View.java

/**

*

*This is called to find out how big a view should be. The parent

*supplies constraint information in the width and height parameters.

*

*

*The actual mesurement work of a view is performed in

*{@link #onMeasure(int, int)},called by this method. Therefore, only

*{@link #onMeasure(int, int)}can and must be overriden by subclasses.

*

*@paramwidthMeasureSpec Horizontal space requirements as imposed by the

* parent

*@paramheightMeasureSpec Vertical space requirements as imposed by the

* parent

*@see#onMeasure(int, int)

publicfinalvoidmeasure(intwidthMeasureSpec, intheightMeasureSpec) {

if((mPrivateFlags& FORCE_LAYOUT)== FORCE_LAYOUT||

widthMeasureSpec !=mOldWidthMeasureSpec||

heightMeasureSpec !=mOldHeightMeasureSpec){

//first clears the measured dimension flag

mPrivateFlags&= ~MEASURED_DIMENSION_SET;

if(ViewDebug.TRACE_HIERARCHY){

ViewDebug.trace(this,ViewDebug.HierarchyTraceType.ON_MEASURE);

}

//measure ourselves, this should set the measured dimension flag back

onMeasure(widthMeasureSpec,heightMeasureSpec); //重点是它

//flag not set, setMeasuredDimension() was not invoked, we raise

//an exception to warn the developer

if((mPrivateFlags& MEASURED_DIMENSION_SET)!= MEASURED_DIMENSION_SET){

thrownewIllegalStateException("onMeasure()did not set the"

+ "measured dimension by calling"

+ "setMeasuredDimension()");

}

mPrivateFlags|= LAYOUT_REQUIRED;

}

mOldWidthMeasureSpec= widthMeasureSpec;

mOldHeightMeasureSpec= heightMeasureSpec;

}

/**

*

*Measure the view and its content to determine the measured width andthe

*measured height. This method is invoked by {@link#measure(int, int)} and

*should be overriden by subclasses to provide accurate and efficient

*measurement of their contents.

*

*

*CONTRACT:When overriding this method, you

*mustcall {@link #setMeasuredDimension(int,int)} to store the

*measured width and height of this view. Failure to do so will triggeran

*<code>IllegalStateException</code>,thrown by

*{@link #measure(int, int)}.Calling the superclass'

*{@link #onMeasure(int, int)}is a valid use.

*

*

*The base class implementation of measure defaults to the backgroundsize,

*unless a larger size is allowed by the MeasureSpec. Subclasses should

*override {@link #onMeasure(int, int)}to provide better measurements of

*their content.

*

*

*If this method is overridden, it is the subclass's responsibility tomake

*sure the measured height and width are at least the view's minimumheight

*and width ({@link#getSuggestedMinimumHeight()} and

*{@link #getSuggestedMinimumWidth()}).

*

*@paramwidthMeasureSpec horizontal space requirements as imposed by theparent.

* The requirements are encoded with

* {@linkandroid.view.View.MeasureSpec}.

*@paramheightMeasureSpec vertical space requirements as imposed by theparent.

* The requirements are encoded with

* {@linkandroid.view.View.MeasureSpec}.

*@see#getMeasuredWidth()

*@see#getMeasuredHeight()

*@see#setMeasuredDimension(int, int)

*@see#getSuggestedMinimumHeight()

*@see#getSuggestedMinimumWidth()

*@seeandroid.view.View.MeasureSpec#getMode(int)

*@seeandroid.view.View.MeasureSpec#getSize(int)

protectedvoidonMeasure(intwidthMeasureSpec, intheightMeasureSpec) {

setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(),widthMeasureSpec),

getDefaultSize(getSuggestedMinimumHeight(),heightMeasureSpec));

}

每个view中一定存在一个必调的方法,measure(int, int) ,它调用onMeasure(int,int)

onMeasure(widthMeasureSpec,heightMeasureSpec)的作用是什么？

测量view和它的content得出widthMeasureSpec heightMeasureSpec 并保存下来。

measure(widthMeasureSpec,heightMeasureSpec)的作用是什么？

通过父布局提供的 widthMeasureSpec， widthMeasureSpec来确定这个View的大小，有多高，有多宽，

特别对于一些实现ViewGroup 的控件在某些情况下，当 android:layout_heightandroid:layout_width 不能达到目的时，可以通过重写 onMeasure(int,int) 来达到调整view高宽的目的。

/**

*Thismehod must be called by {@link#onMeasure(int, int)} to store the

*measured width and measured height. Failing to do so will trigger an

*exception at measurement time.

