1. View的measure过程
measure是final的，但它里面调用了onMeasure方法
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
onMeasure(widthMeasureSpec, heightMeasureSpec);
}
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
// unspecified是getSuggestedMinimumWidth()的size
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
// 精确模式和最大模式的测量宽高都是MeasureSpec中的size
result = specSize;
break;
}
return result;
}
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
如果有背景则返回背景宽度，否则返回mMinWidth值，mMinWidth是android:minWidth的这个属性值所指定的值，没有指定则为0.而mBackground为drawable类型，获取背景宽度如下
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
有原始宽度，则返回原始宽度，否则返回0。那么Drawable在什么情况下有原始宽度？ShapeDrawable就没有原始宽度，BitmapDrawable就有原始宽度。
再看setMeasuredDimension方法
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
...
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
注意：这里mMeasuredWidth和mMeasuredHeight就是最终的测量宽高值
2. 一个问题
从上面的分析中可以知道，View的宽高由specSize决定，所以我们可以得出如下结论：直接继承View的自定义控件要重写onMeasure方法并设置wrap_content时的自身大小，否则在布局中使用wrap_content就相当于使用match_parent。
首先写个例子试一下：
public class DemoTextView extends View {
public DemoTextView(Context context, AttributeSet attrs) {
super(context, attrs);
setBackgroundColor(getResources().getColor(android.R.color.holo_blue_bright));
}
}
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="qingfengmy.developmentofart._4view.ViewActivity">
<qingfengmy.developmentofart._4view.DemoTextView
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</RelativeLayout>
问题确实存在，如果我们这样
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
setMeasuredDimension(200,200);
}
则View的宽高就是200。但TextView却是正常的，可见这些具体的View都已经重写在onMeasure方法，下面简单看下TextView的onMeasure代码
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
int width;
int height;
if (widthMode == MeasureSpec.EXACTLY) {
width = widthSize;
} else {
final Drawables dr = mDrawables;
if (dr != null) {
width = Math.max(width, dr.mDrawableWidthTop);
width = Math.max(width, dr.mDrawableWidthBottom);
}
width += getCompoundPaddingLeft() + getCompoundPaddingRight();
}
if (heightMode == MeasureSpec.EXACTLY) {
height = heightSize;
mDesiredHeightAtMeasure = -1;
} else {
if (heightMode == MeasureSpec.AT_MOST) {
height = Math.min(desired, heightSize);
}
}
int unpaddedHeight = height - getCompoundPaddingTop() - getCompoundPaddingBottom();
setMeasuredDimension(width, height);
}
3. ViewGroup的measure过程
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);
}
}
}
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);
}
ViewGroup没有定义其测量的具体过程，因为ViewGroup是一个抽象类，其测量过程的onMeasure方法需要各个子类去具体实现，这也是因为不同的ViewGroup子类有不同的布局特性，因此ViewGroup无法做统一实现。
4. LinearLayout的onMeasure方法
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
final int count = getVirtualChildCount();
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
for (int i = 0; i < count; ++i) {
// 遍历每个child
final View child = getVirtualChildAt(i);
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
} else {
int oldHeight = Integer.MIN_VALUE;
if (lp.height == 0 && lp.weight > 0) {
oldHeight = 0;
lp.height = LayoutParams.WRAP_CONTENT;
}
measureChildBeforeLayout(
child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
final int childHeight = child.getMeasuredHeight();
final int totalLength = mTotalLength;
// 每次都叠加新的高度
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
}
}
// 算LinearLayout自己的高度
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
}
如果它的布局中的高度采用的match_parent或者具体值，那么高度为specSize；如果它的布局中高度采用的wrap_content，那么它的高度是所有元素的高度总和，但不能超过父容器的剩余空间。
5. 几种获取View宽高的方式
- onWindowFocusChanged方法
当Activity的窗口在得到焦点和失去焦点时均会被调用一次。
@Override
public void onWindowFocusChanged(boolean hasFocus) {
super.onWindowFocusChanged(hasFocus);
if (hasFocus){
// 得到焦点
}
}
- view.post
延时获取 - ViewTreeObserver
当View树的状态发生或者改变View树内部的View可见性时，onGlobalLayout将会被调用。随着View树的状态改变，onGlobalLayout会多次调用。
ViewTreeObserver observer = view.getViewTreeObserver();
observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
}
});
- view.measure
手动通过measure方法来得到View的宽高，根据LayoutParams来分：- match_parent
直接放弃，因为这种需要知道parentSize，我们无法知道，所以直接放弃。 - 固定像素值
int widthMeasureSpec = View.MeasureSpec.makeMeasureSpec(100, View.MeasureSpec.EXACTLY); int heightMeasureSpec = View.MeasureSpec.makeMeasureSpec(100, View.MeasureSpec.EXACTLY); view.measure(widthMeasureSpec,heightMeasureSpec);- wrap_content
注意上面的(1<<30)-1，因为是最大化，所以宽高值应该是最大值，而specSize最大是30ge1，也就是(1<<30)-1.int widthMeasureSpec = View.MeasureSpec.makeMeasureSpec((1<<30)-1, View.MeasureSpec.AT_MOST); int heightMeasureSpec = View.MeasureSpec.makeMeasureSpec((1<<30)-1, View.MeasureSpec.AT_MOST); view.measure(widthMeasureSpec,heightMeasureSpec); - match_parent
