Android中mesure过程详解

我们在编写layout的xml文件时会碰到layout_width和layout_height两个属性,对于这两个属性我们有三种选择:赋值成具体的数值,match_parent或者wrap_content,而measure过程就是用来处理match_parent或者wrap_content,假如layout中规定所有View的layout_width和layout_height必须赋值成具体的数值,那么measure其实是没有必要的,但是google在设计Android的时候考虑加入match_parent或者wrap_content肯定是有原因的,它们会使得布局更加灵活。

  首先我们来看几个关键的函数和参数:
      1public final void measue(int widthMeasureSpec, int heightMeasureSpec);

      2protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec);

      3、protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec)

      4、protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec)

      5、protected void measureChildWithMargins(View child,int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed)

接着我们来看View类中measure和onMeasure函数的源码:

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        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);

            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }

            mPrivateFlags |= LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;
    }

由于函数原型中有final字段,那么measure根本没打算被子类继承,也就是说measure的过程是固定的,而measure中调用了onMeasure函数,因此真正有变数的是onMeasure函数,onMeasure的默认实现很简单,源码如下:

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

onMeasure默认的实现仅仅调用了setMeasuredDimension,setMeasuredDimension函数是一个很关键的函数,它对View的成员变量mMeasuredWidth和mMeasuredHeight变量赋值,而measure的主要目的就是对View树中的每个View的mMeasuredWidth和mMeasuredHeight进行赋值,一旦这两个变量被赋值,则意味着该View的测量工作结束。

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;

        mPrivateFlags |= MEASURED_DIMENSION_SET;
    }

对于非ViewGroup的View而言,通过调用上面默认的measure——>onMeasure,即可完成View的测量,当然你也可以重载onMeasure,并调用setMeasuredDimension来设置任意大小的布局,但一般不这么做,因为这种做法太“专政”,至于为何“专政”,读完本文就会明白。

对于ViewGroup的子类而言,往往会重载onMeasure函数负责其children的measure工作,重载时不要忘记调用setMeasuredDimension来设置自身的mMeasuredWidth和mMeasuredHeight。如果我们在layout的时候不需要依赖子视图的大小,那么不重载onMeasure也可以,但是必须重载onLayout来安排子视图的位置,这在下一篇博客中会介绍。

再来看下measue(int widthMeasureSpec, int heightMeasureSpec)中的两个参数, 这两个参数分别是父视图提供的测量规格,当父视图调用子视图的measure函数对子视图进行测量时,会传入这两个参数,通过这两个参数以及子视图本身的LayoutParams来共同决定子视图的测量规格,在ViewGroup的measureChildWithMargins函数中体现了这个过程,稍后会介绍。

MeasureSpec参数的值为int型,分为高32位和低16为,高32位保存的是specMode,低16位表示specSize,specMode分三种:

1、MeasureSpec.UNSPECIFIED,父视图不对子视图施加任何限制,子视图可以得到任意想要的大小;

2、MeasureSpec.EXACTLY,父视图希望子视图的大小是specSize中指定的大小;

3、MeasureSpec.AT_MOST,子视图的大小最多是specSize中的大小。

以上施加的限制只是父视图“希望”子视图的大小按MeasureSpec中描述的那样,但是子视图的具体大小取决于多方面的。

ViewGroup中定义了measureChildren, measureChild, measureChildWithMargins来对子视图进行测量,measureChildren内部只是循环调用measureChild,measureChild和measureChildWithMargins的区别就是是否把margin和padding也作为子视图的大小,我们主要分析measureChildWithMargins的执行过程:

 protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        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);
    } 

总的来看该函数就是对父视图提供的measureSpec参数进行了调整(结合自身的LayoutParams参数),然后再来调用child.measure()函数,具体通过函数getChildMeasureSpec来进行参数调整,过程如下:

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // 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.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                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.
                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 asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him 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 = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    } 

getChildMeasureSpec的总体思路就是通过其父视图提供的MeasureSpec参数得到specMode和specSize,并根据计算出来的specMode以及子视图的childDimension(layout_width和layout_height中定义的)来计算自身的measureSpec,如果其本身包含子视图,则计算出来的measureSpec将作为调用其子视图measure函数的参数,同时也作为自身调用setMeasuredDimension的参数,如果其不包含子视图则默认情况下最终会调用onMeasure的默认实现,并最终调用到setMeasuredDimension,而该函数的参数正是这里计算出来的。

1)不论父View 的specMode是EXACTLY、UNSPECIFIED、AT_MOST的哪一种,如果子view在xml文件里面把layout_width或者layout_height这是了具体
的dimen值、或者在代码里调用相应的方法为view的宽度或者高度设置了具体的值,那么此时widthSpec的mode 或者heightSpec的mode就是EXACLTY,size就等于layout_width或者
layout_height的值。此时子view的specMode值不受父View的specMode的影响。

2)当父View的specMode为EXACTLY的时候:父View强加给子View一个确切的大小,有如下两种情况
2.1)子View的layout_width或者layout_height设置为MATCH_PARENT的时候,子View的specMode为EXACTLY
2.2)子View的layout_width或者layout_height设置为 WRAP_CONTENT的时候,子View的specMode为AT_MOST
2.3)不论子view为match_parent或者wrap_content,resultSize都等于父类的size.

3)当父view的specMode为AT_MOST的时候:父View强加给一个最大的size给子view,最大的size也就是父view的size
3.1)此时不论子view的为match_parent或者wrap_content,子view的specMode都为AT_MOST
3.2)resultSize的大小被设置为父view的大小

4)当父view的specMode为UNSPECIFIED的时候:
4.1) 此时不论子view的为match_parent或者wrap_content,子view的specMode都为UNSPECIFIED
4.3)此时reusltSize = 0

5)从对UNSPECIFIED分析可以知道,在1)情况之外的情况下,要是想让子view自己决定自己的宽度或者高度的大小,MeasureSpec.makeMeasureSpec(0,UNSPECIFIED)这么调用来获取宽度和高度
的spec

6)根据方法的参数以及方法的实现可以知道,视图的大小是由父视图和子视图共同决定的,前提是子视图没有设置具体的dimen值

总结:从上面的描述看出,决定权最大的就是View的设计者,因为设计者可以通过调用setMeasuredDimension决定视图的最终大小,例如调用setMeasuredDimension(100, 100)将视图的mMeasuredWidth和mMeasuredHeight设置为100,100,那么父视图提供的大小以及程序员在xml中设置的layout_width和layout_height将完全不起作用,当然良好的设计一般会根据子视图的measureSpec来设置mMeasuredWidth和mMeasuredHeight的大小,已尊重程序员的意图。

通过以下方法可以强制计算当前view的宽高,原因不明

int w = View.MeasureSpec.makeMeasureSpec(0,
                    View.MeasureSpec.UNSPECIFIED);
            int h = View.MeasureSpec.makeMeasureSpec(0,
                    View.MeasureSpec.UNSPECIFIED);
            view.measure(w, h);
            mChildHeight = view.getMeasuredHeight();
            mChildWidth = view.getMeasuredWidth();

参考链接

获取view的高度和宽度(在onCreate方法中) - Listening_music的专栏 - 博客频道 - CSDN.NET

本文主要参考

Android中mesure过程详解 – - 那些人追过的年 - 博客园

MeasureSpec学习 - 转 - yuhailong626的专栏 - 博客频道 - CSDN.NET

MeasureSpec的简单说明 - 凌云 - 博客频道 - CSDN.NET

posted @ 2016-05-18 10:21  RicardoMJiang  阅读(263)  评论(0编辑  收藏  举报