RecyclerView简介
RecyclerView是一种新的视图组，目标是为任何基于适配器的视图提供相似的渲染方式。它被作为ListView和GridView控件的继承者，具有更优的灵活性与可替代性。在最新的support-v7版本中提供支持。本文将讲解RecyclerView的简单实现，添加删除条目，点击事件添加与瀑布流的实现。
相关原理与简单实现
添加依赖
在AndroidStudio的build.gradle中添加依赖：
dependencies {
...
compile 'com.android.support:recyclerview-v7:25.3.0'
}
在布局中使用
添加完依赖后就可以在布局中使用RecyclerView了：
RecyclerView.Adapter
RecyclerView封装了一种新型的适配器，与现在使用的适配器大同小异。它强制用户使用RecyclerView提供的ViewHolder，使用时主要需要重写onCreateViewHolder与onBindViewHolder方法。前者用来展现视图及其持有者，且只有真正需要一个新view时才会被回调，不需要检查是否已经被回收。后者用来绑定数据到View上。
import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import java.util.List;
public class MyAdapter extends RecyclerView.Adapter {
private Context context;
private List list;
private LayoutInflater inflater;
public MyAdapter(Context context, List list) {
this.context = context;
this.list = list;
inflater = LayoutInflater.from(context);
}
@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = inflater.inflate(R.layout.item_recyclerview, parent, false);
MyViewHolder holder = new MyViewHolder(view);
return holder;
}
@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
holder.textView.setText(list.get(position));
}
@Override
public int getItemCount() {
return list.size();
}
class MyViewHolder extends RecyclerView.ViewHolder {
TextView textView;
public MyViewHolder(View itemView) {
super(itemView);
textView = (TextView) itemView.findViewById(R.id.item_textview);
}
}
}
LayoutManager
RecyclerView通过布局管理器LayoutManager控制每一个item如何进行排列摆放，何时展示和隐藏。回收或重用一个View时LayoutManager会向适配器请求新的数据来替换旧的数据，这种机制避免了创建过多的View和频繁的调用findViewById方法，目前其自带的主要有以下三种：
- LinearLayoutManager：ListView样式
- GridLayoutManager：GridView样式
- StaggeredGridLayoutManager：瀑布流样式
ItemDecoration
RecyclerView并不能像ListView一样直接在xml布局中修改item分割线样式。需要在Activity动态设置，当然更推荐在单个条目布局中设置margin或者padding来实现分割线效果，这里提供一个分割线。
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.View;
public class DividerItemDecoration extends RecyclerView.ItemDecoration {
private static final int[] ATTRS = new int[]{
android.R.attr.listDivider
};
public static final int HORIZONTAL_LIST = LinearLayoutManager.HORIZONTAL;
public static final int VERTICAL_LIST = LinearLayoutManager.VERTICAL;
private Drawable mDivider;
private int mOrientation;
public DividerItemDecoration(Context context, int orientation) {
final TypedArray a = context.obtainStyledAttributes(ATTRS);
mDivider = a.getDrawable(0);
a.recycle();
setOrientation(orientation);
}
public void setOrientation(int orientation) {
if (orientation != HORIZONTAL_LIST && orientation != VERTICAL_LIST) {
throw new IllegalArgumentException("invalid orientation");
}
mOrientation = orientation;
}
@Override
public void onDraw(Canvas c, RecyclerView parent) {
if (mOrientation == VERTICAL_LIST) {
drawVertical(c, parent);
} else {
drawHorizontal(c, parent);
}
}
public void drawVertical(Canvas c, RecyclerView parent) {
final int left = parent.getPaddingLeft();
final int right = parent.getWidth() - parent.getPaddingRight();
final int childCount = parent.getChildCount();
for (int i = 0; i < childCount; i++) {
final View child = parent.getChildAt(i);
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child .getLayoutParams();
final int top = child.getBottom() + params.bottomMargin;
final int bottom = top + mDivider.getIntrinsicHeight();
mDivider.setBounds(left, top, right, bottom);
mDivider.draw(c);
}
}
public void drawHorizontal(Canvas c, RecyclerView parent) {
final int top = parent.getPaddingTop();
final int bottom = parent.getHeight() - parent.getPaddingBottom();
final int childCount = parent.getChildCount();
for (int i = 0; i < childCount; i++) {
final View child = parent.getChildAt(i);
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child .getLayoutParams();
final int left = child.getRight() + params.rightMargin;
final int right = left + mDivider.getIntrinsicHeight();
mDivider.setBounds(left, top, right, bottom);
mDivider.draw(c);
}
}
@Override
public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {
if (mOrientation == VERTICAL_LIST) {
outRect.set(0, 0, 0, mDivider.getIntrinsicHeight());
} else {
outRect.set(0, 0, mDivider.getIntrinsicWidth(), 0);
}
}
}
ItemAnimator
ItemAnimator会根据适配器上收到的通知来动画显示视图组的修改，比如item的添加与删除。DefaultItemAnimator已经能很好的展现动画效果了。
RecyclerView初始化
若想初始化一个RecyclerView使其进入工作状态，你需要在Activity中做以下的操作：
RecyclerView recyclerView = (RecyclerView)findViewById(R.id.recycler_view_main);
MyAdapter adapter = new MyAdapter(this, list);
//设置RecyclerView保持固定的大小
recyclerView.setHasFixedSize(true);
