深入理解 Android 之 View 的绘制流程(二)_Measure

上一篇中介绍了将xml布局文件加载到Activity中之后，对View进行开始绘制之前的一些操作进行了说明。知道了View的绘制主要是由ViewRoot负责执行，并且对执行的核心方法进行了图示说明，如下：

下面将对ViewRootImpl中performMeasure/performLayout/performDraw，分别进行说明。

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);
        }
}

这个方法里面直接使用的是View的measure方法。

View#measure

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        //判断是否是ViewGroup
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
            int oHeight = insets.top  + insets.bottom;
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }

        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
 // 若mPrivateFlags中包含PFLAG_FORCE_LAYOUT标记，则强制重新布局，
 // 或者 widthMeasureSpec 与mOldWidthMeasureSpec，
 // heightMeasureSpec 与mOldHeightMeasureSpec 不相等。
        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

            resolveRtlPropertiesIfNeeded();
            // 先尝试从缓从中获取，若forceLayout为true或是缓存中不存在或是
            // 忽略缓存，则调用onMeasure()重新进行测量工作
            int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -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 {
                long value = mMeasureCache.valueAt(cacheIndex);
                // Casting a long to int drops the high 32 bits, no mask needed
                setMeasuredDimensionRaw((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }

            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("View with id " + getId() + ": "
                        + getClass().getName() + "#onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;

        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}

在measure方法中，我们知道需要满足几个条件时，才会进行实际的测量工作：
1.mPrivateFlags 标志为PFLAG_FORCE_LAYOUT，表示需要强制重新布局。是通过View.requestLayout方法中实现。
2.widthMeasureSpec 和 heightMeasureSpec 与 mOldWidthMeasureSpec和mOldHeightMeasureSpec 值不相等。
3.缓存中没有保存该测量值，或者忽略缓存中的测量值。
满足上面的条件，才会进入测量的方法onMeasure中。

View#onMeasure

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

在方法中，会调用getSuggestedMinimumWidth()和getSuggestedMinimumHeight()，获取最小的宽高。

View#getSuggestedMinimumWidth & View#getSuggestedMinimumHeight

protected int getSuggestedMinimumWidth() {
        return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

protected int getSuggestedMinimumHeight() {
        return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());

}

根据最小的宽高，得到默认的尺寸。

View#getDefaultSize

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;
}

最后需要通过setMeasuredDimension方法来保存测量的宽高值：
View#setMeasuredDimension

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);
}

View#setMeasuredDimensionRaw

private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;

        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}

以上便完成了对View的测量，之后就可以通过getMeasuredWidth()和getMeasuredHeight()来获取测量之后的宽高值了。
以上只是针对View的测量，我们知道DecorView是继承自FrameLayout的，那么我们来看看FrameLayout类中的onMeasure方法的实现。

FrameLayout#onMeasure

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();

        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        mMatchParentChildren.clear();

        int maxHeight = 0;
        int maxWidth = 0;
        int childState = 0;
        //遍历子View
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                //每个子视图View的宽和高
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                // 通过遍历获取ViewGroup的宽高
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }

        // Account for padding too
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // Check against our foreground's minimum height and width
        final Drawable drawable = getForeground();
        if (drawable != null) {
            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
        }
        //保存测量结果，此方法的调用表示当前View测量的结束
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));

        count = mMatchParentChildren.size();
        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);
            }
        }
}

在方法中，先遍历FrameLayout中的子View，然后调用setMeasuredDimension子View的大小。在给测量后的View加上边距的值。然后调用setMeasuredDimension方法保存ViewGroup的测量值，表示测量结束。继续来分析measureChildWithMargins方法：

ViewGroup#measureChildWithMargins

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);
}

该方法中通过getChildMeasureSpec来获取子View的测量规格，然后通过measure方法，完成对View的测量。
我们可以通过图示显示：

总结：
1.View的measure方法是final类型的，子类不可以重写，子类可以通过重写onMeasure方法来测量自己的大小，当然也可以不重写onMeasure方法使用系统默认测量大小。
2.View测量结束的标志是调用了View类中的setMeasuredDimension成员方法，言外之意是如果你需要在自定义的View中重写onMeasure方法，在你测量结束之前你必须调用setMeasuredDimension方法测量才有效。
3.在Activity生命周期onCreate和onResume方法中调用View.getWidth()和View.getMeasuredHeight()返回值为0的，是因为当前View的测量还没有开始，这里关系到Activity启动过程，上一篇文章说了当ActivityThread类中的performResumeActivity方法执行之后才将DecorView添加到PhoneWindow窗口上，开始测量。在Activity生命周期onCreate在中performResumeActivity还未执行，因此调用View.getMeasuredHeight(）返回值为0。