一个很简单的Item布局,我只要让它由上而下排列,文字居中
 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    android:orientation="vertical"  
    > 
 
    <TextView 
        android:id="@+id/item_0" 
        android:layout_width="match_parent" 
        android:layout_height="wrap_content" 
        android:gravity="center" 
        android:padding="8dp" 
        android:textColor="@color/grayDark"  
        android:textSize="@dimen/font_2xbig" 
        /> 
 
</LinearLayout> 


然后代码这样写:很标准的使用方式吧?
 
recyclerView = (RecyclerView) findViewById(R.id.recyclerView); 
		recyclerView.setHasFixedSize(true); 
		 recyclerView.addItemDecoration(new DividerGridItemDecoration(this)); 
		recyclerView.setItemAnimator(new DefaultItemAnimator()); 
		LinearLayoutManager manager=new LinearLayoutManager(this,LinearLayoutManager.VERTICAL,false); 
//		GridLayoutManager manager = new GridLayoutManager(this, 2); 
		recyclerView.setLayoutManager(manager); 
recyclerView.setAdapter(adapter...); 


问题来了,无论你怎么设置item中各元素的layout_width是match_parent,都无法让文字居中,为什么?

这个问题还得从android的LayoutInflater.from(context).inflate(...)源码下手,

inflater在inflate一个xml时,需要知道parent的类型,才能生成对应的LayoutParams,才可以把xml根节点的attrs(如layout_width)读进去,代码如下:
 
// android.view.LayoutInflater 
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) { 
        synchronized (mConstructorArgs) { 
     
    ... 
 
    // Temp is the root view that was found in the xml 
    final View temp = createViewFromTag(root, name, inflaterContext, attrs); 
 
    ViewGroup.LayoutParams params = null; 
 
    if (root != null) { 
        if (DEBUG) { 
            System.out.println("Creating params from root: " + 
                    root); 
        } 
        // Create layout params that match root, if supplied 
        params = root.generateLayoutParams(attrs); 
        if (!attachToRoot) { 
            // Set the layout params for temp if we are not 
            // attaching. (If we are, we use addView, below) 
            temp.setLayoutParams(params); 
        } 
    } 
 
    ... 
 
} 

如果parent传进去为null,生成的View的LayoutParams为null,在RecyclerView.addView时,发现LayoutParams为null,则生成默认的LayoutParams,
 
// android.view.ViewGroup 
protected LayoutParams generateDefaultLayoutParams() { 
    return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT); 
} 

所以无论无论你怎么写,最外层的LinearLayout宽度为WRAP_CONTENT,如果那三个点的宽度为6dp,那么整个View的宽度也为6dp,所以无法居中。

所以要解决照这个问题需要在inflate的时候将parent传进去,如:
 
result = new DividerHolder(mInflater.inflate(R.layout.divider, parent, false)); 

同时,最后一个参数设置成false,如果不填该参数则抛异常,说先要removeAllViews()


衍生1,为何ListView加进去就是MATCH_PARENT的?
因为AbsListView重写的generateDefaultLayoutParams方法为
 
// android.widget.AbsListView 
@Override 
protected ViewGroup.LayoutParams generateDefaultLayoutParams() { 
    return new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, 
            ViewGroup.LayoutParams.WRAP_CONTENT, 0); 
} 

衍生2,为何高度只能用minHeight控制?
同理,layout.xml根节点的attrs属性没被写到LayoutParams中!所以使用minHeight来控制高度的做法是可笑的!你所要做的是在inflate时把parent传进去!

RecyclerView自定义LayoutManager,打造不规则布局
http://blog.csdn.net/qibin0506/article/details/52676670
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