血泪教训：由于不够了解MeasureSpec而引起的生产事故

夭寿啦，出生产事故了

那天，风和日丽，菜鸡Android开发工程师小张开心的写着 bug 代码 ，忽然，测试经理老石跑过来讲客户报上来一个线上bug，小张忽然菊花一紧，说不可能，个人代码是不可能有问题的。而后老石从胯下 口袋 掏出来手机，三下五除二将bug复现出来了：

情景再现：

从图中咱们明显能看到，底部的服务弹框被裁切挡住了，显示的不完整。

分析缘由：

看到复原的犯罪现场，小张内心一个放松：“还好还好，不是我写的代码。”当着老石的面，小张认可是代码出了问题，答应了老石下个版本修复，老石才不依不挠的走开了。打发走了老石，小张开始分析起来问题了。开始，小张简单的认为是底部的弹框(Dialog)的高度过小了，形成了dialog的内容显示不完整。然鹅，看到出问题的代码，小张开始发现事情并不简单，涉事代码以下：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <!-- 省略无关代码 -->
    <com.xx.ListViewForScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginLeft="20dp"
        android:layout_marginTop="18dip"
        android:layout_marginRight="20dp"
        android:layout_marginBottom="48dp"
        android:divider="@null">
    </com.xx.ListViewForScrollView>
</RelativeLayout>
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看到上面Dialog的布局xml文件中的ListViewForScrollView小张内心不由泛起了嘀咕：“怎么又是自定义的View，又要看一大堆逻辑了，脑袋疼，脑袋疼...”转念一想，不对啊，这个View看名字一看就是继承自ListView，就算是Dialog的高度不够，这个ListViewForScrollView也应该能滑动才对啊，如今不能滑动了是怎么一回事，有问题有问题...

不看代码光靠猜是解决不了问题的，小张下定决心去研究研究这个自定义View，打开这个ListViewForScrollView的代码，小张松了一口气：还好还好，代码量并很少，看起来逻辑并不复杂：

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);
	}
}
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看到这段代码，小张更疑惑了：“这不就是继承的ListView么，也没什么特殊的逻辑啊，为啥它就是不能滑动呢？”
为了解决内心的疑惑，小张决定从这个ListViewForScrollView惟一重写的方法onMeasure(int widthMeasureSpec, int heightMeasureSpec)下手，看能不能发现问题。
刚开始看第一段代码，小张就犯难了，这个MeasureSpec是个什么东东？看起来不理解这个MeasureSpec是解决不了问题了，小张决定了解一下这个MeasureSpec：

关于MeasureSpec，小张想说

咱们都知道，View展现在手机上，须要经历测量-绘制-布局三个流程，对应到View的代码中就是onMeasure()、onDraw()、onLayout()三个方法。在onMeasure()过程当中，咱们就须要借助MeasureSpec对View进行测量。
onMeasure()方法接受两个参数：widthMeasureSpec和heightMeasureSpec，这两个参数都是32位的int类型，由View的父控件传过来。重点来了：View的onMeasure()方法，实际上是由父控件调用，那View的大小难道就只能有父控件决定么？其实否则。咱们刚才也说了，widthMeasureSpec和heightMeasureSpec都是32位的int类型，他们的高两位表示测量模式mode，低30位表示测量大小size。经过MeasureSpec这个静态类，咱们很容易获取View的测量模式和测量大小，其中测量模式有3种：

• UNSPECIFIED
  父控件不对你有任何限制，你想要多大给你多大，想上天就上天。这种状况通常用于系统内部，表示一种测量状态。（这个模式主要用于系统内部屡次Measure的情形，并非真的说你想要多大最后就真有多大）

• EXACTLY
  父控件已经知道你所需的精确大小，你的最终大小应该就是这么大。

• AT_MOST
  你的大小不能大于父控件给你指定的size，但具体是多少，得看你本身的实现。

父控件经过ViewGroup中的静态方法getChildMeasureSpec来获取子控件的测量规格。下面是getChildMeasureSpec()的代码(因为小张terrible的英语水平，就不给你们翻译注释了，你们能够自行有道～)：

/**
     * 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;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }
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该方法经过将父控件的测量规格和childview的布局参数LayoutParams相结合，获得一个最可能符合条件的child view的测量规格。
经过以上代码，你们能够有一个概念：就是一个View的大小是由它的父控件加上自身的LayoutParams决定的。详情以下(图片来源于任玉刚博客)：

解决问题

经过了解View的Measure过程，小张知道了，在ListViewForScrollView的onMeasure()方法里，强行将ListView的测量模式改成了AT_MOST，并将测量大小改成了MAX_VALUE >> 2,接着调用父类ListView的onMeasure()方法，咱们来看看ListView的onMeasure()方法：

