從原始碼角度分析SrcollView巢狀ListView顯示不全的問題
問題描述
在之前開發的時候會碰到列表滑動佈局中ScrollView巢狀ListView的情況，當巢狀了之後發現ListView只能顯示一行資料。碰到這種情況也是
於是乎開始Google，很快就找到解決方法，自定義view繼承自ListView，重寫onMeasure()方法，然後再加入一行程式碼即可解決問題。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
複製程式碼But... why?
問題分析
首先應該可以想到，ScrollView巢狀ListView，ListView相當於ScrollView的子View了，我們應該去看看父View如何去測量子View的，開啟原始碼先看ScrollView的onMeasure方法。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
if (!mFillViewport) {
return;
}
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
if (heightMode == MeasureSpec.UNSPECIFIED) {
return;
}
if (getChildCount() > 0) {
final View child = getChildAt(0);
final int widthPadding;
final int heightPadding;
final int targetSdkVersion = getContext().getApplicationInfo().targetSdkVersion;
final FrameLayout.LayoutParams lp = (LayoutParams) child.getLayoutParams();
if (targetSdkVersion >= VERSION_CODES.M) {
widthPadding = mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin;
heightPadding = mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin;
} else {
widthPadding = mPaddingLeft + mPaddingRight;
heightPadding = mPaddingTop + mPaddingBottom;
}
final int desiredHeight = getMeasuredHeight() - heightPadding;
if (child.getMeasuredHeight() < desiredHeight) {
final int childWidthMeasureSpec = getChildMeasureSpec(
widthMeasureSpec, widthPadding, lp.width);
final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
desiredHeight, MeasureSpec.EXACTLY);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
複製程式碼發現第一行就呼叫了父類的onMeasure方法，點進去看看做了什麼，ScrollView的父類是FrameLayout
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
}
複製程式碼擷取了一部分程式碼，看呀看，發現了一個很可疑的方法
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
複製程式碼看名稱是測量Child的，跟進,發現進入了ViewGroup裡面。
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
複製程式碼一眼望去，感覺應該在裡面計運算元View的寬高，並且把padding值和margin值都算進去了。不管那麼多了，ViewGroup是ScrollView的爺爺了吧，ScrollView裡面應該會重寫這個方法，因為ScrollView裡面也能包裹其他的View，跟進。
@Override
protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int usedTotal = mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin +
heightUsed;
final int childHeightMeasureSpec = MeasureSpec.makeSafeMeasureSpec(
Math.max(0, MeasureSpec.getSize(parentHeightMeasureSpec) - usedTotal),
MeasureSpec.UNSPECIFIED);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
複製程式碼果然，這樣一來ListView被ScrollView包裹，它的寬高應該是在這裡被計算的，看看到底為啥只有一個條目的高度。看到這裡
final int childHeightMeasureSpec = MeasureSpec.makeSafeMeasureSpec(
Math.max(0, MeasureSpec.getSize(parentHeightMeasureSpec) - usedTotal),
MeasureSpec.UNSPECIFIED);
複製程式碼這裡就是計算childHeight的地方，到這裡我們就應該去了解一下MeasureSpec了.
MeasureSpec： 從字面意思翻譯像是測量規格，它在很大程度上決定了一個View的尺寸規則。實際上MeasureSpec 是一個32位的int值，高2位代表SpecMode（測量模式），低30位代表SpecSize（某種測量模式下的規格大小）。重點就在於這個SpecMode。
SpecMode： 測量模式有3種
- UNSPECFIED： 父容器不對View有任何限制，要多大給多大。這種情況一般用於系統內部，表示一種測量狀態。
- EXACTLY： 父容器已經檢測出View所需要的精確大小，這個時候View的最終大小就是SpecSize所指定的值。它對應於LayoutParams中的match_parent和具體的數值這兩種模式。
- AT_MOST： 父容器指定了一個可用大小即SpecSize，View的大小不能大於這個值，具體是什麼值要看不同View的具體實現。它對應於LayoutParams中的wrap_content。
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/** @hide */
@IntDef({UNSPECIFIED, EXACTLY, AT_MOST})
@Retention(RetentionPolicy.SOURCE)
public @interface MeasureSpecMode {}
/**
* Measure specification mode: The parent has not imposed any constraint
* on the child. It can be whatever size it wants.
