从源码角度彻底分析layout_weight使用

转载请注明出处：http://blog.csdn.net/zhaodai11?viewmode=contents

layout_weight是线性布局特有的一个属性，这个属性可以按照比例设置控件的大小。线性布局中控件layout_weight默认值为0。
你想按照比例分配界面高度时：LinearLayout 的orientation属性设置为vertical，子控件高度建议0dp

计算方式 View1高度: 1/(1+2)=1/3 View2 高度: 2/(1+2)=2/3
当时当你把高度设置为wrap_content或者match_parent时，会是什么样子呢？
当设置为wrap_content时：

看起来好像没有区别，好多人因此得出wrap_content和0dp的效果相同，但是这就说明wrap_content和0dp效果一样吗？这个放在最后面讲。
当设置为match_parent时：

当设置为match_parent时，两个控件的高度比正好和上面相反。
这就涉及到了在线性布局中LinearLayout在onMeasure方法中对layout_weight属性的处理。

@Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {    //判断线性布局的方向        if (mOrientation == VERTICAL) {            measureVertical(widthMeasureSpec, heightMeasureSpec);        } else {            measureHorizontal(widthMeasureSpec, heightMeasureSpec);        }    }

其实横向和纵向的原理差不多，这里我们这分析纵向的measureVertical(widthMeasureSpec, heightMeasureSpec);方法。在代码中已经添加相关注释，这里就不多加说明了。

