MeasureSpec 封装了父布局传递给子布局的布局要求，每个MeasureSpec代表了一组宽度和高度的要求
public class XfermodeView extends View {
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int wSize;
int wMode = MeasureSpec.getMode(widthMeasureSpec);
int widthMeasureSize = MeasureSpec.getSize(widthMeasureSpec);
if (wMode ==MeasureSpec.EXACTLY){
//精确模式，用其本身大小
wSize=widthMeasureSize;
}else {
WindowManager wm = (WindowManager) getContext().getSystemService(Context.WINDOW_SERVICE);
DisplayMetrics displayMetrics = new DisplayMetrics();
wm.getDefaultDisplay().getMetrics(displayMetrics);
int widthPixels = displayMetrics.widthPixels;
//手动设置为屏幕宽度的一半
wSize=widthPixels/2;
}
setMeasuredDimension(wSize,heightMeasureSpec);
}
}
3种模式
- UNSPECIFIED: 不对View大小做限制，ScrollView
- EXACTLY :确切的大小，如：200dp、match_parent
- AT_MOST : 大小不可超过父布局剩余大小，若wrap_content
子View 的 MeasureSpec是由父View的MeasureSpec 和自身的LayoutParams共同决定的
public abstract class ViewGroup extends View implements ViewParent, ViewManager {
/**
* Ask all of the children of this view to measure themselves, taking into
* account both the MeasureSpec requirements for this view and its padding.
* We skip children that are in the GONE state The heavy lifting is done in
* getChildMeasureSpec.
*
* @param widthMeasureSpec The width requirements for this view
* @param heightMeasureSpec The height requirements for this view
*/
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) {
//跳过View Visiable是gone
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding.
* The heavy lifting is done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param parentHeightMeasureSpec The height requirements for this view
*/
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
//子View的MeasureSpec 是子View自身的LayoutParams 和父View的MeasureSpec 组成
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
//ViewGroup.java
/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results. For example,
* if the this view knows its size (because its MeasureSpec has a mode of
* EXACTLY), and the child has indicated in its LayoutParams that it wants
* to be the same size as the parent, the parent should ask the child to
* layout given an exact size.
*
* @param spec The requirements for this view
* @param padding The padding of this view for the current dimension and
* margins, if applicable
* @param childDimension How big the child wants to be in the current
* dimension
* @return a MeasureSpec integer for the child
*/
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
//父View布局剩余空间大小
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
// childDimension 是子view的LayoutParams具体的宽高
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY://父View有准确的dp值或者match_parent
if (childDimension >= 0) {//子View 的childDimension大于0，则子View的大小为其本身且mode是EXACTLY
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
//子View的大小为父View布局剩余空间大小
// 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.
//子View的大小为父View布局剩余空间大小
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST://父View是wrap_content
if (childDimension >= 0) {
// Child wants a specific size... so be it
//子View的大小为其本身
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.
//子View的大小为父View布局剩余空间大小
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.
//子View的大小为父View布局剩余空间大小
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
//子View大小为其本身大小
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
//子View大小为0
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
//子View大小为0
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
参考地址：
MeasureSpec详解
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