Android中View测量之MeasureSpec

当View测量自身的大小的时候,会执行measure(int widthMeasureSpec, int heightMeasureSpec)方法,至于measure方法内容怎么执行的,这里先不去探讨。注意方法中两个参数,它们其实是一个int 类型的MeasureSpec。MeasureSpec可以说是View测量过程的前提,所以我们很有必要先来了解一下MeasureSpec。

MeasureSpec 工作原理

MeasureSpec 代表一个32位的int值,高2位代表SpecMode,低30位代表SpecSize。
SpecMode是指测量模式,SpecSize是指在某种测量模式下的大小。

MeasureSpec是View中的一个静态内部类。

public static class MeasureSpec {

    private static final int MODE_SHIFT = 30;
    private static final int MODE_MASK  = 0x3 << MODE_SHIFT;
    public static final int UNSPECIFIED = 0 << MODE_SHIFT;
    public static final int EXACTLY     = 1 << MODE_SHIFT;
    public static final int AT_MOST     = 2 << MODE_SHIFT;

    public static int makeMeasureSpec(int size, int mode) {
        if (sUseBrokenMakeMeasureSpec) {
            return size + mode;
        } else {
            return (size & ~MODE_MASK) | (mode & MODE_MASK);
        }
    }

    public static int getMode(int measureSpec) {
        return (measureSpec & MODE_MASK);
    } 

    public static int getSize(int measureSpec) {
        return (measureSpec & ~MODE_MASK);
    }

}
  • 我们可以把MeasureSpec理解为测量规则,而这个测量规则是由测量模式和和该模式下的测量大小共同组成的。
int MeasureSpec = MeasureSpec.makeMeasureSpec(specSize,SpecMode);
  • 确定了View测量规则后,我们也可以通过测量规则获取测量模式和该模式下的测量大小
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);

SpecMode有三类:

  1. UNSPECIFIED
    父容器不对View有任何限制,要多大给多大,这般情况一般用于系统内部,表示一种测量状态,如ScrollView测量子View时用的就是这个。
  2. EXACTLY
    父容器已经检测出View所需要的大小,这个时候View的最终大小就是SpecSize所测定的值,它对应于LayoutParams中的match_parent和具体的数值(如40dp,60dp)这两种模式。
  3. AT_MOST
    父容器指定了一个可用大小即SpecSize,View的大小不能大于这个值,具体是什么值要看不同View的具体实现。它对应于LayoutParams中的wrap_content.

普通View的MeasureSpec的创建过程

MeasureSpec很重要,上文中我们也了解了MeasureSpec的工作原理,那如何获取MeasureSpec呢?下面就结合源码来分析MeasureSpec的创建过程。

先来看下ViewGroup中的measureChild方法

    protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

在这个方法中,先获取了子View的布局参数,然后通过getChildMeasureSpec方法分别得到子View的宽高测量规则,即childWidthMeasureSpec和childHeightMeasureSpec,最后调用子View的measure方法,至此测量过程就由父View传递到了子View.。MeasureSpec确定后就可以在onMeasure方法确定View的测量宽高了。

我们重点分析的是getChildMeasureSpec方法,源码如下:

    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);  //返回父View的测量模式
        int specSize = MeasureSpec.getSize(spec);  //返回父View的测量大小

        int size = Math.max(0, specSize - padding);  //父View的测量大小 - 父View的padding占用的大小,剩余的即是子View可用的最大空间

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {  //子View大小为具体数值的情况
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {   //子View大小为match_parent的情况
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {   //子View大小为wrap_content的情况
                // 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);
    }

此方法比较清晰,它主要用来通过父View的MeasureSpec和子View的LayoutParams来确定子View的MeasureSpec的,即普通View的MeasureSpec创建过程。

为了让猿宝宝们更清晰的理解getChildMeasureSpec方法,借用《Android开发艺术探索》中一张表格,如下:

MeasureSpec创建过程

DecorView的MeasureSpec创建过程

普通View的MeasureSpec的创建过程阐述了怎样通过父View的MeasureSpec和子View的LayoutParams来确定子View的MeasureSpec。那顶级View,即DecorView的MeasureSpec创建过程又是怎样的呢?ViewRootImp的measureHierarchy方法中有如下代码:

    childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
    childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

接着来看getRootMeasureSpec方法

    private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

从上述源码,我们可以得出如下规则,具体根据它的LayoutParams来划分:

  • LayoutParams.MATCH_PARENT:精确模式 其大小就为屏幕的尺寸大小
  • ViewGroup.LayoutParams.WRAP_CONTENT:最大模式,大小不定,但是不能超过屏幕的大小
  • 具体数值(如40dp):精确模式,大小为LayoutParamas指定的大小。

至此,相信大家对MeasureSpec有一定了解了,当我们确定了View的MeasureSpec后,我们怎么用它来测量View的大小呢?具体请看View工作原理之measure过程解析

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