ViewGroup的onMeasure()的学习记录

ViewGroup的onMeasure()源码分析

（1）viewgroup是抽象类继承view，自己并没有重写自身onMeasure()方法，而是交给内层子view的onMeasure()自己实现，这是因为viewgroup内层子view的属性不确定性，使得无法做一个统一的测量流程。拿linearLayout举例：需要根据不同的情况（vertical和horizont）分别采取不同的方式测量，这样的测量方式就会异于relativeLayout，所以viewgroup不会有个onMeasure()方法，因此继承viewgroup的类（如linearLayout、relativeLayout等）都自己重写onMeasure()方法。

（2）虽然子view的测量是由自身完成，那是在viewgroup哪里调用的呢？来看看下面几段代码。

 /**     * Ask all of the children of this view to measure themselves, taking into     * account both the MeasureSpec requirements for this view and its padding.     * We skip children that are in the GONE state The heavy lifting is done in     * getChildMeasureSpec.     *     * @param widthMeasureSpec The width requirements for this view     * @param heightMeasureSpec The height requirements for this view     */    protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {        final int size = mChildrenCount;        final View[] children = mChildren;        for (int i = 0; i < size; ++i) {            final View child = children[i];            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {                measureChild(child, widthMeasureSpec, heightMeasureSpec);            }        }    }

2.1.可知此方法是获取子view的总数，然后遍历，传入自身的measureSpec调用 measureChild(...)来测量每个子view，接着，继续跟踪measureChild(...)。

 /**     * Ask one of the children of this view to measure itself, taking into     * account both the MeasureSpec requirements for this view and its padding.     * The heavy lifting is done in getChildMeasureSpec.     *     * @param child The child to measure     * @param parentWidthMeasureSpec The width requirements for this view     * @param parentHeightMeasureSpec The height requirements for this view     */    protected void measureChild(View child, int parentWidthMeasureSpec,            int parentHeightMeasureSpec) {        final LayoutParams lp = child.getLayoutParams();        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,                mPaddingLeft + mPaddingRight, lp.width);        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,                mPaddingTop + mPaddingBottom, lp.height);        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);    }

2.2.该方法主要就是计算获得自身的measureSpec，然后调用自身的measure()方法去测量自身的大小。流程首先获取的该子view的layoutParams，然后根据父容器的measureSpec和自身的layoutParams来计算自身的measureSpec，传入padding是子view自己设置的padding，会影响到自身的可用大小，接下来关注getChildMeasureSpec(...)方法。

 /**     * Does the hard part of measureChildren: figuring out the MeasureSpec to     * pass to a particular child. This method figures out the right MeasureSpec     * for one dimension (height or width) of one child view.     *     * The goal is to combine information from our MeasureSpec with the     * LayoutParams of the child to get the best possible results. For example,     * if the this view knows its size (because its MeasureSpec has a mode of     * EXACTLY), and the child has indicated in its LayoutParams that it wants     * to be the same size as the parent, the parent should ask the child to     * layout given an exact size.     *     * @param spec The requirements for this view     * @param padding The padding of this view for the current dimension and     *        margins, if applicable     * @param childDimension How big the child wants to be in the current     *        dimension     * @return a MeasureSpec integer for the child     */    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 = 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;        }        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);    }

2.3.分析下该方法的流程：

（1）首先根据父容器的spec获取父容器的模式和可用大小（specMode和specSize），然后算出默认最大的可用大小为size。

（2）当父容器模式为EXACTLY：

① 子view有设置尺寸，则resultSize为设置的尺寸（若设置值大于父容器设置的值，则只显示父容器可用大小的部分，超出部分不显示），模式为EXACTLY

② 子view设置match_parent，则resultSize为父容器的可用大小，模式为EXACTLY

③ 子view设置warp_content，则resultSize为父容器的可用大小，模式为AT_MOST，根据源码注释（// It can't be bigger than us）可理解

（3）当父容器模式为AT_MOST：

① 子view有设置尺寸，则resultSize为设置的尺寸,模式为EXACTLY（在viewgr测量完子view后会测量自身的大小从而达到拉伸的效果）

② 子view设置match_parent，则resultSize为父容器的可用大小，模式为AT_MOST，根据源码注释（// Child wants to be our size, but our size is not fixed. Constrain child to not be bigger than us.）可理解

③ 子view设置warp_content，则resultSize为父容器的可用大小，模式为AT_MOST，根据源码注释（// It can't be bigger than us）可理解