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* Measure specification mode: The parent has determined an exact size
* for the child. The child is going to be given those bounds regardless
* of how big it wants to be.
*/
public static final int EXACTLY = 1 << MODE_SHIFT;
/**
* Measure specification mode: The child can be as large as it wants up
* to the specified size.
*/
public static final int AT_MOST = 2 << MODE_SHIFT;
複製程式碼在瞭解了測量模式之後，在看這段程式碼
final int childHeightMeasureSpec = MeasureSpec.makeSafeMeasureSpec(
Math.max(0, MeasureSpec.getSize(parentHeightMeasureSpec) - usedTotal),
MeasureSpec.UNSPECIFIED);
複製程式碼在這裡ScrollView對子View的高的測量模式都制定成了MeasureSpec.UNSPECIFIED,也就是說，ListView要多高就給多高，這時候我們就可以去看看ListView到底要了多高。原始碼轉入ListView,看看onMesaure()方法。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// Sets up mListPadding
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
int childWidth = 0;
int childHeight = 0;
int childState = 0;
mItemCount = mAdapter == null ? 0 : mAdapter.getCount();
if (mItemCount > 0 && (widthMode == MeasureSpec.UNSPECIFIED
|| heightMode == MeasureSpec.UNSPECIFIED)) {
final View child = obtainView(0, mIsScrap);
// Lay out child directly against the parent measure spec so that
// we can obtain exected minimum width and height.
measureScrapChild(child, 0, widthMeasureSpec, heightSize);
childWidth = child.getMeasuredWidth();
childHeight = child.getMeasuredHeight();
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (recycleOnMeasure() && mRecycler.shouldRecycleViewType(
((LayoutParams) child.getLayoutParams()).viewType)) {
mRecycler.addScrapView(child, 0);
}
}
if (widthMode == MeasureSpec.UNSPECIFIED) {
widthSize = mListPadding.left + mListPadding.right + childWidth +
getVerticalScrollbarWidth();
} else {
widthSize |= (childState & MEASURED_STATE_MASK);
}
if (heightMode == MeasureSpec.UNSPECIFIED) {
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;
}
if (heightMode == MeasureSpec.AT_MOST) {
// TODO: after first layout we should maybe start at the first visible position, not 0
heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}
setMeasuredDimension(widthSize, heightSize);
mWidthMeasureSpec = widthMeasureSpec;
}
複製程式碼看到這一段程式碼
if (heightMode == MeasureSpec.UNSPECIFIED) {
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;
}
複製程式碼當測量模式是MeasureSpec.UNSPECIFIED的時候，我們只看到了 heightSize = ** + childHeight **..，oh my god, 難道ListView只向他爸爸要了一個孩子的高度，這就可以解釋清楚了為什麼ScrollView巢狀ListView只顯示一行的高度了。
問題解決
解決問題的方法在開頭已經給出了，但是為什麼要這麼做呢。
我們在觀察ListView的onMesaure方法的時候發現，當heightMode == MeasureSpec.UNSPECIFIED時，ListView只測量了一個child的高度，下面看看當heightMode == MeasureSpec.AT_MOST的時候會發生什麼。
if (heightMode == MeasureSpec.AT_MOST) {
// TODO: after first layout we should maybe start at the first visible position, not 0
heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}
複製程式碼看這個方法貌似又去測量每個孩子的高度，然後賦值給heightSize了，看看方法的具體實現。
/**
* Measures the height of the given range of children (inclusive) and
* returns the height with this ListView's padding and divider heights
* included. If maxHeight is provided, the measuring will stop when the
* current height reaches maxHeight.
*
* @return The height of this ListView with the given children.
