86
87 return child;
88 }
第28行调用了getView！根据Java多态的特性，实际执行的getView将会是我们自定义BaseAdapter中的那个getView方法。
好，现在虽然找到getView的直接调用者了，问题来了，何时去触发makeAndAddView并调用getView呢？
我们首先来看ListView中的fillDown：
1 /**
2 填充从pos到list底部所有的item。里面调用到了makeAndAddView方 法：
3 * Fills the list from pos down to the end of the list view.
4 *
5 * @param pos The first position to put in the list
6 *
7 * @param nextTop The location where the top of the item associated with pos
8 * should be drawn
9 *
10 * @return The view that is currently selected, if it happens to be in the
11 * range that we draw.
12 */
13 private View fillDown(int pos, int nextTop) {
14 View selectedView = null;
15
16 int end = (mBottom - mTop);
17 if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
18 end -= mListPadding.bottom;
19 }
20
21 while (nextTop < end && pos < mItemCount) {
22 // is this the selected item?
23 boolean selected = pos == mSelectedPosition;
24 View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
25
26 nextTop = child.getBottom() + mDividerHeight;
27 if (selected) {
28 selectedView = child;
29 }
30 pos++;
31 }
32
33 return selectedView;
34 }
有fillDown自然就有fillUp：
1 /**
2 * Fills the list from pos up to the top of the list view.
3 *
4 * @param pos The first position to put in the list
5 *
6 * @param nextBottom The location where the bottom of the item associated
7 * with pos should be drawn
8 *
9 * @return The view that is currently selected
10 */
11 private View fillUp(int pos, int nextBottom) {
12 View selectedView = null;
13
14 int end = 0;
15 if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
16 end = mListPadding.top;
17 }
18
19 while (nextBottom > end && pos >= 0) {
20 // is this the selected item?
21 boolean selected = pos == mSelectedPosition;
22 View child = makeAndAddView(pos, nextBottom, false, mListPadding.left, selected);
23 nextBottom = child.getTop() - mDividerHeight;
24 if (selected) {
25 selectedView = child;
26 }
27 pos--;
28 }
29
30 mFirstPosition = pos + 1;
31
32 return selectedView;
33 }
还有fillFromTop、fillFromMiddle、fillAboveAndBelow、fillFromSelection等，这些方法都是用来填充ListView的，区别是参照的起始位置不同而已。
好了，现在填充的方法有了，那么谁来触发这些方法呢？
通过查找ListView源码，发现刚才的那些填充方法在layoutChildren()中基本上都被调用到，而layoutChildren的调用者是onFocusChanged、setSelectionInt、父类AbsListView中的onTouchMove等，说明当ListView的焦点发生变化时、选中某一项、或者滑动ListView时都会触发ListView的layoutChildren()重新装载应当显示的item。
1 @Override
2 protected void layoutChildren() {
3 final boolean blockLayoutRequests = mBlockLayoutRequests;
4 if (blockLayoutRequests) {
5 return;
6 }
7
8 mBlockLayoutRequests = true;
9
10 try {
11 super.layoutChildren();
12
13 invalidate();
我们仔细阅读layoutChildren()方法，发现在第13行，调用了invalidate();，也就是重绘了ListView。
到此为止我们已经很清楚getView的调用时机了，根据掌握的知识点，我们很自然能想到，当初始化一个ListView时，getView的调用也是避免不了的。这是因为ListView在初始化时肯定会绑定一个adapter，即调用语句listview.setAdapter(adapter)，我们看一下setAdapter的源码：
1 @Override
2 public void setAdapter(ListAdapter adapter) {
3 if (mAdapter != null && mDataSetObserver != null) {
4 mAdapter.unregisterDataSetObserver(mDataSetObserver);
5 }
6
7 resetList();
8 mRecycler.clear();
9
10 if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
11 mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter);
12 } else {
13 mAdapter = adapter;
14 }
15
16 mOldSelectedPosition = INVALID_POSITION;
17 mOldSelectedRowId = INVALID_ROW_ID;
18
19 // AbsListView#setAdapter will update choice mode states.
20 super.setAdapter(adapter);
21
22 if (mAdapter != null) {
23 mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
24 mOldItemCount = mItemCount;
25 mItemCount = mAdapter.getCount();
26 checkFocus();
27
28 mDataSetObserver = new AdapterDataSetObserver();
29 mAdapter.registerDataSetObserver(mDataSetObserver);
30
31 mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());
32
33 int position;
34 if (mStackFromBottom) {
35 position = lookForSelectablePosition(mItemCount - 1, false);
36 } else {
37 position = lookForSelectablePosition(0, true);
38 }
39 setSelectedPositionInt(position);
40 setNextSelectedPositionInt(position);
41
42 if (mItemCount == 0) {
43 // Nothing selected
44 checkSelectionChanged();
45 }
46 } else {
47 mAreAllItemsSelectable = true;
48 checkFocus();
49 // Nothing selected
50 checkSelectionChanged();
51 }
52
53 requestLayout();
54 }
通读setAdapter源码，我们发现其中并未出现生成新子视图，即调用mAdapter.getView的语句或方法，说明此时ListView并未包含子视图。那么疑问来了，ListView是如何在初始化的时候生成子视图的？往后看，我们发现在第53行调用了requestLayout请求布局重绘，我们知道requestLayout最终会去调用ListView或父类的onMeasure、onLayout、onDraw方法，因此我们猜测会不会是在onMeasure、onLayout、onDraw某个方法中生成了子视图？
答案是肯定的，ListView.onMeasure中有AbsListView.obtainView的调用语句（这里还需深入研究，因为涉及到ListView高度探测），也即生成了新的子视图。此外，ListVIew.onLayout过程与普通视图的layout过程完全不同，该方法调用了layoutChildren();，即对ListView中的子视图进行重新布局，具体过程请参考《Android ListView初始化简单分析》一文。
由此说明调用requestLayout可以生成ListView的子视图，这里联想到adapter.notifyDataSetChanged也会调用requestLayout，从而能都实现ListView的刷新。
以上过程只是个人探索，并非绝对正确，如有差错敬请批评指正，谢谢。
参考文献：
时间： 2024-10-16 14:58:12