（4）最后得到子view的可用大小和模式，调用MeasureSpec.makeMeasureSpec(...)，接着看看该方法做了些什么，不过再此之前先看看MeasureSpec是什么一个东西。

/**     * A MeasureSpec encapsulates the layout requirements passed from parent to child.     * Each MeasureSpec represents a requirement for either the width or the height.     * A MeasureSpec is comprised of a size and a mode. There are three possible     * modes:     * <dl>     * <dt>UNSPECIFIED</dt>     * <dd>     * The parent has not imposed any constraint on the child. It can be whatever size     * it wants.     * </dd>     *     * <dt>EXACTLY</dt>     * <dd>     * The parent has determined an exact size for the child. The child is going to be     * given those bounds regardless of how big it wants to be.     * </dd>     *     * <dt>AT_MOST</dt>     * <dd>     * The child can be as large as it wants up to the specified size.     * </dd>     * </dl>     *     * MeasureSpecs are implemented as ints to reduce object allocation. This class     * is provided to pack and unpack the <size, mode> tuple into the int.     */    public static class MeasureSpec {        private static final int MODE_SHIFT = 30;        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;        /**         * Measure specification mode: The parent has not imposed any constraint         * on the child. It can be whatever size it wants.         */        public static final int UNSPECIFIED = 0 << MODE_SHIFT;        /**         * Measure specification mode: The parent has determined an exact size         * for the child. The child is going to be given those bounds regardless         * of how big it wants to be.         */        public static final int EXACTLY     = 1 << MODE_SHIFT;        /**         * Measure specification mode: The child can be as large as it wants up         * to the specified size.         */        public static final int AT_MOST     = 2 << MODE_SHIFT;......}

（5）先看其说明和成员变量，可以看出它是模式和可用大小的一个封装，高2位代表模式，低30位代表可用大小

① UNSPECIFIED：父容器不限制子view的大小，子view可以是任意大小，一般用在系统内部测量，我们无需太多关注

② EXACTLY：父容器决定了一个精确的size，定义了界限，子view会被赋予这哥界限而忽略自身的想实现的size，对应着match_parent

③ AT_MOST：父容器定义了一个精确的size，子view可以在这个size内想多大就多大，但是不能超过该size

（6）接着回到MeasureSpec.makeMeasureSpec(...)方法看看做了什么处理

 /**         * Creates a measure specification based on the supplied size and mode.         *         * The mode must always be one of the following:         * <ul>         *  <li>{@link android.view.View.MeasureSpec#UNSPECIFIED}</li>         *  <li>{@link android.view.View.MeasureSpec#EXACTLY}</li>         *  <li>{@link android.view.View.MeasureSpec#AT_MOST}</li>         * </ul>         *         * <p><strong>Note:</strong> On API level 17 and lower, makeMeasureSpec's         * implementation was such that the order of arguments did not matter         * and overflow in either value could impact the resulting MeasureSpec.         * {@link android.widget.RelativeLayout} was affected by this bug.         * Apps targeting API levels greater than 17 will get the fixed, more strict         * behavior.</p>         *         * @param size the size of the measure specification         * @param mode the mode of the measure specification         * @return the measure specification based on size and mode         */        public static int makeMeasureSpec(int size, int mode) {            if (sUseBrokenMakeMeasureSpec) {                return size + mode;            } else {                return (size & ~MODE_MASK) | (mode & MODE_MASK);            }        }

（7）由上可知，其实就是把size和mode封装成一个measureSpec

2.4. 回到getChildMeasureSpec(...),它的返回值就是测量后封装好的measureSpec。再回到2.2 的measureChild(...)把算出的子view的宽高spec返回给子view去测量自身，这样viewgroup的测量子view的方法measureChildren(int widthMeasureSpec, int heightMeasureSpec)源码就分析完了。

另：measureChildWithMargins(...)与measureChild(...)代码流程分析基本一样，只是多了margin和已使用的宽高。

 /**     * Ask one of the children of this view to measure itself, taking into     * account both the MeasureSpec requirements for this view and its padding     * and margins. The child must have MarginLayoutParams The heavy lifting is     * done in getChildMeasureSpec.     *     * @param child The child to measure     * @param parentWidthMeasureSpec The width requirements for this view     * @param widthUsed Extra space that has been used up by the parent     *        horizontally (possibly by other children of the parent)     * @param parentHeightMeasureSpec The height requirements for this view     * @param heightUsed Extra space that has been used up by the parent     *        vertically (possibly by other children of the parent)     */    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);    }