場景
在某些場合使用ScrollView去嵌套ListView的時候，ListView會只顯示第一個item。
one_item.png
解決辦法網上有很多，這篇文章主要從源碼角度分析下這個現象產生的原因和解決辦法的原理。
分析
先看下ScrollView關系圖：
ScrollView繼承關系圖.png
ScrollView繼承自FrameLayout且在onMeasure()調用了父類的onMeasure方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
//調用父類FrameLayout的onMeasure方法
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
//mFillViewport的默認值為false，所以默認以下方法會return
if (!mFillViewport) {
return;
}
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
if (heightMode == MeasureSpec.UNSPECIFIED) {
return;
}
if (getChildCount() > 0) {
final View child = getChildAt(0);
int height = getMeasuredHeight();
if (child.getMeasuredHeight() < height) {
final FrameLayout.LayoutParams lp = (LayoutParams) child.getLayoutParams();
int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
height -= mPaddingTop;
height -= mPaddingBottom;
int childHeightMeasureSpec =
MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
ScrollView首先會調用FrameLayout的onMeasure()方法，當為mFillViewport為true時后續代碼才會繼續執行，會根據子View的高度和ScrollView本身的高度決定是否重新measure子View使其充滿ScrollView。而ScrollView的mFillViewport默認值為false，所以默認情況下ScrollView的onMeasure()基本類似于父類的FrameLayout。
再來追蹤下FrameLayout的onMeasure方法：
@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);
}
}
}
}
// Account for padding too
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
final 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();
int childWidthMeasureSpec;
int childHeightMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
lp.leftMargin - lp.rightMargin,
MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
if (lp.height == LayoutParams.MATCH_PARENT) {
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
lp.topMargin - lp.bottomMargin,
MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
FrameLayout的onMeasure()方法有點長，影響ListView的measure的代碼主要在for循環內的代碼。
//measure Child的Margins
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
繼續追蹤下measureChildWithMargins()方法：
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding
* and margins. The child must have MarginLayoutParams The heavy lifting is
* done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param widthUsed Extra space that has been used up by the parent
* horizontally (possibly by other children of the parent)
* @param parentHeightMeasureSpec The height requirements for this view
* @param heightUsed Extra space that has been used up by the parent
* vertically (possibly by other children of the parent)
*/
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);
}
在對childView進行measure時會根據childView的child的margin值和heightUsed重新計算childHeightMeasureSpec ，來完成對child的measure。這個就是ViewGroup對childView的measure。而在ScrollView中重寫了measureChildWithMargins()方法，measure方法有點不同。
@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);
//改變childView的measureMode
final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
lp.topMargin + lp.bottomMargin, MeasureSpec.UNSPECIFIED);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
從代碼中可以清晰的看到，不管childView是什么模式的measureMode，都會被外層包裹的ScrollView改變成MeasureSpec.UNSPECIFIED。而這就是導致我們嵌套的ListView為什么只能展示1個item的源頭。
接著分析下ListView的onMeasure()方法：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// Sets up mListPadding
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
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);
measureScrapChild(child, 0, widthMeasureSpec);
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;
}
從上述的分析得知這里的heightMeasureSpec里面的measureMode 已經被ScrollView強制改為了MeasureSpec.UNSPECIFIED，
所以這里的heightMode = MeasureSpec.UNSPECIFIED。
就會這些下面這個if語句
if (heightMode == MeasureSpec.UNSPECIFIED) {
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;
}
所以我們的嵌套ListView的height就是ListView在垂直高度上的padding和child（也就是我們的item）的高度。
大致流程如下：
ScrollView嵌套ListView.png
解決辦法
1.自定義ListView填充全部的item
網上一種解決辦法是自定義一個ListView去繼承原生的ListView，并復寫onMeasure()方法。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2,
MeasureSpec.AT_MOST);
super.onMeasure(widthMeasureSpec, expandSpec);
}
從代碼中可以看出，主要是手動改變ListView的heightMeasureSpec。從上面ListView的onMeasure()源碼中，可以看到ListView除了對 MeasureSpec.UNSPECIFIED 這種情況外還對 MeasureSpec.AT_MOST 做了特殊處理。而這個自定義的ListView便是將ListView的heightMeasureSpecMode強制改為 MeasureSpec.AT_MOST ，并把 int 的最大值作為heightMeasureSpecSize。
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);
}
再來跟進下ListView中的measureHeightOfChildren()方法。截取其中一段代碼
for (i = startPosition; i <= endPosition; ++i) {
child = obtainView(i, isScrap);
measureScrapChild(child, i, widthMeasureSpec);
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;
}
ListView會遍歷所有的item，returnedHeight 會累加所有的 dividerHeight 和 child（也就是item）的高度。如果 returnedHeight >= maxHeight 就會跳出循環，而這個 maxHeight 就是我們自定義ListView中的onMeasure()方法中傳入的 heightMeasureSpec的heightMeasureSpecSize（int的最大值）。所以這個for循環就能順利遍歷完全部的item。運行效果就如下：
展示全部item.png
2.添加ScrollView屬性
在本文最前面曾提到過,ScrollView中有個mFillViewpor的變量。當為true時才會繼續執行ScrollView自己的measure方法，會根據子View的高度和ScrollView本身的高度決定是否重新measure子View使其充滿ScrollView。有以下幾種方法可以設置該值。
/**
* Indicates this ScrollView whether it should stretch its content height to fill
* the viewport or not.
*
* @param fillViewport True to stretch the content's height to the viewport's
* boundaries, false otherwise.
*
* @attr ref android.R.styleable#ScrollView_fillViewport
*/
public void setFillViewport(boolean fillViewport) {
if (fillViewport != mFillViewport) {
mFillViewport = fillViewport;
requestLayout();
}
}
以及在layout文件中設置屬性
/**
* When set to true, the scroll view measure its child to make it fill the currently
* visible area.
*/
@ViewDebug.ExportedProperty(category = "layout")
private boolean mFillViewport;
只需要給ScrillView在Layout文件中添加屬性 android:fillViewport="true" ，或者在java文件中設置 mScrollView.setFillViewport(true); 。
3.事件分發
一般情況下不會在ScrollView中直接去嵌套一個ListView。如果在ScrollView中嵌套一個LinearLayout去實現一些業務布局，LinearLayout里面再去嵌套一個ListView，我們可以去手動處理其中的事件分發了。如果是類似于一下布局形式的需要注意一些設置了。
布局.png
這里分2種情況：1是ListView擴充全部item，2是ListView顯示一定的高度，仍然處于平時的滑動狀態。
第一種情況的做法已經在上述說明過了，這里講解下第二種情況。
在這里需要注意的是ListView的高度不能使用match_parent屬性來填充LinearLayout的剩余高度。ScrollView會將LinearLayout的heightMeasureSpecMode改變為MeasureSpec.UNSPECIFIED。如果ListView沒有設置精確的dp的話，heightMeasureSpecMode會遵循parentSize也就是MeasureSpec.UNSPECIFIED。
遵循規則如下
measure.png
