Scrolview 嵌套ListView 导致ListView条目不能展开的问题及原理详解

以前开发遇到ScrollView嵌套ListView的问题，虽然一脸懵逼啥也不懂，但是只要Google，问题总是可以解决，但是如果不深究只是拷贝的话，那么以后遇到还是一脸懵逼，今天无意中遇到了有人问这样的问题，那么就我就来解答一下部分人的疑惑。
1、 Question——问题
ScrollView嵌套ListView的时候ListView的条目只显示一个，而且快速滑动的时候也会出问题。
2、 How to do—我们该怎么做
我们Google到的这类问题的解决方法一般是Override ListView的onMeasure(int widthMeasureSpec,int heightMeasureSpec)方法，代码如下：

public class ListViewForScrollView extends ListView {    public ListViewForScrollView(Context context) {        super(context);    }    public ListViewForScrollView(Context context, AttributeSet attrs) {        super(context, attrs);    }    public ListViewForScrollView(Context context, AttributeSet attrs,                                 int defStyle) {        super(context, attrs, defStyle);    }    @Override    /**     * 重写该方法，达到使ListView适应ScrollView的效果     */    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2,                MeasureSpec.AT_MOST);        super.onMeasure(widthMeasureSpec, expandSpec);    }}

3、Why ：我们为什么要这么写呢？
要想知道我们为什么这么写，就得先知道为什么会出现这样的问题，看完各种各样的说话以后，再经过我们的大脑处理，我们可以确定，原来是由于ScrollView事件消费以及ListView的高度不确定导致的，那么为什么我们通过上面的简单的代码就搞定了呢？让我们走进MeasureSpec源码去一探究竟。。。

/**     * 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 &lt;size, mode&gt; tuple into the int.     */    public static class MeasureSpec {        private static final int MODE_SHIFT = 30;        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;        /** @hide */        @IntDef({UNSPECIFIED, EXACTLY, AT_MOST})        @Retention(RetentionPolicy.SOURCE)        public @interface MeasureSpecMode {}        /**         * 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;        /**         * 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(@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);            }        }        /**         * Like {@link #makeMeasureSpec(int, int)}, but any spec with a mode of UNSPECIFIED         * will automatically get a size of 0. Older apps expect this.         *         * @hide internal use only for compatibility with system widgets and older apps         */        public static int makeSafeMeasureSpec(int size, int mode) {            if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {                return 0;            }            return makeMeasureSpec(size, mode);        }        /**         * Extracts the mode from the supplied measure specification.         *         * @param measureSpec the measure specification to extract the mode from         * @return {@link android.view.View.MeasureSpec#UNSPECIFIED},         *         {@link android.view.View.MeasureSpec#AT_MOST} or         *         {@link android.view.View.MeasureSpec#EXACTLY}         */        @MeasureSpecMode        public static int getMode(int measureSpec) {            //noinspection ResourceType            return (measureSpec & MODE_MASK);        }        /**         * Extracts the size from the supplied measure specification.         *         * @param measureSpec the measure specification to extract the size from         * @return the size in pixels defined in the supplied measure specification         */        public static int getSize(int measureSpec) {            return (measureSpec & ~MODE_MASK);        }        static int adjust(int measureSpec, int delta) {            final int mode = getMode(measureSpec);            int size = getSize(measureSpec);            if (mode == UNSPECIFIED) {                // No need to adjust size for UNSPECIFIED mode.                return makeMeasureSpec(size, UNSPECIFIED);            }            size += delta;            if (size < 0) {                Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size +                        ") spec: " + toString(measureSpec) + " delta: " + delta);                size = 0;            }            return makeMeasureSpec(size, mode);        }        /**         * Returns a String representation of the specified measure         * specification.         *         * @param measureSpec the measure specification to convert to a String         * @return a String with the following format: "MeasureSpec: MODE SIZE"         */        public static String toString(int measureSpec) {            int mode = getMode(measureSpec);            int size = getSize(measureSpec);            StringBuilder sb = new StringBuilder("MeasureSpec: ");            if (mode == UNSPECIFIED)                sb.append("UNSPECIFIED ");            else if (mode == EXACTLY)                sb.append("EXACTLY ");            else if (mode == AT_MOST)                sb.append("AT_MOST ");            else                sb.append(mode).append(" ");            sb.append(size);            return sb.toString();        }}

代码也不多，100多行，最开始是对MeasureSpec这个类的介绍，大概就是说这个类封装了父布局传递给子布局的一些要求，一个MeasureSpec类需要两个参数，一个是size ，一个是mode,这里给出了三种mode：
1. UNSPECIFIED：我们看注释可以了解到，意思就是说，老子没要求，儿子爱咋滴咋滴
2. EXACTLY：意思就是说，老子是确定的大小，儿子也必须是确定的大小
3. AT_MOST：意思就是说，老子给个最大值，儿子愿意多大就多大，但是别超过老子
然后有3个方法比较重要：
1.public static int makeMeasureSpec(int size, int mode) ，这个方法的作用是根据大小和模式来生成一个int值，这个int值封装了模式和大小信息
2.public static int getMode(int measureSpec)，这个方法的作用是通过一个int值来获取里面的模式信息
3.public static int getSize(int measureSpec)，这个方法的作用是通过一个int值来获取里面的大小信息
在Android里面，一个控件所占的模式和大小是通过一个整数int来表示的，这里很多同学就疑惑了，一个int值是怎么来表示模式的大小的，这里来看一张图片：

private static final int MODE_SHIFT = 30;
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;

不确定模式是0左移30位，也就是int类型的最高两位是00
精确模式是1左移30位，也就是int类型的最高两位是01
最大模式是是2左移30位，也就是int类型的最高两位是10
现在我们知道了，我们肯定是要AT_MOST这个模式了，但是值我们要多大呢？当然是要最大值了，最大值该怎么表示呢？前面我们说过了，Android控件封装了int，用int的最高的两位表示模式，其余的位数表示控件的大小，那么我们要最大值该怎么表示呢？自然就是Integer.MAX_VALUE >> 2这个了，所以最后的表达是：MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2
, MeasureSpec.AT_MOST);，如果不懂移位运行的，不用担心，本博主都给你准备好学习的资料了，点进去，看明白就o了：http://blog.csdn.net/HayJohn/article/details/61929889