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当我们在使用到ScrollView和ListView的时候可能会出现显示不全的问题。那我们可以进行以下分析
ScrollView在测量子布局的时候会用UNSPECIFIED。通过源码观察，
在ScrollView的onMeasure方法中

    @Overrideprotected 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);}}}

点击super.onMeasure()方法可以观察到进入了FrameLayout中

  @Overrideprotected 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);}}}}// Account for padding toomaxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();// Check against our minimum height and widthmaxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());// Check against our foreground's minimum height and widthfinal Drawable drawable = getForeground();if (drawable != null) {maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());}setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),resolveSizeAndState(maxHeight, heightMeasureSpec,childState << MEASURED_HEIGHT_STATE_SHIFT));count = mMatchParentChildren.size();if (count > 1) {for (int i = 0; i < count; i++) {final View child = mMatchParentChildren.get(i);final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();final int childWidthMeasureSpec;if (lp.width == LayoutParams.MATCH_PARENT) {final int width = Math.max(0, getMeasuredWidth()- getPaddingLeftWithForeground() - getPaddingRightWithForeground()- lp.leftMargin - lp.rightMargin);childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width, MeasureSpec.EXACTLY);} else {childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,getPaddingLeftWithForeground() + getPaddingRightWithForeground() +lp.leftMargin + lp.rightMargin,lp.width);}final int childHeightMeasureSpec;if (lp.height == LayoutParams.MATCH_PARENT) {final int height = Math.max(0, getMeasuredHeight()- getPaddingTopWithForeground() - getPaddingBottomWithForeground()- lp.topMargin - lp.bottomMargin);childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);} else {childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,getPaddingTopWithForeground() + getPaddingBottomWithForeground() +lp.topMargin + lp.bottomMargin,lp.height);}child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}}}

里面有个measureChildWithMargins方法

    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);}

这个时候发现ScrollView有重写这个方法

    @Overrideprotected 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);}

其中childHeightMeasureSpec的模式是MeasureSpec.UNSPECIFIED。这个表示尽可能的大，很少能用到。
而我们的ListView也会调用onMeaure方法

 @Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {// Sets up mListPaddingsuper.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 0heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);}setMeasuredDimension(widthSize, heightSize);mWidthMeasureSpec = widthMeasureSpec;}

其中的heightMeasureSpec也就是我们在ScrollView中child.measure(childWidthMeasureSpec, childHeightMeasureSpec);在传递过来的。所以也是模式为MeasureSpec.UNSPECIFIED。而当heightMode == MeasureSpec.UNSPECIFIED时会调用

 if (heightMode == MeasureSpec.UNSPECIFIED) {heightSize = mListPadding.top + mListPadding.bottom + childHeight +getVerticalFadingEdgeLength() * 2;}

这样在heightSize = mListPadding.top + mListPadding.bottom + childHeight +getVerticalFadingEdgeLength() * 2只是加了一个childHeight 所以展示的只有一个item的大小。
所以要解决这个问题应该让listView进入heightMode == MeasureSpec.AT_MOST里。

public class MyListView extends ListView
{public MyListView(Context context) {super(context);}public MyListView(Context context, AttributeSet attrs) {super(context, attrs);}public MyListView(Context context, AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);}@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {//解决显示不全的问题heightMeasureSpec=MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE>>2,MeasureSpec.AT_MOST);super.onMeasure(widthMeasureSpec, heightMeasureSpec);}
}

网上有的博客换成MyListView这么做就实现了，那么为什么呢？
首先当我们的模式为AT_MOST

      if (heightMode == MeasureSpec.AT_MOST) {// TODO: after first layout we should maybe start at the first visible position, not 0heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);}

调用ListView的measureHeightOfChildren

   @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P, trackingBug = 115609023)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 listint returnedHeight = mListPadding.top + mListPadding.bottom;final int dividerHeight = mDividerHeight;// The previous height value that was less than maxHeight and contained// no partial childrenint prevHeightWithoutPartialChild = 0;int i;View child;// mItemCount - 1 since endPosition parameter is inclusiveendPosition = (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 childreturnedHeight += dividerHeight;}// Recycle the view before we possibly return from the methodif (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 returnedHeightreturn returnedHeight;}

因为returnedHeight一直在累加，这样我们让maxHeight为最大值，这样他就不会进入if (returnedHeight >= maxHeight) ，而是返回returnedHeight。
onMeasure方法中的widthMeasureSpec和heightMeasureSpec会包含两个信息，第一个信息是模式：2位，第二个信息是值：30位。一共32位


而这里为什么Integer.MAX_VALUE 要右移两位

        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);}}

因为Integer.MAX_VALUE右移两位等于一个30位的值，这样执行makeMeasureSpec就可以构建成一个32位的值。这样去替换ListView的heightMeasureSpec即可。

    /*** A constant holding the maximum value an {@code int} can* have, 2<sup>31</sup>-1.*/@Native public static final int   MAX_VALUE = 0x7fffffff;