*/
final int measureHeightOfChildren(int widthMeasureSpec, int startPosition, int endPosition,
int maxHeight, int disallowPartialChildPosition) {
final ListAdapter adapter = mAdapter;
if (adapter == null) {
return mListPadding.top + mListPadding.bottom;
}
// Include the padding of the list
int returnedHeight = mListPadding.top + mListPadding.bottom;
final int dividerHeight = mDividerHeight;
// The previous height value that was less than maxHeight and contained
// no partial children
int prevHeightWithoutPartialChild = 0;
int i;
View child;
// mItemCount - 1 since endPosition parameter is inclusive
endPosition = (endPosition == NO_POSITION) ? adapter.getCount() - 1 : endPosition;
final AbsListView.RecycleBin recycleBin = mRecycler;
final boolean recyle = recycleOnMeasure();
final boolean[] isScrap = mIsScrap;
for (i = startPosition; i <= endPosition; ++i) {
child = obtainView(i, isScrap);
measureScrapChild(child, i, widthMeasureSpec, maxHeight);
if (i > 0) {
// Count the divider for all but one child
returnedHeight += dividerHeight;
}
// Recycle the view before we possibly return from the method
if (recyle && recycleBin.shouldRecycleViewType(
((LayoutParams) child.getLayoutParams()).viewType)) {
recycleBin.addScrapView(child, -1);
}
returnedHeight += child.getMeasuredHeight();
if (returnedHeight >= maxHeight) {
// We went over, figure out which height to return. If returnedHeight > maxHeight,
// then the i'th position did not fit completely.
return (disallowPartialChildPosition >= 0) // Disallowing is enabled (> -1)
&& (i > disallowPartialChildPosition) // We've past the min pos
&& (prevHeightWithoutPartialChild > 0) // We have a prev height
&& (returnedHeight != maxHeight) // i'th child did not fit completely
? prevHeightWithoutPartialChild
: maxHeight;
}
if ((disallowPartialChildPosition >= 0) && (i >= disallowPartialChildPosition)) {
prevHeightWithoutPartialChild = returnedHeight;
}
}
// At this point, we went through the range of children, and they each
// completely fit, so return the returnedHeight
return returnedHeight;
}
複製程式碼從註釋我們就可以看出，這個方法return的是The height of this ListView with the given children. 也就是ListView包含孩子的高度。看程式碼中
returnedHeight += child.getMeasuredHeight();
複製程式碼returnHeight也是在不斷的累加每個孩子的高度，所以最終會得到ListView的真實高度。
所以，現在的問題就是如何指定ListView的MeasureSpec.SpecMode == MeasureSpec.AT_MOST 再來看開頭的解決方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
複製程式碼makeMeasureSpec方法就能重新指定測量規則，第一個引數提供一個30位的SpecSize,第二個引數提供一個2位的SpecMode，然後該方法將其合併成一個新的MeasureSpec.
public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
@MeasureSpecMode int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
複製程式碼走完這個方法之後在呼叫父類的也就是ListView的onMeasure方法，重新獲取測量模式和測量尺寸，這次拿到的測量模式就是AT_MOST，就可以去測量每個孩子的高度累加了。
問題 為什麼要使用Integer.MAX_VALUE。
這是整型的最大值，很大很大有32位，我們在上面
measureHeightOfChildren這個方法的時候，注意一段程式碼
if (returnedHeight >= maxHeight) {
// We went over, figure out which height to return. If returnedHeight > maxHeight,
// then the i'th position did not fit completely.
return (disallowPartialChildPosition >= 0) // Disallowing is enabled (> -1)
&& (i > disallowPartialChildPosition) // We've past the min pos
&& (prevHeightWithoutPartialChild > 0) // We have a prev height
&& (returnedHeight != maxHeight) // i'th child did not fit completely
? prevHeightWithoutPartialChild
: maxHeight;
}
return returnedHeight;
複製程式碼當returnedHeight >= maxHeight的時候，它會返回一個我們並不想要的值，我們需要的是returnedHeight,所以我們不能讓這個判斷成立，就選擇了整型最大值。這個maxHeight追溯一下，就是我們設定的Integer.MAX_VALUE >> 2, 右移兩位就剩下30位剛好和2位的模式配對。
結束語
此致 敬禮
