在自定義view中多半都會去重寫onMeasure方法，進行view的測量，測量出大小後，再在onDraw方法中進行繪製，下面是一段簡易的自定義view的代碼：
public class MyTextView extends View {
//在new一個MyTextView對象的時候會調用
public MyTextView(Context context) {
this(context,null);
}
//在xml佈局文件中使用MyTextView會調用
public MyTextView(Context context, AttributeSet attrs) {
this(context, attrs,0);
}
//在xml佈局文件中使用MyTextView並給MyTextView設置style樣式會調用
public MyTextView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
//獲取寬高模式
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode=MeasureSpec.getMode(heightMeasureSpec);
//獲取寬高大小
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
}
}
通過MeasureSpec.getMode()可以獲取到寬高模式，系統提供了下面三個模式的常量值：
//在xml佈局中設置爲wrap_content
MeasureSpec.AT_MOST;
//在xml佈局中設置爲具體的值比如100dp或者match_parent或者fill_parent
MeasureSpec.EXACTLY;
//在實際開發中很少用到，ScrollView、ListView等源碼中有使用到
MeasureSpec.UNSPECIFIED;
在項目開發中有時候會用到ScrollView和ListView的嵌套（當然現在很少用到了），就會碰到ListView顯示條目不全的問題，其實就是ScrollView在測量時將ListView的測量模式設置爲MeasureSpec.UNSPECIFIED，ListView在測量時的判斷所導致的；ScrollView是一個佈局容器，肯定是繼承自ViewGoup的，在ViewGroup中會發現measureChild()方法，該方法是用來測量佈局容器中子view的方法，在ViewGoup中的measureChild()方法中並沒有指定子view的測量模式，
//這個是ViewGroup中的measureChild方法源碼
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
//調用view中的measure方法去測量子view
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
ViewGroup中沒有做任何動作，但是ScrollView中重寫了ViewGroup中的measureChild()方法，
//這是ScrollView中measureChild的源碼，
@Override
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
ViewGroup.LayoutParams lp = child.getLayoutParams();
int childWidthMeasureSpec;
int childHeightMeasureSpec;
//在這裏指定了子view的height mode 爲MeasureSpec.UNSPECIFIED
childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft
+ mPaddingRight, lp.width);
final int verticalPadding = mPaddingTop + mPaddingBottom;
childHeightMeasureSpec = MeasureSpec.makeSafeMeasureSpec(
Math.max(0, MeasureSpec.getSize(parentHeightMeasureSpec) - verticalPadding),
MeasureSpec.UNSPECIFIED);
//進行子view的測量
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
在這裏首先要明白的是childMeasure(childeWidthMeasureSpec,childHeightMeasureSpec);中的兩個參數childeWidthMeasureSpec和childHeightMeasureSpec；childeWidthMeasureSpec和childHeightMeasureSpec它是包含兩部分的，前兩位是mode，後30爲是值（size）；接下來就會調用view中的measure方法及onMeasure()方法，並把寬高值和寬高模式傳入；但是ListView的話將View中的onMeasure方法進行了重寫；
//這裏是ListView中onMeasure方法源碼
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// Sets up mListPadding
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
//獲取寬高模式
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
//獲取寬高大小
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);
}
}
//在這裏對寬度模式進行了判斷
if (widthMode == MeasureSpec.UNSPECIFIED) {
widthSize = mListPadding.left + mListPadding.right + childWidth +
getVerticalScrollbarWidth();
} else {
widthSize |= (childState & MEASURED_STATE_MASK);
}
//在這裏對高度模式進行了判斷
if (heightMode == MeasureSpec.UNSPECIFIED) {
//如果高度的模式是MeasureSpec.UNSPECIFIED 計算的heightSize大小就是top+bottom+單個item條目高度(childHeight)+分割線的高度
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
//如果高度模式是MeasureSpec.AT_MOST就會去計算所有item條目的高度，並賦值個heightSize
heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}
//設置計算好的寬高
setMeasuredDimension(widthSize, heightSize);
mWidthMeasureSpec = widthMeasureSpec;
}
在ScrollView和ListView嵌套的時候，ScrollView給ListView高度模式設置的是MeasureSpec.UNSPECIFIED，同時ListView又對onMeasure方法進行了重寫，所以就出現了ScrollView嵌套ListView條目顯示不全的問題，其實只需將ListView的高度模式設置爲MeasureSpec.AT_MOST就可以去測量計算所有item的高度了；
public class MyListView extends ListView {
public MyListView(Context context) {
this(context,null);
}
public MyListView(Context context, AttributeSet attrs) {
this(context, attrs,0);
}
public MyListView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//指定ListView的高度模式爲MeasureSpecAT_MOST 並指定大小爲Integer最大值右移兩位
heightMeasureSpec=MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE>>2,MeasureSpec.AT_MOST);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
}
這樣子問題就解決了，寬高模式設置爲MeasureSpec.AT_MOST容易理解，大小設置爲Integer的最大值右移兩位；
//這裏是ListView中測量所有item高度的源碼 maxHeight就是heightSize
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
//定義的返回height變量
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();
//maxHeight值是Integer.MAX_VALUE>>2爲，所有returnedHeight的值是永遠小於maxHeight的，這個if條件是永遠不成立的，這樣就可以返回returnedHeight計算出來的值，也就是設置height大小爲Integer.MAX_VALUE>>2的原因
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;
}
這裏涉及到java中的左移，右移運算符；
<< : 左移運算符，num << 1,相當於num乘以2
>> : 右移運算符，num >> 1,相當於num除以2
>>> : 無符號右移，忽略符號位，空位都以0補齊