Android-实例理解View measure过程

这里有两个自定义view，HorizontalScrollViewEx（作用类似水平方向的viewpager）,CircleView（简单的画一个圆）。

代码如下：

public class HorizontalScrollViewEx extends ViewGroup {
private static final String TAG = "HorizontalScrollViewEx";

private int mChildrenSize;
private int mChildWidth;
private int mChildIndex;

//     分别记录上次滑动的坐标
private int mLastX = 0;
private int mLastY = 0;
// 分别记录上次滑动的坐标(onInterceptTouchEvent)
private int mLastXIntercept = 0;
private int mLastYIntercept = 0;

private Scroller mScroller;
private VelocityTracker mVelocityTracker;

public HorizontalScrollViewEx(Context context) {
super(context);
init();
}

public HorizontalScrollViewEx(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}

public HorizontalScrollViewEx(Context context, AttributeSet attrs,
int defStyle) {
super(context, attrs, defStyle);
init();
}

private void init() {
mScroller = new Scroller(getContext());
mVelocityTracker = VelocityTracker.obtain();
}

@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
boolean intercepted = false;
int x = (int) event.getX();
int y = (int) event.getY();

switch (event.getAction()) {
case MotionEvent.ACTION_DOWN: {
intercepted = false;
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
intercepted = true;
}
break;
}
case MotionEvent.ACTION_MOVE: {
int deltaX = x - mLastXIntercept;
int deltaY = y - mLastYIntercept;
if (Math.abs(deltaX) > Math.abs(deltaY)) {
intercepted = true;
} else {
intercepted = false;
}
break;
}
case MotionEvent.ACTION_UP: {
intercepted = false;
break;
}
default:
break;
}

Log.d(TAG, "intercepted=" + intercepted);
mLastX = x;
mLastY = y;
mLastXIntercept = x;
mLastYIntercept = y;

return intercepted;
}

@Override
public boolean onTouchEvent(MotionEvent event) {
mVelocityTracker.addMovement(event);
int x = (int) event.getX();
int y = (int) event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN: {
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
}
break;
}
case MotionEvent.ACTION_MOVE: {
int deltaX = x - mLastX;
int deltaY = y - mLastY;
scrollBy(-deltaX, 0);
break;
}
case MotionEvent.ACTION_UP: {
int scrollX = getScrollX();
int scrollToChildIndex = scrollX / mChildWidth;
mVelocityTracker.computeCurrentVelocity(1000);
float xVelocity = mVelocityTracker.getXVelocity();
if (Math.abs(xVelocity) >= 50) {
mChildIndex = xVelocity > 0 ? mChildIndex - 1 : mChildIndex + 1;
} else {
mChildIndex = (scrollX + mChildWidth / 2) / mChildWidth;
}
mChildIndex = Math.max(0, Math.min(mChildIndex, mChildrenSize - 1));
int dx = mChildIndex * mChildWidth - scrollX;
smoothScrollBy(dx, 0);
mVelocityTracker.clear();
break;
}
default:
break;
}

mLastX = x;
mLastY = y;
return true;
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int measuredWidth = 0;
int measuredHeight = 0;
final int childCount = getChildCount();
Log.e(TAG, "width:" + MeasureSpec.getSize(widthMeasureSpec));
//测量子view的大小
measureChildren(widthMeasureSpec, heightMeasureSpec);

int widthSpaceSize = MeasureSpec.getSize(widthMeasureSpec);
int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int heightSpaceSize = MeasureSpec.getSize(heightMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
if (childCount == 0) {
setMeasuredDimension(0, 0);
} else if (heightSpecMode == MeasureSpec.AT_MOST) {
Log.e(TAG, "heightMode:"+heightSpecMode);
final View childView = getChildAt(0);

//TODO layout_height为wrap_content ==> specMode为AT_MOST,将使用childView.getMeasuredHeight()作为其高度

setMeasuredDimension(300, childView.getMeasuredHeight());
} else if (widthSpecMode == MeasureSpec.AT_MOST) {
Log.e(TAG, "widthMode:"+widthSpecMode);
final View childView = getChildAt(0);
measuredWidth = childView.getMeasuredWidth() * childCount;
setMeasuredDimension(measuredWidth, heightSpaceSize);
} else {
Log.e(TAG, "heightMode:"+heightSpecMode);
Log.e(TAG, "widthMode:"+widthSpecMode);
final View childView = getChildAt(0);
measuredWidth = childView.getMeasuredWidth() * childCount;
measuredHeight = childView.getMeasuredHeight();
setMeasuredDimension(measuredWidth, measuredHeight);
}
}

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
int childLeft = 0;
final int childCount = getChildCount();
mChildrenSize = childCount;

for (int i = 0; i < childCount; i++) {
final View childView = getChildAt(i);
if (childView.getVisibility() != View.GONE) {
final int childWidth = childView.getMeasuredWidth();
mChildWidth = childWidth;
childView.layout(childLeft, 0, childLeft + childWidth,
childView.getMeasuredHeight());
childLeft += childWidth;
}
}
}

private void smoothScrollBy(int dx, int dy) {
mScroller.startScroll(getScrollX(), 0, dx, 0, 500);
invalidate();
}

