Android RecyclerView 中的 item 如何居中？

Question

图中有三个item = = 三个点也算一个。不管怎么弄我都没法把那仨点居中....

我用的 new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false)

分割线：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/divider"
    android:layout_gravity="center_horizontal">

    <TextView
        android:width="7dp"
        android:height="7dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:layout_gravity="center_horizontal"
        android:background="@drawable/round"
        android:layout_marginBottom="3dp"/>

    <TextView
        android:layout_marginBottom="3dp"
        android:width="7dp"
        android:height="7dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:layout_gravity="center_horizontal"
        android:background="@drawable/round" />

    <TextView
        android:width="7dp"
        android:height="7dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:layout_gravity="center_horizontal"
        android:background="@drawable/round" />
</LinearLayout>

RecyclerView:

    <LinearLayout
        android:orientation="vertical"
        android:layout_below="@+id/type"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/result_container"
        android:layout_above="@+id/actions">

        <android.support.v7.widget.RecyclerView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:id="@+id/scan_result"
            />

    </LinearLayout>

Answer 1

result = new DividerHolder(mInflater.inflate(R.layout.divider, null));

把parent传进去，如：

result = new DividerHolder(mInflater.inflate(R.layout.divider, parent, false));

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。

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！

Answer 2

把LinearLayout的android:layout_gravity="center_horizontal"改为android:gravity="center_horizontal"。另外，三个TextView的android:layout_gravity="center_horizontal"可以去掉，对于LinearLayout的子布局，这个属性无效

Answer 3

没仔细看代码，直接丢到AS里，文字是可以居中的。。。
Ps. 为什么不用RelativeLayout ，多好控制。。。