layout的作用是ViewGroup用来确定子元素的位置，在这个过程中会用到两个核心方法:layout()和onLayout。layout方法的作用是确定View本身的位置，而onLayout方法则会确定所有子元素的位置。当ViewGroup的位置确定后，它在onLayout中会遍历所有的子元素并调用其layout方法，在layout方法中onLayout方法又会被调用。下面是View的layout源码
@SuppressWarnings({"unchecked"})
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
从源码可以看出这个方法的大致流程:先通过setFrame()方法来设置View的四个元素的位置，即初始化mLeft、mTop、mRight、mBottom这四个值。View的四个顶点确定了，View在父容器中的位置也就确定了；接着会调用onLayout()方法，这个方法的用途是父容器确定子元素的位置。
因为不同的ViewGroup子类的布局方法不同，所以ViewGroup中没有实现onLayout()方法。下面是LinearLayout中的onLayout()方法源码。
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r, b);
} else {
layoutHorizontal(l, t, r, b);
}
}
追踪查看layoutVertical()方法源码
void layoutVertical(int left, int top, int right, int bottom) {
final int paddingLeft = mPaddingLeft;
int childTop;
int childLeft;
// Where right end of child should go
final int width = right - left;
int childRight = width - mPaddingRight;
// Space available for child
int childSpace = width - paddingLeft - mPaddingRight;
final int count = getVirtualChildCount();
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
switch (majorGravity) {
case Gravity.BOTTOM:
// mTotalLength contains the padding already
childTop = mPaddingTop + bottom - top - mTotalLength;
break;
// mTotalLength contains the padding already
case Gravity.CENTER_VERTICAL:
childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
break;
case Gravity.TOP:
default:
childTop = mPaddingTop;
break;
}
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
int gravity = lp.gravity;
if (gravity < 0) {
gravity = minorGravity;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp.rightMargin;
break;
case Gravity.LEFT:
default:
childLeft = paddingLeft + lp.leftMargin;
break;
}
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
可以看出，此方法会遍历所以子元素并调用setChildFrame()方法来为子元素指定对应的位置。而setChildFrame方法，仅仅是调用子元素的layout方法而已，这样父元素在layout方法中完成自己的定位以后，就通过onLayout方法去调用子元素的layout方法，子元素又会通过自己的layout方法来确定自己的位置，就这样一层一层的传递下去就完成了整个View树的layout过程。下面是setChildFrame源码
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}
setChildFrame中的width和height实际上就是子元素的测量宽高。
那么View的测量宽高和最终宽高有什么区别？
这个问题的答案其实也就是View的getMeasuredWidth()和getWidth()两个方法的区别，getMeasuredHeight();和getHeight()区别和前两者一样。
@ViewDebug.ExportedProperty(category = "layout")
public final int getWidth() {
return mRight - mLeft;
}
@ViewDebug.ExportedProperty(category = "layout")
public final int getHeight() {
return mBottom - mTop;
}
从getWidth和getHeight的源码再结合mLeft、mRight、mTop、mBottom这四个变量赋值过程来看，getWidth和getHeight返回的值刚好就是View的测量宽高。
所以上面问题的答案是:在View的默认实现中，View的测量宽高和最终宽高是相等的，区别在于测量宽高形成与View的measure过程，而最终宽高形成于View的layout过程，即两者的赋值时机不同。
宽高不相等的反例:
@Override
public void layout(int l, int t, int r, int b) {
super.layout(l, t, r+100, b+100);
}
重写View的layout方法，并且修改宽高。这样最终宽高和测量宽高就不相等。
参考:《android开发艺术探索》
