View的MeasureSpec确定过程(onMeasure分析

作者: 201e7f3210f4 | 来源:发表于2016-05-07 15:16 被阅读730次

本文首发在我的个人博客ghui.me欢迎指教

我们在自定义View时通常会去重写View的onMeasure方法，此方法提供的有一个默认实现：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

这个方法有两个int型的参数：widthMeasureSpec,heightMeasureSpec。这篇文章将会分析它们的确定过程。首先看一下MeasureSpec是什么？

MeasureSpec

MeasureSpec是View类的一个静态内部类，源码不太多，这里贴出分析之：

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);
        }

        static int adjust(int measureSpec, int delta) {
            final int mode = getMode(measureSpec);
            if (mode == UNSPECIFIED) {
                // No need to adjust size for UNSPECIFIED mode.
                return makeMeasureSpec(0, UNSPECIFIED);
            }
            int size = getSize(measureSpec) + delta;
            if (size < 0) {
                Log.e(VIEW_LOG_TAG,
                 "MeasureSpec.adjust: new size would be negative! (" + size +") spec: " 
                 + toString(measureSpec) + " delta: " + delta);
                size = 0;
            }
            return makeMeasureSpec(size, mode);
        }

        public static String toString(int measureSpec) {
            int mode = getMode(measureSpec);
            int size = getSize(measureSpec);

            StringBuilder sb = new StringBuilder("MeasureSpec: ");

            if (mode == UNSPECIFIED)
                sb.append("UNSPECIFIED ");
            else if (mode == EXACTLY)
                sb.append("EXACTLY ");
            else if (mode == AT_MOST)
                sb.append("AT_MOST ");
            else
                sb.append(mode).append(" ");

            sb.append(size);
            return sb.toString();
        }
    }

widthMeasureSpec 与 heightMeasureSpec是一个int型的变量，java中int型变量由4个字节(32bit)组成，其中高2位用来封装MeasureMode,MeasureMode一共有3种:

UNSPECIFIED = 0 << MODE_SHIFT; 即: 00000000 00000000 00000000 00000000 父容器不对子View有任何限制
EXACTLY = 1 << MODE_SHIFT; 即: 01000000 00000000 00000000 00000000 父容器已经测量出子View所需要的大小,即measureSpec中封装的specsize
AT_MOST = 2 << MODE_SHIFT; 即: 10000000 00000000 00000000 00000000 父窗口限定了一个最大值给子View即specsize

低30位用来封装size.

public static int makeMeasureSpec(int size, int mode) 此方法用来根据size与mode组合出特定的measureSpec.可以看到在api17之后是通过二进制 & | 计算的，这样效率更高。
static int adjust(int measureSpec, int delta) 此方法是用来对measurespec中的size进行调整的和 <inset>相关。
public static String toString(int measureSpec) 此方法大概是用来方便开发者从一个int值中打印mode与size，方便调试。

过程分析

MeasureSpec大概就是这样，为了说明MeasureSpec的创建过程，我们写一个CustomLinearLayout里面套一个CustomView,通过debug来分析一下，布局代码大致如下:

<?xml version="1.0" encoding="utf-8"?>
<me.ghui.customviewgroup.CustomLinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">
    <me.ghui.customviewgroup.CustomView android:layout_width="match_parent"
        android:layout_height="200px"
        android:background="@android:color/holo_blue_light"/>
</me.ghui.customviewgroup.CustomLinearLayout>

然后我们给CustomLinearLayout,CustomView的onMeasure方法加入一些log,如下：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    Log.e("ghui", "CustomLinearLayout." + "widthMeasureSpec:" 
    + MeasureSpec.toString(widthMeasureSpec)
    + ", heightMeasureSpec: " + MeasureSpec.toString(heightMeasureSpec));
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    Log.e("ghui", "CustomView." + "widthMeasureSpec:" 
    + MeasureSpec.toString(widthMeasureSpec)
    + ", heightMeasureSpec: " + MeasureSpec.toString(heightMeasureSpec));
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

