LinearLayout源码解析

为什么学习

自从学了Android自定义控件的一些知识，总是处于似懂非懂状态，说都说了上来，自己在项目里封装了一些自定义控件，但是还是缺乏一个很直观的了解。所以去了解学习下Android是如何封装控件的，就从简单的入手，分析下LinearLayout是如何实现的

什么是LinearLayout

作为最基础的布局，所以从事过Android开发的同学都应该非常了解
中文解释应该叫做线性布局，相比如RelativeLayout，LinearLayout更简单，在没有weight的情况也每次只要测量一次就够，而RelativeLayout每次都需要测量两次

一些LinearLayout需要注意的属性

orientation 纵向排布或者水平排布

weight 权重，用于分配LinearLayout剩下的空间（会详细介绍）

measureWithLargestChild 这个属性不常见，如果赋值为true的话，所有
weight子View都会采用最大View的最小尺寸（为什么Android要设计这个属性，我也不是很理解）

源码分析

一般所有控件类的源码，都会从 measure, layout和draw3个方法入手，查看他们的回调函数onMeasure, onLayout和onDraw
只要明白这3个流程，一般控件的整个实现也就明白了
LinearLayout作为一个ViewGroup的子类，主要作为一个布局容器出现，所以我们需要重点查看写onMeasure方法

@Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        if (mOrientation == VERTICAL) {            measureVertical(widthMeasureSpec, heightMeasureSpec);        } else {            measureHorizontal(widthMeasureSpec, heightMeasureSpec);        }    }

从上面代码看到，LinearLayout的onMeasure方法实现非常简洁，根据布局方向分为measureVertical和measureHorizontal。下后面的onLayout和onDraw也是如此。鉴于内部实现基本一模一样，我在这只分析纵向的实现

/**     * Measures the children when the orientation of this LinearLayout is set     * to {@link #VERTICAL}.     *     * @param widthMeasureSpec Horizontal space requirements as imposed by the parent.     * @param heightMeasureSpec Vertical space requirements as imposed by the parent.     *     * @see #getOrientation()     * @see #setOrientation(int)     * @see #onMeasure(int, int)     */    void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {        mTotalLength = 0;        int maxWidth = 0;        int childState = 0;        int alternativeMaxWidth = 0;        int weightedMaxWidth = 0;        boolean allFillParent = true;        float totalWeight = 0;        final int count = getVirtualChildCount();        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);            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;            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.                final int totalLength = mTotalLength;                mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);                skippedMeasure = true;            } else {                int oldHeight = Integer.MIN_VALUE;                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) {                   lp.height = oldHeight;                }                final int childHeight = child.getMeasuredHeight();                final int totalLength = mTotalLength;                mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +                       lp.bottomMargin + getNextLocationOffset(child));                if (useLargestChild) {                    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;        }        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;        if (skippedMeasure || delta != 0 && totalWeight > 0.0f) {            float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;            mTotalLength = 0;            for (int i = 0; i < count; ++i) {                final View child = getVirtualChildAt(i);                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                    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                    if ((lp.height != 0) || (heightMode != MeasureSpec.EXACTLY)) {                        // child was measured once already above...                        // base new measurement on stored values                        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);        }    }

整个方法代码不长，就300行左右，但是思路十分清晰 我们根据流程，一步一步来看

首先是初始化了一堆变量
我们挑几个重要的看

//记录内部使用的高度，别被字面意思误导了以为是LinearLayout的高度 mTotalLength = 0; //权重值的总和 float totalWeight = 0; //子view的数量， final int count = getVirtualChildCount(); //其实调用的都是getChildCount()，外面套一层getVirtualChildCount()    //可能是为了让读者更好的理解 int getVirtualChildCount() {        return getChildCount();    } //LinearLayout的高度模式和宽度模式 //如果这部分知识不理解的需要去看下Measure的过程       final int widthMode = MeasureSpec.getMode(widthMeasureSpec); final int heightMode=MeasureSpec.getMode(heightMeasureSpec);