*@parammeasuredWidth The measured width of this view. May be a complex

*bit mask as defined by {@link#MEASURED_SIZE_MASK} and

*{@link #MEASURED_STATE_TOO_SMALL}.

*@parammeasuredHeight The measured height of this view. May be a complex

*bit mask as defined by {@link#MEASURED_SIZE_MASK} and

*{@link #MEASURED_STATE_TOO_SMALL}.

protectedfinalvoidsetMeasuredDimension(intmeasuredWidth, intmeasuredHeight) {

mMeasuredWidth= measuredWidth;

mMeasuredHeight= measuredHeight;

mPrivateFlags|= MEASURED_DIMENSION_SET;

}

setMeasuredDimension在onMeasure(int,int) 中调用，用来存储跟新测量的宽和高，若不调，可能产生异常。

转载：

MeasureSpec介绍及使用详解

MeasureSpec是view中的内部类

一个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。一个MeasureSpec由大小和模式组成。它有三种模式：UNSPECIFIED(未指定),父元素部队自元素施加任何束缚，子元素可以得到任意想要的大小；EXACTLY(完全)，父元素决定自元素的确切大小，子元素将被限定在给定的边界里而忽略它本身大小；AT_MOST(至多)，子元素至多达到指定大小的值。

1父布局传下来的

2子元素布局文件或代码中设置的

　　它常用的三个函数：

　　1.staticintgetMode(int measureSpec):根据提供的测量值(格式)提取模式(上述三个模式之一)

　　2.staticintgetSize(int measureSpec):根据提供的测量值(格式)提取大小值(这个大小也就是我们通常所说的大小)

　　3.staticintmakeMeasureSpec(int size,int mode):根据提供的大小值和模式创建一个测量值(格式)

　　这个类的使用呢，通常在view组件的onMeasure方法里面调用但也有少数例外，看看几个例子：

　　a.首先一个我们常用到的一个有用的函数，View.resolveSize(intsize,intmeasureSpec)

publicstatic intresolveSize(intsize, int measureSpec) {

returnresolveSizeAndState(size,measureSpec, 0) & MEASURED_SIZE_MASK;

}

publicstatic intresolveSizeAndState(intsize, int measureSpec, intchildMe asuredState) {

intresult = size;

intspecMode = MeasureSpec.getMode(measureSpec);

intspecSize = MeasureSpec.getSize(measureSpec);

switch(specMode) {

caseMeasureSpec.UNSPECIFIED:

result = size;

break;

caseMeasureSpec.AT_MOST:

if(specSize < size) {

result = specSize |MEASURED_STATE_TOO_SMALL;

} else{

result = size;

}

break;

caseMeasureSpec.EXACTLY:

result = specSize;

break;

}

returnresult | (childMeasuredState&MEASURED_STATE_MASK);

}

简单说一下，这个方法的主要作用就是根据你提供的大小和MeasureSpec，返回你想要的大小值，这个里面根据传入模式的不同来做相应的处理。

　　再看看MeasureSpec.makeMeasureSpec方法，实际上这个方法很简单：

publicstatic intmakeMeasureSpec(int size, intmode) {

returnsize + mode;

}

这样大家不难理解size跟measureSpec区别了。

******************************************

布局步骤

1measure onMeasure(measureWidth, measureHight) 计算大小

2layout onLayout(chang, l, t, r, b) 给自己及子view分配size,position

3draw ondraw(canvas) 画图

dispatchDraw 空方法体

getWidth()getMeasureWidth() 区别

getWidth()：view的布局完成后，view的宽度

getMeasureWidth()：这是一个过程量，得到的是在最近一次調用measure()方法測量後得到的view的寬度，它僅僅用在測量和layout的計算中。