/** 源码来自8.0.0_r4,不一样版本可能略有不一样 **/
@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        /** 此处省略不相关代码 **/
        //......
        // 测量模式为AT_MOST，测量大小为MAX_VALUE >> 2,也就是536870911
        if (heightMode == MeasureSpec.AT_MOST) {
            heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
        }

        setMeasuredDimension(widthSize, heightSize);
        /** 继续省略 **/
        //......
    }
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经过上面的代码咱们能够看到，最终这个自定义的ListViewForScrollView的高度是由measureHeightOfChildren这个方法肯定的，小张接着查看这个方法：

/**
     * Measures the height of the given range of children (inclusive) and
     * returns the height with this ListView's padding and divider heights * included. If maxHeight is provided, the measuring will stop when the * current height reaches maxHeight. * * @param widthMeasureSpec The width measure spec to be given to a child's
     *            {@link View#measure(int, int)}.
     * @param startPosition The position of the first child to be shown.
     * @param endPosition The (inclusive) position of the last child to be
     *            shown. Specify {@link #NO_POSITION} if the last child should be
     *            the last available child from the adapter.
     * @param maxHeight The maximum height that will be returned (if all the
     *            children don't fit in this value, this value will be * returned). * @param disallowPartialChildPosition In general, whether the returned * height should only contain entire children. This is more * powerful--it is the first inclusive position at which partial * children will not be allowed. Example: it looks nice to have * at least 3 completely visible children, and in portrait this * will most likely fit; but in landscape there could be times * when even 2 children can not be completely shown, so a value * of 2 (remember, inclusive) would be good (assuming * startPosition is 0). * @return The height of this ListView with the given children. */ final int measureHeightOfChildren(int widthMeasureSpec, int startPosition, int endPosition, int maxHeight, int disallowPartialChildPosition) { final ListAdapter adapter = mAdapter; if (adapter == null) { return mListPadding.top + mListPadding.bottom; } // Include the padding of the list int returnedHeight = mListPadding.top + mListPadding.bottom; final int dividerHeight = mDividerHeight; // The previous height value that was less than maxHeight and contained // no partial children int prevHeightWithoutPartialChild = 0; int i; View child; // mItemCount - 1 since endPosition parameter is inclusive endPosition = (endPosition == NO_POSITION) ? adapter.getCount() - 1 : endPosition; final AbsListView.RecycleBin recycleBin = mRecycler; final boolean recyle = recycleOnMeasure(); final boolean[] isScrap = mIsScrap; for (i = startPosition; i <= endPosition; ++i) { child = obtainView(i, isScrap); measureScrapChild(child, i, widthMeasureSpec, maxHeight); if (i > 0) { // Count the divider for all but one child returnedHeight += dividerHeight; } // Recycle the view before we possibly return from the method if (recyle && recycleBin.shouldRecycleViewType( ((LayoutParams) child.getLayoutParams()).viewType)) { recycleBin.addScrapView(child, -1); } returnedHeight += child.getMeasuredHeight(); if (returnedHeight >= maxHeight) { // We went over, figure out which height to return. If returnedHeight > maxHeight, // then the i'th position did not fit completely.
                return (disallowPartialChildPosition >= 0) // Disallowing is enabled (> -1)
                            && (i > disallowPartialChildPosition) // We've past the min pos && (prevHeightWithoutPartialChild > 0) // We have a prev height && (returnedHeight != maxHeight) // i'th child did not fit completely
                        ? prevHeightWithoutPartialChild
                        : maxHeight;
            }

            if ((disallowPartialChildPosition >= 0) && (i >= disallowPartialChildPosition)) {
                prevHeightWithoutPartialChild = returnedHeight;
            }
        }

        // At this point, we went through the range of children, and they each
        // completely fit, so return the returnedHeight
        return returnedHeight;
    }
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经过查看该方法，小张知道了这个方法的意义就是测量给定范围的高度，并返回包含此ListView的填充和分隔符高度的高度。若是提供了maxHeight，当当前高度达到maxHeight时，测量将中止。而对于ListViewForScrollView，maxHeight就是上面onMeasure()中咱们传过来的536870911(即MAX_VALUE >> 2)，若是ListView的itemView的总高度不超过这个值，那么ListView的高度就是全部itemView高度加上分隔符加上padding的高度，不然就为536870911。因此问题的缘由就很清楚了，ListViewForScrollView的高度在大多数状况下都为咱们指定的536870911，而这种状况下ListView是不能滑动的。

解决问题

经过研究ListViewForScrollView，小张发现这个自定义View是用来解决ScrollView嵌套ListView带来的滑动冲突等问题的，而出问题的Dialog根布局是RelativeLayout，并非ScrollView，因此小张将布局文件中的ListViewForScrollView换成了ListView，问题圆满解决了。

吃一堑长一智

经过此次的血泪教训，小张明白了：

1. View的绘制流程
2. MeasureSpec的做用
3. 使用自定义View的时候要充分理解其做用