初始化变量之后，就开始遍历所有子View了，然后对子View进行测量

             //首先把子View取出來            final View child = getVirtualChildAt(i);            //如果子View是null就继续测量下一个子View            if (child == null) {                mTotalLength += measureNullChild(i);                continue;            }            //如果子View是GONE的也不算在总高度里面，这里也能看出GONE和INVISIBLE的区别            if (child.getVisibility() == View.GONE) {               i += getChildrenSkipCount(child, i);               continue;            }            //如果有分割线，就把分割线高度加上            if (hasDividerBeforeChildAt(i)) {                mTotalLength += mDividerHeight;            }

然后

//有时候我们在代码里面通过Inflater服务，动态加载一个布局，然后去设置他的LayoutParams，如果不引用父容器的LayoutParams就会报一个强转错误，原因就在这个 父容器在add,measure的时候都会把子View的LayoutParams强转成自己的类型LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();//计入总权重totalWeight += lp.weight;//这里就值得注意下了如果当前的LinearLayout是EXACTLY模式，且子view的高度为0，且权重大于0//这个子view只有在LinearLayout高度有剩余的时候，才会根据权重的占比去平分剩余空间//上文说的二次测量也就指的这部分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.    final int totalLength = mTotalLength;    mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);    skippedMeasure = true;    }

//如果不是上述的情况else {    int oldHeight = Integer.MIN_VALUE;        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     //这里其实官方的注释讲了也挺清楚的，到了这步，当前的LinearLayout的模式     //肯定是UNSPECIFIED或者MOST，因为EXACTLY模式会进入上一个判断     //然后把子View的高度赋值成-1(WRAP_CONTENT)        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).  //这里就开始测算子View了  //如果当前的LinearLayout不是EXACTLY模式，且子View的weight大于0，优先会把当前LinearLayout的全部可用高度用于子View测量        measureChildBeforeLayout(          child, i, widthMeasureSpec, 0, heightMeasureSpec,          totalWeight == 0 ? mTotalLength : 0);// 重置子控件高度，然后进行精确赋值        if (oldHeight != Integer.MIN_VALUE) {            lp.height = oldHeight;        }       final int childHeight = child.getMeasuredHeight();       final int totalLength = mTotalLength;       //加上子View的margin值       mTotalLength = Math.max(totalLength, totalLength+childHeight + lp.topMargin +lp.bottomMargin + getNextLocationOffset(child));      if (useLargestChild) {           largestChildHeight = Math.max(childHeight, largestChildHeight);           }     }

//totalWeight == 0 ? mTotalLength : 0//这里的totalHeight就是有这个决定的//如果为0，就会把所有可用高度给子Viewvoid measureChildBeforeLayout(View child, int childIndex,            int widthMeasureSpec, int totalWidth, int heightMeasureSpec,int totalHeight) {        measureChildWithMargins(child, widthMeasureSpec, totalWidth, heightMeasureSpec, totalHeight);    }//接着看这个方法是如何实现的protected void measureChildWithMargins(View child,            int parentWidthMeasureSpec, int widthUsed,            int parentHeightMeasureSpec, int heightUsed) {    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();    //这个就和普通的ViewGroup实现方法一样了，根据子View的LayoutParams和父容器的MeasureSpec共同决定了子View的大小    //getChildMeasureSpec是属于ViewGroup的方法    final int childWidthMeasureSpec = getChildMeasureSpec(        parentWidthMeasureSpec,        mPaddingLeft + mPaddingRight + lp.leftMargin +     final int childHeightMeasureSpec getChildMeasureSpec(        parentHeightMeasureSpec,        mPaddingTop + mPaddingBottom + lp.topMargin        + lp.bottomMargin + heightUsed, lp.height);        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);    }

