在使用android提供的组件以列表的格式显示数据时,使用过ListView组件和RecyclerView组件。目前一般推荐使用RecyclerView,因为RecyclerView本身的缓存和效率比ListView高,且支持灵活的布局方式,所以会被大家采用。相信大家在使用ListView时,如果要显示的数据多,肯定多会想到优化Adaper的getView()方法,下面给出一个例子:
public class UsersAdapter extends ArrayAdapter<User> {
private static class ViewHolder {
TextView name;
TextView home;
}
public UsersAdapter(Context context, ArrayList<User> users) {
super(context, R.layout.item_user, users);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
User user = getItem(position);
ViewHolder viewHolder;
if (convertView == null) {
viewHolder = new ViewHolder();
LayoutInflater inflater = LayoutInflater.from(getContext());
convertView = inflater.inflate(R.layout.item_user, parent, false);
viewHolder.name = (TextView) convertView.findViewById(R.id.tvName);
viewHolder.home = (TextView)convertView.findViewById(R.id.tvHome);
convertView.setTag(viewHolder);
} else {
viewHolder = (ViewHolder) convertView.getTag();
}
viewHolder.name.setText(user.name);
viewHolder.home.setText(user.hometown);
return convertView;
}
}
在这边,我们在ListView需要用到的Adapter中定义了一个内部类ViewHolder,它存储了我们要加载的view的所有子view结构,如果这个view已经被加载过只是暂时被回收, 当需要再次展示的话我们就不需要重新加载整个view,也不需要通过findViewById()来寻找要加载的view的子view,可以直接找到这个view,将要展示的数据设置即可返回显示。
但是在RecyclerView中,我们并不需要做这么多,我们先看一个RecyclerView的简单使用步骤:
定义一个RecyclerView的布局文件以及要展示的item的布局文件
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<android.support.v7.widget.RecyclerView
android:id="@+id/t_classList"
android:layout_width="match_parent"
android:layout_height="match_parent">
</android.support.v7.widget.RecyclerView>
</LinearLayout>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_marginStart="20dp"
android:layout_marginLeft="20dp">
<TextView
android:id="@+id/t_class_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:textSize="19sp" />
<View
android:layout_width="match_parent"
android:layout_height="0.5px"
android:background="@color/whiteGray"
android:layout_marginTop="6dp"
android:layout_marginBottom="6dp"
android:typeface="serif"/>
</LinearLayout>
定义一个adapter
public class ClassAdapter extends RecyclerView.Adapter<ClassAdapter.ClassViewHolder> {
private List<ClassVO> classVOs;
static class ClassViewHolder extends RecyclerView.ViewHolder {
View classView;
TextView className;
public ClassViewHolder(View itemView) {
super(itemView);
classView = itemView;
className = (TextView) itemView.findViewById(R.id.t_class_name);
}
}
public ClassAdapter(List<ClassVO> classVOs){
this.classVOs = classVOs;
}
@Override
public ClassViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.fragement_class_item, null, false);
final ClassViewHolder holder = new ClassViewHolder(view);
holder.classView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
int position = holder.getAdapterPosition();
ClassVO classVO = classVOs.get(position);
Toast.makeText(view.getContext(), "you click class "+classVO.getId(), Toast.LENGTH_SHORT).show();
}
});
return holder;
}
@Override
public void onBindViewHolder(ClassViewHolder holder, int position) {
ClassVO classVO = classVOs.get(position);
holder.className.setText(classVO.getName());
}
@Override
public int getItemCount() {
return classVOs.size();
}
}
设置RecyclerView的布局和adapter
recyclerView = (RecyclerView) classListView.findViewById(R.id.t_classList);
layoutManager = new LinearLayoutManager(this.getContext());
recyclerView.setLayoutManager(layoutManager);
if (classVOs!=null){
adapter = new ClassAdapter(classVOs);
}
recyclerView.setAdapter(adapter);
既然我们要说的是RecyclerView中的ViewHolder,但是我们的使用步骤中并没有单独提出ViewHolder,因为在我们上面说的使用步骤的第二步———“定义一个adapter”,就涉及到ViewHolder。
当我们自定义一个adapter时,继承了RecyclerView的一个内部类:
/**
* Base class for an Adapter
*
* <p>Adapters provide a binding from an app-specific data set to views that are displayed
* within a {@link RecyclerView}.</p>
*/
public static abstract class Adapter<VH extends ViewHolder> {
private final AdapterDataObservable mObservable = new AdapterDataObservable();
private boolean mHasStableIds = false;
/**
* Called when RecyclerView needs a new {@link ViewHolder} of the given type to represent
* an item.
* <p>
* This new ViewHolder should be constructed with a new View that can represent the items
* of the given type. You can either create a new View manually or inflate it from an XML
* layout file.
