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 ，突然，测试经理老石跑过来说客户报上来一个线上bug，小张突然菊花一紧，说不可能，我的代码是不可能有问题的。然后老石从胯下 口袋 掏出来手机，三下五除二将bug复现出来了：
情景再现：
从图中我们明显能看到，底部的服务弹框被裁切挡住了，显示的不完整。
分析原因：
看到复原的犯罪现场，小张心里一个放松：“还好还好，不是我写的代码。”当着老石的面，小张承认是代码出了问题，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)的高度太小了，造成了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，涉事代码如下：
<?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>
复制代码看到上面Dialog的布局xml文件中的ListViewForScrollView小张心里不禁泛起了嘀咕：“怎么又是自定义的View，又要看一大堆逻辑了，脑壳疼，脑壳疼...”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，现在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，代码量并不多，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);
}
}
复制代码看到这段代码，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？”
为了解决心里的疑惑，小张决定从这个ListViewForScrollView唯一重写的方法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
我们都知道，View展示在手机上，需要经历测量-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()过程中，我们就需要借助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，其实是由父控件调用，那View的大小难道就只能有父控件决定么？其实不然。我们刚才也说了，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。通过MeasureSpec这个静态类，我们很容易获取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。这种情况一般用于系统内部，表示一种测量状态。（这个模式主要用于系统内部多次Measure的情形，并不是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，得看你自己的实现。
父控件通过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(由于小张terrible的英语水平，就不给大家翻译注释了，大家可以自行有道～)：
/**
* 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);
}
复制代码该方法通过将父控件的测量规格和childview的布局参数LayoutParams相结合，得到一个最可能符合条件的child view的测量规格。
通过以上代码，大家可以有一个概念：就是一个View的大小是由它的父控件加上自身的LayoutParams决定的。详情如下(图片来源于任玉刚博客)：
解决问题
通过了解View的Measure过程，小张知道了，在ListViewForScrollView的onMeasure()方法里，强行将ListView的测量模式改为了AT_MOST，并将测量大小改为了MAX_VALUE >> 2,接着调用父类ListView的onMeasure()方法，我们来看看ListView的onMeasure()方法：
/** 源码来自8.0.0_r4,不同版本可能略有不同 **/
@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 **/
//......
}
复制代码通过上面的代码我们可以看到，最终这个自定义的ListViewForScrollView的高度是由measureHeightOfChildren这个方法确定的，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;
}
复制代码通过查看该方法，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。如果提供了maxHeight，当当前高度达到maxHeight时，测量将停止。而对于ListViewForScrollView，maxHeight就是上面onMeasure()中我们传过来的536870911(即MAX_VALUE >> 2)，如果ListView的itemView的总高度不超过这个值，那么ListView的高度就是所有itemView高度加上分隔符加上padding的高度，否则就为536870911。所以问题的原因就很清楚了，ListViewForScrollView的高度在大多数情况下都为我们指定的536870911，而这种情况下ListView是不能滑动的。
解决问题
通过研究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，并不是ScrollView，所以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
通过这次的血泪教训，小张明白了：
- View的绘制流程
- MeasureSpec的作用
- 使用自定义View的时候要充分理解其作用
