关于view

问题：

1.view的getWidth()和getMeasuredWidth()有什么区别

2.如何在oncreate()中拿到view的宽度和高度

回答：

(1)

getWidth 获取的是这个view最终显示的大小，这个大小有可能等于原始的大小也有可能不等于原始大小

return mRight - mLeft;（在onlayout()过程中获取）

getMeasuredWidth 此视图的原始测量宽度,也就是这个view在XML文件中配置或者是代码中设置的大小。

return mMeasuredWidth & MEASURED_SIZE_MASK;(在onMeasure()中获取)

区别：一般情况下getWidth 和getMeasuredWidth 值是一样的但自身或父view在onlayout调用完毕后

再设置onMeasure的参数，getMeasuredWidth将被改变，getWidth 不变.

详细过程：见下方

(2)

3种方法 ：

1.View.MeasureSpec.makeMeasureSpec(200,View.MeasureSpec.EXACTLY); view_test.measure(w, h);

2.view_test.post

3.view_test.getViewTreeObserver().addOnGlobalLayoutListener

详细过程：见下方

执行过程：

I/viewActivity: view1 getWidth:0 getMeasuredWidth:0

I/viewActivity: onCreate

I/viewActivity: onStart

I/viewActivity: onResume

I/viewActivity: onAttachedToWindow

I/viewActivity: onMeasure getWidth:0 getMeasuredWidth:1079

I/viewActivity: onMeasure getWidth:0 getMeasuredWidth:1080

I/viewActivity: onSizeChanged getWidth:1080 getMeasuredWidth:1080

I/viewActivity: onLayout getWidth:1080 getMeasuredWidth:1080

view_test.getViewTreeObserver

view_test.post

I/viewActivity: onDraw getWidth:1080 getMeasuredWidth:1080

（1.1）getMeasuredWidth 的获取 return mMeasuredWidth & MEASURED_SIZE_MASK;

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),

getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));

/* getSuggestedMinimumWidth（）返回视图应该使用的最小宽度。

return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());

*/

}

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {

boolean optical = isLayoutModeOptical(this);

if (optical != isLayoutModeOptical(mParent)) {

Insets insets = getOpticalInsets();

int opticalWidth = insets.left + insets.right;

int opticalHeight = insets.top + insets.bottom;

measuredWidth += optical ? opticalWidth : -opticalWidth;

measuredHeight += optical ? opticalHeight : -opticalHeight;

}

setMeasuredDimensionRaw(measuredWidth, measuredHeight);

}

private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {

mMeasuredWidth = measuredWidth;

mMeasuredHeight = measuredHeight;

mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;

}

-----------------getDefaultSize解释：

public static int getDefaultSize(int size, int measureSpec) {

int result = size;

int specMode = MeasureSpec.getMode(measureSpec);

int specSize = MeasureSpec.getSize(measureSpec);

switch (specMode) {

case MeasureSpec.UNSPECIFIED:

result = size;

break;

case MeasureSpec.AT_MOST:

case MeasureSpec.EXACTLY:

result = specSize;

break;

}

return result;

}

public static int getMode(int measureSpec) {

//noinspection ResourceType

return (measureSpec & MODE_MASK);

}

public static int getSize(int measureSpec) {

return (measureSpec & ~MODE_MASK);

}

（1.2）getWidth()的获取return mRight - mLeft;

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;

//isLayoutModeOptical(mParent)判断是是否是viewgroup

boolean changed = isLayoutModeOptical(mParent) ?

setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

.............

}

private boolean setOpticalFrame(int left, int top, int right, int bottom) {

Insets parentInsets = mParent instanceof View ?

((View) mParent).getOpticalInsets() : Insets.NONE;

Insets childInsets = getOpticalInsets();

return setFrame(

left + parentInsets.left - childInsets.left,

top + parentInsets.top - childInsets.top,

right + parentInsets.left + childInsets.right,

bottom + parentInsets.top + childInsets.bottom);

}

protected boolean setFrame(int left, int top, int right, int bottom) {

.............

if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {

int oldWidth = mRight - mLeft;

int oldHeight = mBottom - mTop;

int newWidth = right - left;

int newHeight = bottom - top;

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);

}

.............

return changed;

}

（2.1） view.measure

int w = View.MeasureSpec.makeMeasureSpec(200,View.MeasureSpec.EXACTLY);

int h = View.MeasureSpec.makeMeasureSpec(200,View.MeasureSpec.EXACTLY);

view_test.measure(w, h);

// view_test.layout(0, 0, w, h);

int measuredwidth =view_test.getMeasuredWidth();//测量值已获得

int width =view_test.getWidth(); //getWidth在调用layout后获得

MeasureSpec：由32位2进制组成，前两位为mode后面为size,size最大值为（1<<30）-1即2^30-1

（2.2） view.post

利用handler通信机制，发送一个runble到message queue中layout完成时，自动发送消息通知UI线程

view_test.post(new Runnable() {

@Override

public void run() {

Log.i(TAG,"TestPost getWidth:"+view_test.getWidth());

}

});

public boolean post(Runnable action) {

final AttachInfo attachInfo = mAttachInfo;

if (attachInfo != null) { //在onCreate的方法中执行的时候，attachInfo==null

return attachInfo.mHandler.post(action);

}

在onCreate中调用View.post的时候，post的Runnable被保存起来，在下一次遍历布局的时候重新加到消息队列里（Android 7.0是在attach window的时候重新加到消息队列里），而且布局遍历本身也是加到消息队列执行的，所以我们post的Runnable所在的消息肯定是在布局遍历之后，所以在Runnable里必定可以获取到正确的宽高。

（2.3） view.getViewTreeObserver

监听view的onlayout绘制过程，一旦layout触发变化，立即回调onlayoutchange方法

view_test.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {

@Override

public void onGlobalLayout() {

Log.i(TAG,"TestViewTreeObserver getWidth:"+view_test.getWidth());

//移除监听避免每一次 view变化都触发该事件

view_test.getViewTreeObserver().removeOnGlobalLayoutListener(this);

}

});