* <p>
* The new ViewHolder will be used to display items of the adapter using
* {@link #onBindViewHolder(ViewHolder, int, List)}. Since it will be re-used to display
* different items in the data set, it is a good idea to cache references to sub views of
* the View to avoid unnecessary {@link View#findViewById(int)} calls.
*
* @param parent The ViewGroup into which the new View will be added after it is bound to
* an adapter position.
* @param viewType The view type of the new View.
*
* @return A new ViewHolder that holds a View of the given view type.
* @see #getItemViewType(int)
* @see #onBindViewHolder(ViewHolder, int)
*/
public abstract VH onCreateViewHolder(ViewGroup parent, int viewType);
/**
* Called by RecyclerView to display the data at the specified position. This method should
* update the contents of the {@link ViewHolder#itemView} to reflect the item at the given
* position.
* <p>
* Note that unlike {@link android.widget.ListView}, RecyclerView will not call this method
* again if the position of the item changes in the data set unless the item itself is
* invalidated or the new position cannot be determined. For this reason, you should only
* use the <code>position</code> parameter while acquiring the related data item inside
* this method and should not keep a copy of it. If you need the position of an item later
* on (e.g. in a click listener), use {@link ViewHolder#getAdapterPosition()} which will
* have the updated adapter position.
*
* Override {@link #onBindViewHolder(ViewHolder, int, List)} instead if Adapter can
* handle efficient partial bind.
*
* @param holder The ViewHolder which should be updated to represent the contents of the
* item at the given position in the data set.
* @param position The position of the item within the adapter's data set.
*/
public abstract void onBindViewHolder(VH holder, int position);
/**
* Called by RecyclerView to display the data at the specified position. This method
* should update the contents of the {@link ViewHolder#itemView} to reflect the item at
* the given position.
* <p>
* Note that unlike {@link android.widget.ListView}, RecyclerView will not call this method
* again if the position of the item changes in the data set unless the item itself is
* invalidated or the new position cannot be determined. For this reason, you should only
* use the <code>position</code> parameter while acquiring the related data item inside
* this method and should not keep a copy of it. If you need the position of an item later
* on (e.g. in a click listener), use {@link ViewHolder#getAdapterPosition()} which will
* have the updated adapter position.
* <p>
* Partial bind vs full bind:
* <p>
* The payloads parameter is a merge list from {@link #notifyItemChanged(int, Object)} or
* {@link #notifyItemRangeChanged(int, int, Object)}. If the payloads list is not empty,
* the ViewHolder is currently bound to old data and Adapter may run an efficient partial
* update using the payload info. If the payload is empty, Adapter must run a full bind.
* Adapter should not assume that the payload passed in notify methods will be received by
* onBindViewHolder(). For example when the view is not attached to the screen, the
* payload in notifyItemChange() will be simply dropped.
*
* @param holder The ViewHolder which should be updated to represent the contents of the
* item at the given position in the data set.
* @param position The position of the item within the adapter's data set.
* @param payloads A non-null list of merged payloads. Can be empty list if requires full
* update.
*/
可以看到,RecyclerView内部已经帮我们定义好了ViewHolder抽象类,当我们自定义Adapter时,需要定义好要继承ViewHolder的类(Adapter<VH extends ViewHolder>)。
ViewHolder的作用我们在上文介绍使使用ListView时已经说到,简而言之就是提高消息,下面我们看一看源码中对ViewHolder的介绍:
/**
* A ViewHolder describes an item view and metadata about its place within the RecyclerView.
*
* <p>{@link Adapter} implementations should subclass ViewHolder and add fields for caching
* potentially expensive {@link View#findViewById(int)} results.</p>
*
* <p>While {@link LayoutParams} belong to the {@link LayoutManager},
* {@link ViewHolder ViewHolders} belong to the adapter. Adapters should feel free to use
* their own custom ViewHolder implementations to store data that makes binding view contents
* easier. Implementations should assume that individual item views will hold strong references
* to <code>ViewHolder</code> objects and that <code>RecyclerView</code> instances may hold
* strong references to extra off-screen item views for caching purposes</p>
*/
public static abstract class ViewHolder {
......
}
可以看出两点重要的地方:
Adapter implementations should subclass ViewHolder and add fields for caching potentially expensive {@link View#findViewById(int)} results(adapter应当拥有ViewHolder的子类,并且ViewHolder内部应当存储一些子view,避免时间代价很大的findViewById操作)
Adapters should feel free to use their own custom ViewHolder implementations to store data that makes binding view content easier
其RecyclerView内部定义的ViewHolder类包含很多复杂的属性,内部使用场景也有很多,而我们经常使用的也就是onCreateViewHolder()方法和onBindViewHolder()方法,onCreateViewHolder()方法在RecyclerView需要一个新类型。item的ViewHolder时调用来创建一个ViewHolder,而onBindViewHolder()方法则当RecyclerView需要在特定位置的item展示数据时调用。
参考文章:
**粗体** _斜体_ [链接](http://example.com) `代码` - 列表 > 引用
。你还可以使用@
来通知其他用户。