void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {        mTotalLength = 0;//所有子控件高度之和        int maxWidth = 0;//子控件的最大宽度        int childState = 0;子控件的测量状态        int alternativeMaxWidth = 0; // 子控件中layout_weight<=0的View的最大宽度        int weightedMaxWidth = 0;// 子控件中layout_weight>0的View的最大宽度        boolean allFillParent = true;//所有子控件宽度是否全部为fillParent        float totalWeight = 0;//子控件所有layout_weight之和        final int count = getVirtualChildCount();//获取所有子控件的数量        //获取宽度和高度的测量模式        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);        boolean matchWidth = false;        boolean skippedMeasure = false;        final int baselineChildIndex = mBaselineAlignedChildIndex;                final boolean useLargestChild = mUseLargestChild;        int largestChildHeight = Integer.MIN_VALUE;        // See how tall everyone is. Also remember max width.        for (int i = 0; i < count; ++i) {            final View child = getVirtualChildAt(i);//获取对应子控件            if (child == null) {                mTotalLength += measureNullChild(i);                continue;            }            if (child.getVisibility() == View.GONE) {               i += getChildrenSkipCount(child, i);               continue;            }            if (hasDividerBeforeChildAt(i)) {                mTotalLength += mDividerHeight;            }            LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();            totalWeight += lp.weight;            //判断是否需要测量子控件            if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {                // Optimization: don't bother measuring children who are going to use                // leftover space. These views will get measured again down below if                // there is any leftover space.                //如果LinearLayout测量规格为MeasureSpec.EXACTLY，说明，LinearLayout的高度已经确定，并不需要依赖于子控件的高度，并且子控件的高度为0，weight>0，说明子控件的高度，依赖于LinearLayout的剩余空间来计算的                final int totalLength = mTotalLength;                mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);                skippedMeasure = true;            } else {            //进入说明LinearLayout高度不确定，依赖于子控件高度                int oldHeight = Integer.MIN_VALUE;//当LinearLayout高度不确定，子控件高度为0，weight>0，会强制设置子控件的高度计算模式为WRAP_CONTENT来配合LinearLayout高度计算。                if (lp.height == 0 && lp.weight > 0) {                    // heightMode is either UNSPECIFIED or AT_MOST, and this                    // child wanted to stretch to fill available space.                    // Translate that to WRAP_CONTENT so that it does not end up                    // with a height of 0                    oldHeight = 0;                    lp.height = LayoutParams.WRAP_CONTENT;                }                // Determine how big this child would like to be. If this or                // previous children have given a weight, then we allow it to                // use all available space (and we will shrink things later                // if needed).                measureChildBeforeLayout(                       child, i, widthMeasureSpec, 0, heightMeasureSpec,                       totalWeight == 0 ? mTotalLength : 0);                if (oldHeight != Integer.MIN_VALUE) {                // 测量完成之后，重新设置 LayoutParams.height                   lp.height = oldHeight;                }                final int childHeight = child.getMeasuredHeight();                //重新计算子控件高度之和                final int totalLength = mTotalLength;                mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +                       lp.bottomMargin + getNextLocationOffset(child));                if (useLargestChild) {                //比较高度，设置最大子控件高度                    largestChildHeight = Math.max(childHeight, largestChildHeight);                }            }            /**             * If applicable, compute the additional offset to the child's baseline             * we'll need later when asked {@link #getBaseline}.             */            if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {               mBaselineChildTop = mTotalLength;            }            // if we are trying to use a child index for our baseline, the above            // book keeping only works if there are no children above it with            // weight.  fail fast to aid the developer.            if (i < baselineChildIndex && lp.weight > 0) {                throw new RuntimeException("A child of LinearLayout with index "                        + "less than mBaselineAlignedChildIndex has weight > 0, which "                        + "won't work.  Either remove the weight, or don't set "                        + "mBaselineAlignedChildIndex.");            }            boolean matchWidthLocally = false;            if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {                // The width of the linear layout will scale, and at least one                // child said it wanted to match our width. Set a flag                // indicating that we need to remeasure at least that view when                // we know our width.                matchWidth = true;                matchWidthLocally = true;            }            final int margin = lp.leftMargin + lp.rightMargin;            final int measuredWidth = child.getMeasuredWidth() + margin;            maxWidth = Math.max(maxWidth, measuredWidth);            childState = combineMeasuredStates(childState, child.getMeasuredState());            allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;            if (lp.weight > 0) {                /*                 * Widths of weighted Views are bogus if we end up                 * remeasuring, so keep them separate.                 */                weightedMaxWidth = Math.max(weightedMaxWidth,                        matchWidthLocally ? margin : measuredWidth);            } else {                alternativeMaxWidth = Math.max(alternativeMaxWidth,                        matchWidthLocally ? margin : measuredWidth);            }            i += getChildrenSkipCount(child, i);        }        if (mTotalLength > 0 && hasDividerBeforeChildAt(count)) {            mTotalLength += mDividerHeight;        }// 这里是处理useLargestChild相关操作        if (useLargestChild &&                (heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {            mTotalLength = 0;            for (int i = 0; i < count; ++i) {                final View child = getVirtualChildAt(i);                if (child == null) {                    mTotalLength += measureNullChild(i);                    continue;                }                if (child.getVisibility() == GONE) {                    i += getChildrenSkipCount(child, i);                    continue;                }                final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)                        child.getLayoutParams();                // Account for negative margins                final int totalLength = mTotalLength;                mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +                        lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));            }        }        // Add in our padding        mTotalLength += mPaddingTop + mPaddingBottom;        int heightSize = mTotalLength;        // Check against our minimum height        heightSize = Math.max(heightSize, getSuggestedMinimumHeight());        // Reconcile our calculated size with the heightMeasureSpec        int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);        heightSize = heightSizeAndState & MEASURED_SIZE_MASK;        // Either expand children with weight to take up available space or        // shrink them if they extend beyond our current bounds. If we skipped        // measurement on any children, we need to measure them now.        int delta = heightSize - mTotalLength;//计算剩余高度        if (skippedMeasure || delta != 0 && totalWeight > 0.0f) {        // 限定weight总和范围，假如我们给过weighSum范围，那么子控件的weight总和受此影响            float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;            mTotalLength = 0;            for (int i = 0; i < count; ++i) {                final View child = getVirtualChildAt(i);                //判断如果子控件不可见 跳过                if (child.getVisibility() == View.GONE) {                    continue;                }                LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();                float childExtra = lp.weight;                if (childExtra > 0) {                    // Child said it could absorb extra space -- give him his share                    // 计算 weight 属性分配的大小，可能为负值                    int share = (int) (childExtra * delta / weightSum);                    weightSum -= childExtra;                    delta -= share;                    final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,                            mPaddingLeft + mPaddingRight +                                    lp.leftMargin + lp.rightMargin, lp.width);                    // TODO: Use a field like lp.isMeasured to figure out if this                    // child has been previously measured                    //注意  子控件高度计算                    if ((lp.height != 0) || (heightMode != MeasureSpec.EXACTLY)) {                        // child was measured once already above...                        // base new measurement on stored values                        //子控件高度子控件真是高度为weight分配后的高度+本身高度                        int childHeight = child.getMeasuredHeight() + share;                        if (childHeight < 0) {                            childHeight = 0;                        }                        child.measure(childWidthMeasureSpec,                                MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY));                    } else {                        // child was skipped in the loop above.                        // Measure for this first time here                              child.measure(childWidthMeasureSpec,                                MeasureSpec.makeMeasureSpec(share > 0 ? share : 0,                                        MeasureSpec.EXACTLY));                    }                    // Child may now not fit in vertical dimension.                    childState = combineMeasuredStates(childState, child.getMeasuredState()                            & (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));                }                final int margin =  lp.leftMargin + lp.rightMargin;                final int measuredWidth = child.getMeasuredWidth() + margin;                maxWidth = Math.max(maxWidth, measuredWidth);                boolean matchWidthLocally = widthMode != MeasureSpec.EXACTLY &&                        lp.width == LayoutParams.MATCH_PARENT;                alternativeMaxWidth = Math.max(alternativeMaxWidth,                        matchWidthLocally ? margin : measuredWidth);                allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;                final int totalLength = mTotalLength;                mTotalLength = Math.max(totalLength, totalLength + child.getMeasuredHeight() +                        lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));            }            // Add in our padding            mTotalLength += mPaddingTop + mPaddingBottom;            // TODO: Should we recompute the heightSpec based on the new total length?        } else {            alternativeMaxWidth = Math.max(alternativeMaxWidth,                                           weightedMaxWidth);            // We have no limit, so make all weighted views as tall as the largest child.            // Children will have already been measured once.            if (useLargestChild && heightMode != MeasureSpec.EXACTLY) {                for (int i = 0; i < count; i++) {                    final View child = getVirtualChildAt(i);                    if (child == null || child.getVisibility() == View.GONE) {                        continue;                    }                    final LinearLayout.LayoutParams lp =                            (LinearLayout.LayoutParams) child.getLayoutParams();                    float childExtra = lp.weight;                    if (childExtra > 0) {                        child.measure(                                MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),                                        MeasureSpec.EXACTLY),                                MeasureSpec.makeMeasureSpec(largestChildHeight,                                        MeasureSpec.EXACTLY));                    }                }            }        }        if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {            maxWidth = alternativeMaxWidth;        }        maxWidth += mPaddingLeft + mPaddingRight;        // Check against our minimum width        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),                heightSizeAndState);        if (matchWidth) {            forceUniformWidth(count, heightMeasureSpec);        }    }

