姓名:李昕洲 学号:16030120026
转载自:http://blog.csdn.net/gongxiaoou/article/details/78806188
【嵌牛导读】:大家或多或少了解过View,本文将为你揭晓View的工作原理之layout过程。
【嵌牛鼻子】:View树、layout过程。
【嵌牛提问】:View树从上到下的布局过程如何？ getMeasuredWidth和getWidth的本质区别是什么？
【嵌牛正文】:
layout和onLayout方法的作用
layout用来确定View自己的位置，onLayout用来确定各个子View的位置
在View类中只有layout的实现，没有onLayout的实现，因为不同的实现类有不同特殊情况。
如下为View类中的onLayout
view sourceprint?
/**
*布置子类
* Called from layout when this view should
* assign a size and position to each of its children.
*
* Derived classes with children should override
* this method and call layout on each of
* their children.
* @param changed This is a new size or position for this view
* 相对于父控件的左，上，右，下值
* @param left Left position, relative to parent
* @param top Top position, relative to parent
* @param right Right position, relative to parent
* @param bottom Bottom position, relative to parent
*/
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
下面是layout方法，这里我们只关心我们要的代码其他的省略。
@SuppressWarnings({"unchecked"})
public void layout(int l, int t, int r, int b) {
...省略代码...
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);
...省略代码...
}
}
layout流程大致如下：首先通过setFrame设置View的四个顶点在父View的位置，那么此View的位置就确定了；然后调用onLayout方法确定各个子View的位置。
下面是setFrame方法（看注释部分即可）：
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
//位置发生了变化
changed = true;
// Remember our drawn bit
int drawn = mPrivateFlags & PFLAG_DRAWN;
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
//判断尺寸是否发生了变化
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
// Invalidate our old position
invalidate(sizeChanged);
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
mPrivateFlags |= PFLAG_HAS_BOUNDS;
if (sizeChanged) {
//改变尺寸，但是我们发现它的具体操作也是交给了子类根据具体情况实现
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
// If we are visible, force the DRAWN bit to on so that
// this invalidate will go through (at least to our parent).
// This is because someone may have invalidated this view
// before this call to setFrame came in, thereby clearing
// the DRAWN bit.
mPrivateFlags |= PFLAG_DRAWN;
invalidate(sizeChanged);
// parent display list may need to be recreated based on a change in the bounds
// of any child
invalidateParentCaches();
}
// Reset drawn bit to original value (invalidate turns it off)
mPrivateFlags |= drawn;
mBackgroundSizeChanged = true;
if (mForegroundInfo != null) {
mForegroundInfo.mBoundsChanged = true;
}
notifySubtreeAccessibilityStateChangedIfNeeded();
}
return changed;
}
// Remember our drawn bit
int drawn = mPrivateFlags & PFLAG_DRAWN;
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
//判断尺寸是否发生了变化
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
// Invalidate our old position
invalidate(sizeChanged);
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
mPrivateFlags |= PFLAG_HAS_BOUNDS;
if (sizeChanged) {
//改变尺寸，但是我们发现它的具体操作也是交给了子类根据具体情况实现
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
// If we are visible, force the DRAWN bit to on so that
// this invalidate will go through (at least to our parent).
// This is because someone may have invalidated this view
// before this call to setFrame came in, thereby clearing
// the DRAWN bit.
mPrivateFlags |= PFLAG_DRAWN;
invalidate(sizeChanged);
// parent display list may need to be recreated based on a change in the bounds
// of any child
invalidateParentCaches();
}
// Reset drawn bit to original value (invalidate turns it off)
mPrivateFlags |= drawn;
mBackgroundSizeChanged = true;
if (mForegroundInfo != null) {
mForegroundInfo.mBoundsChanged = true;
}
notifySubtreeAccessibilityStateChangedIfNeeded();
}
return changed;
}
下面以LinearLayout为例子来分析onLayout方法。
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);
}
}
这里我们以mOrientation == VERTICAL为例分析。
void layoutVertical(int left, int top, int right, int bottom) {
final int paddingLeft = mPaddingLeft;
int childTop;
int childLeft;
// 父View的宽度
final int width = right - left;
//得到所有子View的最右边界
int childRight = width - mPaddingRight;
//所有子View占用的横向空间
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;
//根据子View在父View中的Gravity（上，下，左，右，中）来计算子所有View的上边界
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;
}
//重点
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
//获取单个子View的测试宽高
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);
//获取每个子View的左边界
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;
}
//逐个累计各个子View的竖直方向占用空间为布置下一个子View做准备
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
//最终让每个子View各自完成自己的layout
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
下面是setChildFrame方法，其实就是让每个子View完成自己的layout。
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}
上面基本就将整个View树的layout展示了一下。
下面我们来解释getMeasuredWidth和getWidth的本质区别（高度方向原理一样）
先看layoutHorizontal—->setChildFrame
void layoutHorizontal(int left, int top, int right, int bottom) {
...省略代码...
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
...省略代码...
}
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}
从中我们发现父View给子View布置的宽高（childWidth, childHeight）就是它的测量宽高getMeasuredWidth()，getMeasuredHeight()。
再看layout和setFrame方法和getWidth和getHeight
public void layout(int l, int t, int r, int b) {
...省略代码...
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
...省略代码...
}
protected boolean setFrame(int left, int top, int right, int bottom) {
...省略代码...
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
...省略代码...
}
public final int getWidth() {
return mRight - mLeft;
}
public final int getHeight() {
return mBottom - mTop;
}
从上面可以看出getMeasuredWidth和getWidth其实值是一样的，只是获取的时间点不同，measuredWidth（测量宽度）形成于View的measure过程中，而View的width（真实宽度）形成于layout过程中。
补充说明：我们可以撑的没事重写layout如下,这会造成无法正常显示等错误，这只是为了证明可以让测量宽/高度不等于最终宽/高度。
public void layout(int l, int t, int r, int b) {
super.layout(l, t, r+10, b+10);
}
而且有的View需要多次measure过程，那么在这个过程中测量宽/高度不等于最终宽/高度，但是最终测量宽/高度等于最终宽/高度public void layout(int l, int t, int r, int b) {
super.layout(l, t, r+10, b+10);
}
而且有的View需要多次measure过程，那么在这个过程中测量宽/高度不等于最终宽/高度，但是最终测量宽/高度等于最终宽/高度