//在这两段代码之间还有些杂七杂八的处理，如果读者有兴趣可以自己阅读分析下//当测量完子View的大小后，总高度会再加上padding的高度// Add in our padding        mTotalLength += mPaddingTop + mPaddingBottom;        int heightSize = mTotalLength;        // Check against our minimum height        //如果设置了minimumheight属性，会根据当前使用高度和最小高度进行比较        //然后取两者中大的值        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.        //到了这里，会再对带weight属性的子View进行一次测绘        //首先计算属于高度        int delta = heightSize - mTotalLength;        if (skippedMeasure || delta != 0 && totalWeight > 0.0f) {        //如果设置了weightSum就会使用你设置的weightSum，否则采用当前所有子View的权重和。所以如果要手动设置weightSum的时候，千万别计算错误哦            float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;            mTotalLength = 0;        //这里的代码就和第一次测量很像了            for (int i = 0; i < count; ++i) {                final View child = getVirtualChildAt(i);                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                    //子控件的weight占比*剩余高度                    int share = (int) (childExtra * delta / weightSum);                    weightSum -= childExtra;                    //剩余高度减去分配出去的高度                    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                    //如果是当前LinearLayout的模式是EXACTLY                    //那么这个子View是没有被测量过的，就需要测量一次                    //如果不是EXACTLY的，在第一次循环里就被测量一些了                    if ((lp.height != 0) || (heightMode != MeasureSpec.EXACTLY)) {                        // child was measured once already above...                        // base new measurement on stored values                        //如果是非EXACTLY模式下的子View就再加上                        //weight分配占比*剩余高度                        int childHeight = child.getMeasuredHeight() + share;                        if (childHeight < 0) {                            childHeight = 0;                        }                       //重新测量一次，因为高度发生了变化                        child.measure(childWidthMeasureSpec,                                MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY));                    } else {                        // child was skipped in the loop above.                        // Measure for this first time here                              //如果是EXACTLY模式下的                        //这里只会把weight占比所拥有的高度分配给你的子View                        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?        }

上述就是onMeasure的主要分析
注意点
1.会根据当前LinearLayout的模式分成2条支线，如果是EXACTLY模式下的weight不为0，且高度设置为0的子View优先级是最低的。如果LinearLayout剩余空间不足，就会不显示
但是如果是AT_MOST的weight不为0，且高度设置为0就会优先获得高度
2.为LinearLayout动态添加子View的时候，子View的LayoutParams一定要是LinearLayout的内部类（适用于其他ViewGroup子类）

举个例子

所有的源码解析都是我们自己根据代码的推论，配合几个demo跑下会理解了更深

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical">    <TextView        android:layout_width="match_parent"        android:layout_height="300dp"        android:layout_weight="2"        android:background="@android:color/holo_orange_dark" />    <TextView        android:layout_width="match_parent"        android:layout_height="fill_parent"        android:background="@android:color/holo_blue_dark" /></LinearLayout>

在分析之前大伙儿先预测下结果，思索下再往下看

假设我们的屏幕是1000dp的高度

父容器LinearLayout是EXACTLY模式，但是TextView1本身的height是300dp，所以会进入第一次测量
TextView1 在第一次测算时拿到了300dp的高度

然后TextView因为是match，所以会拿到1000dp的高度

然后由于有weight的子view，所以进入第二次测量

int delta = heightSize - mTotalLength;

delta = 1000 - 1300; // 结果是-300

int share = (int) (childExtra * delta / weightSum);

share = 2.0 * -300 / 2.0 //share的结果也是-300

MeasureSpec.makeMeasureSpec(share > 0 ? share : 0,
MeasureSpec.EXACTL);
所以最后TextView1的高度是0.不会显示在屏幕上，屏幕上应该被Textview2充满。
不信的话，大家可以试一下

好了LinearLayout的主要测量就讲到这了

onLayout()和onDraw()相对来说就比较简单，大家自己看下就好了