我们在CustomView的onMeasure方法中加入一个断点，通过android studio观察onMeasure方法的调用栈信息，截图如下：

从上面可以发现CustomView的onMeasure方法调用过程源起其Parent CustomLinearLayout中的onMeasure.在这条调用链上最后一个ViewGroup的方法是measureChildWithMargins(),源码如下:

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);
}

可以看到此方法的最后一行中会调用child.measure方法将计算好的childWidthMeasureSpec 与 childHeightMeasureSpec传给子View,再往上看会发现这两个参数的计算过程被完全封装到了
getChildMeasureSpec中，此方法的源码如下:

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);
}

从中可以看出子View的MeasureSpec的计算过程被分为了3部分：

parentMeasureMode = EXACTLY

a. childDimension >=0 即子View设置的有具体的大小(如：100px)，这种情况下childSpecSize即为其自身LayoutParams里指定的大小，childSpecMode为EXACTLY

b. childDimension = MATCH_PARENT 即子View的LayoutParams里设置的为MATCH_PARENT,这种情况下childSpecSize即为parentLeftSize(父容器剩余的空间)，childSpecMode为EXACTLY

c. childDimension = WRAP_CONTENT 即子View的LayoutParams里设置的为WRAP_CONTENT,这种情况下childSpecSize为parentLeftSize(父容器剩余的空间),childSpecMode为AT_MOST
parentMeasureMode = AT_MOST

a. childDimension >=0 即子View设置的有具体的大小(如：100px)，这种情况下childSpecSize即为其自身LayoutParams里指定的大小，childSpecMode为EXACTLY (同 1.a)

b. childDimension = MATCH_PARENT 即子View的LayoutParams里设置的为MATCH_PARENT,这种情况下childSpecSize即为parentLeftSize(父容器剩余的空间)，childSpecMode为AT_MOST (同 1.c)

c. childDimension = WRAP_CONTENT 即子View的LayoutParams里设置的为WRAP_CONTENT,这种情况下childSpecSize即为parentLeftSize(父容器剩余的空间),childSpecMode为AT_MOST(同1.c)
parentMeasureMode = UNSPECIFIED

a. childDimension >=0 即子View设置的有具体的大小(如：100px)，这种情况下childSpecSize即为其自身LayoutParams里指定的大小，childSpecMode为EXACTLY (同 1.a)

b. childDimension = MATCH_PARENT 即子View的LayoutParams里设置的为MATCH_PARENT,这种情况下childSpecSize为0，childSpecMode为UNSPECIFIED

c. childDimension = WRAP_CONTENT 即子View的LayoutParams里设置的为WRAP_CONTENT,这种情况下childSpecSize为0，childSpecMode为UNSPECIFIED(同3.b)

整理一下可以得到如下表格:

总结

View的measureSpec由其父容器的measureSpec及自身的LayoutParams共同决定
若子View为具体的大小(如100px),则不管其父容器的specMode为哪种，子View对应的specMode均为EXACTLY,specSize均为childSize
若子View的LayoutParams中的宽或高为wrap_content,则不管其父容器的specMode为哪种，子View对应的specMode均为AT_MOST,specSize均为parentLeftSize
子View的前期measure过程实际上在其父容器的onMeasure中就基本完成了，父容器会把计算好的measureSpec传递给子View，子View在自己的onMeasure中可以得到这些值，子View可以根据这些值设置自己的大小，当然也可以不参考它们。最终在onMeasure方法通过调用
setMeasuredDimension方法设置view的最终measureSize。但View的真实大小是在Layout阶段才确定下来的，通过child.layout(left,top,right,bottom).View的measure size与 layout size不必相等，但绝大多数情况下是相等的。

View的MeasureSpec确定过程(onMeasure分析

MeasureSpec

过程分析

总结

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