@Override
public void computeScroll() {
if (mScroller.computeScrollOffset()) {
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
postInvalidate();
}
}

@Override
protected void onDetachedFromWindow() {
mVelocityTracker.recycle();
super.onDetachedFromWindow();
}
}

public class CircleView extends View {

private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
private final int mCustomWidth = 400;
private final int mCustomHeight = 400;

public CircleView(Context context) {
super(context);

mPaint.setColor(Color.RED);
}

public CircleView(Context context, AttributeSet attrs) {
super(context, attrs);

mPaint.setColor(Color.RED);
}

public CircleView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
mPaint.setColor(Color.RED);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);

int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);

//宽高都是AT_MOST
if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST)
{
setMeasuredDimension(mCustomWidth, mCustomHeight);
}
else if (widthSpecMode == MeasureSpec.AT_MOST)
{
setMeasuredDimension(mCustomWidth, heightSpecSize);
}
else if (heightSpecMode == MeasureSpec.AT_MOST)
{
setMeasuredDimension(widthSpecSize, mCustomHeight);
}

}

@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);

int width = getWidth();
int height = getHeight();

int radius = Math.min(width, height) / 2;

canvas.drawCircle(width / 2, height / 2, radius, mPaint);
}
}

布局文件：activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">

<com.dhn.customview.HorizontalScrollViewEx
android:layout_width="wrap_content"
android:layout_height="wrap_content">

<com.dhn.customview.CircleView
android:layout_width="100dp"
android:layout_height="match_parent" />

<TextView
android:layout_width="100dp"
android:layout_height="match_parent"
android:background="#AAff0000"
android:text="t1" />

<TextView
android:layout_width="100dp"
android:layout_height="match_parent"
android:background="#AAff00"
android:text="t1" />

</com.dhn.customview.HorizontalScrollViewEx>

</RelativeLayout>

问题：

布局文件中父View的高为wrap_content，子View为match_parent。这时它们的高度该如何确定呢？

因为HorizontalScrollViewEx的高设置为wrap_content所以它的高的SpecMode为AT_MOST（它的parent view和自身的LayoutParam在getChildMeasureSpec中计算得出），我们在自定义的onMeasure（）方法中将它的高设置为第一个子View的测量高度(第146行)，那么第一个子View的childView.getMeasuredHeight()为多少呢。子View的测量在132行measureChildren(widthMeasureSpec, heightMeasureSpec)中进行，进入该方法：

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}

第7行对第一个子View调用measureChild(child, widthMeasureSpec, heightMeasureSpec)

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);

child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

这里的getChildMeasureSpec(parentWidthMeasureSpec,mPaddingLeft + mPaddingRight, lp.width);返回第一个子View的MeasureSpec，该方法如下：

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);

int size = Math.max(0, specSize - padding);

int resultSize = 0;
int resultMode = 0;

switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;

// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;

// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

子View的MeasureSpec由父View的MeasureSpec和自身的LayoutParams 共同决定。这里父view高的MeasureSpec.Mode为AT_MOST，子View高的LayoutParams 为math_parent所以子View高的MeasureSpec也为AT_MOST。

接下来第10行调用第一个子View的measure()方法：

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int oWidth  = insets.left + insets.right;
int oHeight = insets.top  + insets.bottom;
widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}

// Suppress sign extension for the low bytes
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);

final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
final boolean isExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY &&
MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
final boolean matchingSize = isExactly &&
getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec) &&
getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
if (forceLayout || !matchingSize &&
(widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec)) {

// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

resolveRtlPropertiesIfNeeded();

int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}

// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
throw new IllegalStateException("onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}

mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}

mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;

mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}

第33行调用onMeasure(widthMeasureSpec, heightMeasureSpec);对于CircleView我们自定义了该方法：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);

int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);

//宽高都是AT_MOST
if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST)
{
setMeasuredDimension(mCustomWidth, mCustomHeight);
}
else if (widthSpecMode == MeasureSpec.AT_MOST)
{
setMeasuredDimension(mCustomWidth, heightSpecSize);
}
else if (heightSpecMode == MeasureSpec.AT_MOST)
{
setMeasuredDimension(widthSpecSize, mCustomHeight);
}

}

在这里为高设置了我们自定义的值mCustomHeight=400,最终circleView的测量高度为400。

这样HorizontalScrollViewEx的高度也就确定为400。

结论：

自定义Veiw是需要在onMeasure方法中处理AT_MOST这种情况。像TextView的onMeasrue实现中，都对AT_MOST进行了处理。这样当出现parent view为wrap_content，child view为math_parent时才可以确定大小。