2.View绘制分析笔记之onMeasure

今天主要学习记录一下Android View绘制三部曲的第一步，onMeasure，测量。

起源

在Activity中，所有的View都是DecorView的子View，然后DecorView又是被ViewRootImpl所控制，当Activity显示的时候，ViewRootImpl的performTranversals方法开始运行，这个方法很长，不过核心的三个流程就是依次调用performMeasure、performLayout、performDraw三个方法，从而完成DecorView的绘制。

ViewRootImpl#performMeasure

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
    try {
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

这里直接调用了mView的measure方法，参数是两个经过设置的MeasureSpec，接下来我们分析一下MeasureSpec是如何设置的。

MeasureSpec

这个MeasureSpec不是实际测绘值，而是父View传递给子View的布局要求，MeasureSpec涵盖了对子View大小和模式的要求。其中，三种模式要求分别是：

• UNSPECIFIED：对子View无任何要求，想要测绘多少由子View决定。
• EXACTLY：父View已确定了自己确切的大小。子View将在这个边界内测绘自己的宽高。
• AT_MOST：父View对子View没有要求，子View可以达到它想要的大小。

首先这个MeasureSpec是个32位的int值，其中31，32两位代表的是三种模式的要求，分别是00….、01….、11….，makeMeasureSpec方法中，sUseBrokenMakeMeasureSpec默认是false，所以一般执行(size & ~MODE_MASK) | (mode & MODE_MASK)这个语句，这个意思就是说，MeasureSpec的高两位代表的是模式，低30位代表父View的尺寸。
下面是对应的方法：

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(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
                                      @MeasureSpecMode int mode) {
    if (sUseBrokenMakeMeasureSpec) {
        return size + mode;
    } else {
        return (size & ~MODE_MASK) | (mode & MODE_MASK);
    }
}

View#measure

计算完MeasureSpec，DecorView就该执行measure方法了。

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    ···
    final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
    ···
    final boolean needsLayout = specChanged
            && (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);

    if (forceLayout || needsLayout) {
        ···
        int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
        if (cacheIndex < 0 || sIgnoreMeasureCache) {
            // measure ourselves, this should set the measured dimension flag back
            onMeasure(widthMeasureSpec, heightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        } else {
            ···
        }
        ···
        }
        ···
    }
    ···
}

measure方法是final的，所以不能重写，不过measure方法最主要的作用就是调用了onMeasure方法，由于DecorView是继承的FrameLayout，所以本篇文章我们主要分析FrameLayout的onMeasure方法。

FrameLayout#onMeasure

这个onMeasure方法，可能是View绘制中最难理解的了，所以我们逐步分析。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    //获取子View个数
    int count = getChildCount();
    
    //判断是否是确定宽高的
    //如果宽高都确定，那么boolean为false
    //如果有一个不确定，那么boolean为true
    final boolean measureMatchParentChildren =
            MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
            MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
    mMatchParentChildren.clear();

    int maxHeight = 0;
    int maxWidth = 0;
    int childState = 0;
    
    //遍历所有不为GONE的子View，并加以计算
    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (mMeasureAllChildren || child.getVisibility() != GONE) {
            //计算各个子View宽高，包括Margin参数以及padding参数
            //该方法详细分析见下文
            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            maxWidth = Math.max(maxWidth,
                    child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
            maxHeight = Math.max(maxHeight,
                    child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
            //用按位或的方法合并所有子View的State
            //getMeasuredState方法详细解析在下面
            childState = combineMeasuredStates(childState, child.getMeasuredState());
            //如果宽高有不确定的（即warp_content模式），将子View中宽或高是match_parent的添加到mMatchParentChildren中。
            if (measureMatchParentChildren) {
                if (lp.width == LayoutParams.MATCH_PARENT ||
                        lp.height == LayoutParams.MATCH_PARENT) {
                    mMatchParentChildren.add(child);
                }
            }
        }
    }

    // 计算padding
    maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
    maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

    // 与最小宽高作比较，二者取较大的
    maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
    maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

    // 与前景图宽高作比较，二者取较大的
    final Drawable drawable = getForeground();
    if (drawable != null) {
        maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
        maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
    }
    
    //计算并保存measured宽高
    //resolveSizeAndState方法分析在下面
    //setMeasuredDimension方法分析在下面
    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
            resolveSizeAndState(maxHeight, heightMeasureSpec,
                    childState << MEASURED_HEIGHT_STATE_SHIFT));
                    
    //计算macth_parent的子View的个数
    count = mMatchParentChildren.size();
    //只有FrameLayout中宽或者高有warp_content属性，
    //并且match_parent的子view个数大于1才会执行下面代码。
    //因为如果宽高都是match_parent的，或者设置好dp数值的，则mMatchParentChildren永远是空的。
    