结论：
由上面的源码分析可以得出下面的公式：
当View layout_height!=0时

delta = heightSize - mTotalLength; share = (int) (childExtra * delta / weightSum); height = share+child.getMeasuredHeight() 剩余空间高度 = LinearLayout高度-所有子控件高度之和 子控件真实高度 = （子控件所占比例）*（剩余空间高度）/总比例 + 子控件原本高度

当View layout_height=0时

delta = heightSize - mTotalLength; share = (int) (childExtra * delta / weightSum); height = share>0？share:0 剩余空间高度 = LinearLayout高度-所有子控件高度之和 子控件按比例分配的高度 share = （子控件所占比例）*（剩余空间高度）/总比例 子控件真实高度 h = 当share>0时 为share 当share<0时，直接为0

所以当上面两个子控件高度为match_parent时，

假设 LinearLayout 高度为H  //View1 weight = 1 View2 weight=2剩余空间 delta = H-(H+H)=-H//View1 高度h1  View2 高度h2h1 = 1*(-H)/(1+2)+H=2/3*Hh2 = 2*(-H)/(1+2)+H = 1/3*Hh1/h2 = 2/1

其实通过上面两个公式，我们也能理解当为wrap_content时为什么看起来效果和为0时一样，但其实不一样。主要是因为View高度不为0时，View本身的高度也会影响最终高度的计算。
下面我们来验证一下:
图1

图2

通过上面TextView1设置padding =100dp在wrap_content时会对View本身高度测量产生影响，和mTotalLength的计算产生影响

//mTotalLength计算final int childHeight = child.getMeasuredHeight();final int totalLength = mTotalLength;mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +lp.bottomMargin + getNextLocationOffset(child));View最终高度计算 相关代码 int childHeight = child.getMeasuredHeight() + share;                        if (childHeight < 0) {                            childHeight = 0;                        }child.measure(childWidthMeasureSpec,                                MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY));

此外例如TextView字体大小等会影响高度的属性，也会导致wrap_content和0dp结果不一样。

所以如果你想正常使用layout_weight属性的话，最好将对应的layout_width或者layout_height设置为0dp。这样是不会出现任何偏差的。如果你想有特殊的运用，建议将上面的源码读懂，注意各种细节。

最后在举一个有意思的例子：

上面这个为android:layout_height="wrap_content"时比例和正常相反了。

大家可以思考一下再看答案。

原因在View的源码中：
View的onMeasure方法：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));    }//注意在这个里面MeasureSpec.AT_MOST MeasureSpec.EXACTLY处理方式是一样的，所以wrap_content 按照match_partent的方式处理了。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;    }

有错误的地方欢迎反馈,大家一起进步。