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导航：Android Measure流程(一)juejin.cn/post/716850…
Measure中的Spec参数从何而来
当然是debug了
直接de
从上图的调用栈可以看出由performMeasure调用的DecorView的measure->onMeasure DercorView的onMeasure主要是计算measureSpec，由于DercorView继承了FrameLayout,计算好measureSpec后调用了super.onMeasure,也就是FrameLayout的onMeasure。
我们看一下FrameLayout的onMeasure
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
//清除mMatchParentChildren数组
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//mMeasureAllChildren意思是是否考虑全部子节点默认为false即只考虑可见子节点
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//这里是关键，也正是这里调用到了LinearLayout.measure(由于没有重写measure所以我们看LinearLayout.onMeasure)
//measureChildWithMargins中的参数：heigth/wiethUsed意思是这一部分被被当前layout占用，不允许childView占用，可以看作padding
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
//设置自己的测量结果
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
//如果parent的长度和或宽度都不是确定的，同时如果child存在MATCH_PARENT，则会重新测量。因为当前layout的长度或者宽度可能用传进来的measureSpec确定了，比如上面的child都指定了自己的大小，那么当前layout的宽度和高度就确定了。
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
也就是说如果framelayout的直接子view不对自己warp_parent作限制的话，而且framlayout也是warp_parent,那么该view会填充整个framlayout，那么framlayout的测量结果也会和parent一样。
几个很重要的measure函数
-
viewGroup中的measureChildWithMargins (View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed)
具体实现：
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
作用：使参数中的child测量自己，如同函数名称一样，同时需要考虑到parent的填充和child的边距还有width/heightUsed（parent需要额外占据的大小）。
但是关于这些繁重的操作其实是内部调用了下面这个函数，由它来完成。
-
viewGroup中的getChildMeasureSpec(int spec, int padding, int childDimension)
具体实现：
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//解析parent的spec
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
//减去不能占用的大小，同时限制大小不能是负数
int size = Math.max(0, specSize - padding);
//初始化测量
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// parent的大小已经指定
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {//如果child也已经指定大小（因为warp(match)_parent是负数）
resultSize = childDimension;//那么就直接设置为自己的大小
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
resultSize = size;//和parent的可用宽度一样大
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = size;//child不确定自己的大小，由于不会超过parent的可用大小，则先指定为size
resultMode = MeasureSpec.AT_MOST;
}
break;
// parent未指定大小
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {//child大小已经确定
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
.
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未指定大小
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let them 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 = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//返回测量后的Spec,这个函数的作用就是把seize和mode转换成spec
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
从调用栈可知，measure是从viewroootimpl开始的， 下文我们再关注一下viewroootimpl