举例：有时会通过 child.getMeasureWidth()的值根据 Gravity重心得出 view的4个lt r b 再layout

如Gridview 中存在一个方法：

privatevoidsetupChild(View child, intposition, inty, booleanflow, intchildrenLeft,

booleanselected, booleanrecycled, intwhere) {

booleanisSelected = selected && shouldShowSelector();

finalbooleanupdateChildSelected = isSelected != child.isSelected();

finalintmode = mTouchMode;

finalbooleanisPressed = mode > TOUCH_MODE_DOWN && mode <TOUCH_MODE_SCROLL &&

mMotionPosition ==position;

finalbooleanupdateChildPressed = isPressed != child.isPressed();

booleanneedToMeasure = !recycled || updateChildSelected ||child.isLayoutRequested();

//Respect layout params that are already in the view. Otherwise make

//some up...

AbsListView.LayoutParams p =(AbsListView.LayoutParams)child.getLayoutParams();

if(p == null){

p = newAbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,

ViewGroup.LayoutParams.WRAP_CONTENT,0);

}

p.viewType =mAdapter.getItemViewType(position);

if(recycled && !p.forceAdd) {

attachViewToParent(child,where, p);

} else{

p.forceAdd = false;

addViewInLayout(child,where, p, true);

}

if(updateChildSelected) {

child.setSelected(isSelected);

if(isSelected) {

requestFocus();

}

if(updateChildPressed) {

child.setPressed(isPressed);

}

if(mChoiceMode != CHOICE_MODE_NONE&& mCheckStates != null){

if(child instanceofCheckable) {

((Checkable)child).setChecked(mCheckStates.get(position));

} elseif(getContext().getApplicationInfo().targetSdkVersion

>=android.os.Build.VERSION_CODES.HONEYCOMB){

child.setActivated(mCheckStates.get(position));

}

if(needToMeasure) {

intchildHeightSpec = ViewGroup.getChildMeasureSpec(

MeasureSpec.makeMeasureSpec(0,MeasureSpec.UNSPECIFIED),0, p.height);

intchildWidthSpec = ViewGroup.getChildMeasureSpec(

MeasureSpec.makeMeasureSpec(mColumnWidth,MeasureSpec.EXACTLY),0, p.width);

child.measure(childWidthSpec,childHeightSpec);

} else{

cleanupLayoutState(child);

}

finalintw = child.getMeasuredWidth();

final int h =child.getMeasuredHeight();

intchildLeft;

finalintchildTop = flow ? y : y - h;

finalintlayoutDirection = getResolvedLayoutDirection();

finalintabsoluteGravity = Gravity.getAbsoluteGravity(mGravity,layoutDirection);

switch(absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK){

caseGravity.LEFT:

childLeft =childrenLeft;

break;

caseGravity.CENTER_HORIZONTAL:

childLeft = childrenLeft+ ((mColumnWidth - w) / 2);

break;

case Gravity.RIGHT:

childLeft = childrenLeft+ mColumnWidth - w;

break;

default:

childLeft =childrenLeft;

break;

}

if(needToMeasure) {

finalintchildRight = childLeft + w;

finalintchildBottom = childTop + h;

child.layout(childLeft,childTop, childRight, childBottom);

} else{

child.offsetLeftAndRight(childLeft- child.getLeft());

child.offsetTopAndBottom(childTop- child.getTop());

}

if(mCachingStarted) {

child.setDrawingCacheEnabled(true);

}

if(recycled &&(((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition)

!= position) {

child.jumpDrawablesToCurrentState();

}

ViewGroup.java

1 measureChildren(measureWidth,measureHight) measureChild(child, widthMeasureSpec, heightMeasureSpec)

2继承layout(l,t, r, b) onLayout(chang,l, t, r, b)

3继承 dispatchDraw drawChild

view.java

MeasureSpec介绍及使用详解

MeasureSpec是view中的内部类

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