本文将带你了解Android应用开发之RecyclerView 中的 item 如何居中问题，希望本文对大家学Android有所帮助。
一个很简单的Item布局，我只要让它由上而下排列，文字居中
Xml代码
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
>
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"
/>
然后代码这样写：很标准的使用方式吧？
Java代码 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...); 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：
Java代码 // 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); } } ... } // 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,
Java代码 // android.view.ViewGroup protected LayoutParams generateDefaultLayoutParams() { return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT); } // 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：
Java代码 result = new DividerHolder(mInflater.inflate(R.layout.divider, parent, false)); 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
Java代码 // android.widget.AbsListView @Override protected ViewGroup.LayoutParams generateDefaultLayoutParams() { return new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT, 0); } // 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
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