//设置适配器
recyclerView.setAdapter(adapter);
//设置RecyclerView ListView样式布局管理
recyclerView.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false));
//设置RecyclerView的Item分割线
recyclerView.addItemDecoration(new DividerItemDecoration(this, DividerItemDecoration.VERTICAL_LIST));
//设置RecyclerView的动画
recyclerView.setItemAnimator(new DefaultItemAnimator());
//设置RecyclerView GridView样式
//recyclerView.setLayoutManager(new GridLayoutManager(MainActivity.this, 3));
//设置RecyclerView 水平GridView样式
//recyclerView.setLayoutManager(new StaggeredGridLayoutManager(5, StaggeredGridLayoutManager.HORIZONTAL));
//设置RecyclerView 瀑布流样式
//recyclerView.setLayoutManager(new StaggeredGridLayoutManager(3,StaggeredGridLayoutManager.VERTICAL));
点击事件与添加删除Item
美中不足的是RecyclerView并没有提供像ListView一样的Item点击与Item长点击事件，不提供咱们就自己造，通过接口回调来实现。
Adapter中
import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import java.util.ArrayList;
public class MyAdapter extends RecyclerView.Adapter {
private Context context;
private LayoutInflater inflater;
protected ArrayList datas;
private onItemClickedListener onItemClickedListener;
public MyAdapter(Context context, ArrayList datas) {
this.context = context;
this.datas = datas;
inflater = LayoutInflater.from(context);
}
public void setOnItemClickedListener(MyAdapter.onItemClickedListener onItemClickedListener) {
this.onItemClickedListener = onItemClickedListener;
}
@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
holder.textView.setText(datas.get(position));
}
@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = inflater.inflate(R.layout.item_recycler_view, parent, false);
MyViewHolder myViewHolder = new MyViewHolder(view);
return myViewHolder;
}
@Override
public int getItemCount() {
return datas.size();
}
/**
* 添加条目
*/
public void addItem(int position) {
datas.add(position, "xulei");
// notifyDataSetChanged();
notifyItemInserted(position);//调用这个才有动画效果
}
/**
* 移除条目
*/
public void removeItem(int position) {
datas.remove(position);
notifyItemRemoved(position);
}
class MyViewHolder extends RecyclerView.ViewHolder {
TextView textView;
public MyViewHolder(View itemView) {
super(itemView);
//初始化控件
textView = (TextView) itemView.findViewById(R.id.item_textview);
//设置当前条目单击监听
itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
if (onItemClickedListener != null)
onItemClickedListener.onClick(view, getAdapterPosition());//可立刻获取到当前position
// onItemClickedListener.onClick(view, getLayoutPosition());//需等当前视图更新完才能获取到当前position，<16ms。
}
});
//设置当前条目长按监听
itemView.setOnLongClickListener(new View.OnLongClickListener() {
@Override
public boolean onLongClick(View view) {
if (onItemClickedListener != null)
onItemClickedListener.onLongClick(view, getAdapterPosition());
return false;
}
});
}
}
/**
* 点击回调的接口
*/
interface onItemClickedListener {
void onClick(View view, int position);
void onLongClick(View view, int position);
}
}
Activity中
在Activity中实例化Adapter之后添加如下代码：
adapter.setOnItemClickedListener(new MyAdapter.onItemClickedListener() {
@Override
public void onClick(View view, int position) {
adapter.addItem(position);
Toast.makeText(MainActivity.this, "点击click:" + position, Toast.LENGTH_SHORT).show();
}
@Override
public void onLongClick(View view, int position) {
adapter.removeItem(position);
Toast.makeText(MainActivity.this, "长按click:" + position, Toast.LENGTH_SHORT).show();
}
});
瀑布流的实现
想实现瀑布流的样式通过使用RecyclerView也很容易就能实现。
首先在Activity中设置LayoutManager时选择：
recyclerView.setLayoutManager(new StaggeredGridLayoutManager(3,StaggeredGridLayoutManager.VERTICAL));
修改Adapter
瀑布流自然是每个条目的高度不同才能出现瀑布的效果（水平布局则是宽度不同），那么只需在每个条目绑定数据时动态改变下其高度即可，贴出瀑布流Adapter代码：
import android.content.Context;
import android.view.ViewGroup;
import java.util.ArrayList;
import java.util.List;
public class StaggerAdapter extends MyAdapter {
private List heights;
public StaggerAdapter(Context context, ArrayList datas) {
super(context, datas);
heights = new ArrayList<>();
for (int i = 0; i < datas.size(); i++) {
heights.add((int) (100 + Math.random() * 300));
}
}
@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
ViewGroup.LayoutParams layoutParams = holder.itemView.getLayoutParams();
layoutParams.height = heights.get(position);
// layoutParams.width = heights.get(position);
holder.itemView.setLayoutParams(layoutParams);
holder.textView.setText(datas.get(position));
}
}
总结
通过实践发现RecyclerView相较于ListView与GridView确实强大很多，更加的灵活与方便，提高了开发效率。但也有不足之处，如并未封装点击事件的回调，确实是比较头疼。期待Google的完善。
附上GitHub源码：
RecyclerViewDemo
RecyclerViewDevelop