    //在这里会重新计算传递给子View的MeasureSpec值，并重新测绘子View。
    //关于MeasureSpec值的计算，可以参考下文ViewGroup#getChildMeasureSpec的表格。
    //这里需要注意的是，match_parent行所有的结果均改为：EXACTLY + parentSize
    if (count > 1) {
        for (int i = 0; i < count; i++) {
            final View child = mMatchParentChildren.get(i);
            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

            final int childWidthMeasureSpec;
            if (lp.width == LayoutParams.MATCH_PARENT) {
                final int width = Math.max(0, getMeasuredWidth()
                        - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                        - lp.leftMargin - lp.rightMargin);
                childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                        width, MeasureSpec.EXACTLY);
            } else {
                childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                        getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                        lp.leftMargin + lp.rightMargin,
                        lp.width);
            }

            final int childHeightMeasureSpec;
            if (lp.height == LayoutParams.MATCH_PARENT) {
                final int height = Math.max(0, getMeasuredHeight()
                        - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                        - lp.topMargin - lp.bottomMargin);
                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        height, MeasureSpec.EXACTLY);
            } else {
                childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                        getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                        lp.topMargin + lp.bottomMargin,
                        lp.height);
            }

            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
    }
}

ViewGroup#measureChildWithMargins

该方法作用是测绘子View，在父View的onMeasure中循环调用，达到遍历的效果。

protected void measureChildWithMargins(View child,
        int parentWidthMeasureSpec, int widthUsed,
        int parentHeightMeasureSpec, int heightUsed) {
    //获取子View的LayoutParams
    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
    
    //计算子View的MeasureSpec
    //getChildMeasureSpec方法分析见下文
    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);
    //调用子View的measure方法，对子View进行测绘
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

ViewGroup#getChildMeasureSpec

此方法通过父View的MeasureSpec值以及LayoutParams的宽高，来生成子View的MeasureSpec值，具体代码如下：

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
    //计算父View的size和mode
    int specMode = MeasureSpec.getMode(spec);
    int specSize = MeasureSpec.getSize(spec);
    
    //计算刨除padding的size
    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 = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.UNSPECIFIED;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size.... find out how
            // big it should be
            resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.UNSPECIFIED;
        }
        break;
    }
    //noinspection ResourceType
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

上面代码，其实通过一张表格，就能很清楚的解释转换规律

竖列代表LayoutParams的宽或高属性横行父View传递的Mode	EXACTLY	AT_MOST	UNSPECIFIED
精确值	EXACTLY + childSize	EXACTLY + childSize	EXACTLY + childSize
match_parent	EXACTLY + parentSize	AT_MOST + parentSize	UNSPECIFIED + parentSize
warp_content	AT_MOST + parentSize	AT_MOST + parentSize	UNSPECIFIED + parentSize

View#getMeasuredState

public final int getMeasuredState() {
    return (mMeasuredWidth&MEASURED_STATE_MASK)
            | ((mMeasuredHeight>>MEASURED_HEIGHT_STATE_SHIFT)
                    & (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
}

该方法返回一个表达View宽高measure_state值的整数，第8位代表height是否是MEASURED_STATE_TOO_SMALL，第24为代表width是否是MEASURED_STATE_TOO_SMALL的。
例如：
宽是MEASURED_STATE_TOO_SMALL的
返回:0000 0001 0000 0000 0000 0000 0000 0000
高是MEASURED_STATE_TOO_SMALL的
返回:0000 0000 0000 0000 0000 0001 0000 0000
宽高均是MEASURED_STATE_TOO_SMALL的
返回:0000 0001 0000 0000 0000 0001 0000 0000

View#resolveSizeAndState

public static final int MEASURED_STATE_TOO_SMALL = 0x01000000;
public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
    //计算父view传递的size和mode
    final int specMode = MeasureSpec.getMode(measureSpec);
    final int specSize = MeasureSpec.getSize(measureSpec);
    final int result;
    switch (specMode) {
        case MeasureSpec.AT_MOST:
            if (specSize < size) {
                //如果父View给的size小于自身测绘出的size，
                //则在第24位上加上measure_state标记。
                result = specSize | MEASURED_STATE_TOO_SMALL;
            } else {
                result = size;
            }
            break;
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        case MeasureSpec.UNSPECIFIED:
        default:
            result = size;
    }
    //如果child在第24为上有measure_state标记，则在result的第24位上也加上measure_state标记，然后返回result。
    return result | (childMeasuredState & MEASURED_STATE_MASK);
}

View#setMeasuredDimension

该方法将计算好的measuredWidth和measuredHeight设置给成员变量mMeasuredWidth及mMeasuredHeight，并且将flag设置成PFLAG_MEASURED_DIMENSION_SET。
在4.3版本以上，如果设置了optical模式，则还要对width、height进一步修改，然后再设置mMeasuredWidth、mMeasuredHeight。
mMeasuredWidth和mMeasuredHeight的值，不光包括size，同时还包括state，具体请看getMeasuredWidth、getMeasuredWidthAndState、getMeasuredHeight、getMeasuredHeightAndState方法

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;
}

时序图

图为onMeasure时序图

小结

到这里，关于View的测绘我们大概的走了一遍。Measure的原理就是通过遍历，从上至下，利用传递的MeasureSpec以及子View的LayoutParams，依次进行测绘。不同的layout可能会进行多次的measure，所以熟读源码，合理布局，可以帮我们避免不必要的measure开销，达到提升性能的效果。

最后，感谢阅读，也希望可以和大家多多交流，共同进步。

系列文章

Android 视图及View绘制分析笔记之setContentView
View绘制分析笔记之onMeasure
View绘制分析笔记之onLayout
View绘制分析笔记之onDraw