做iOS开发时,明白UITableView和UICollectionView的常用性和重要性,它们都是UIScrollView的子类,因为加入cell的重用机制,使得手机能够流畅的显示超过屏幕的内容.同样Android开发中,相对应的就是ListView和RecyclerView.RecyclerView通过设置layoutManager的orientation属性,实现横向滑动,性能也优于ListView.下面通过对同一个Demo的实现,来对比学习和掌握二者.
先看一下最终效果,其中点击SEND按钮,将发送消息并滚动到最后.
首先使用ListView,在layout中创建每一条消息的布局item.xml,代码如下:
使用LinearLayout布局,创建layout_left和layout_right分别用于显示接受的消息和发送的消息.
在activity_main.xml中,使用LinearLayout布局,添加ListView控件和新的LinearLayout布局,添加EidtText和Button用于输入消息和发送消息.
在java文件夹下的文件包中,增加存储消息的Message类.type表示消息的类型,用于控制item中layout的显示.content表示消息的具体内容.
public class Message {
public static final int TYPE_RECEIVED = 1;
public static final int TYPE_SENT = 0;
private String content;
private int type;
public Message(int type, String content) {
this.type = type;
this.content = content;
}
public int getType() {
return this.type;
}
public String getContent() {
return this.content;
}
}
添加继承自ArrayAdapter类的MsgAdapter.重写构造方法
public ArrayAdapter(@NonNull Context context, @LayoutRes int resource,
@NonNull Listobjects)
记录resouce用于getView中View的创建.
已经创建好的item.xml在getView方法中应用,创建用于显示消息的View.
public class MsgAdapter extends ArrayAdapter {
private int resourceId;
public MsgAdapter(Context context, int resouceId, List messages) {
super(context, resouceId, messages);
this.resourceId = resouceId;
}
@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
View view = LayoutInflater.from(parent.getContext()).inflate(resourceId, parent, false);
LinearLayout leftLayout = (LinearLayout)view.findViewById(R.id.layout_left);
LinearLayout rightLayout = (LinearLayout)view.findViewById(R.id.layout_right);
Message msg = getItem(position);
ImageView imageView;
TextView textView;
if (msg.getType() == Message.TYPE_RECEIVED) {
leftLayout.setVisibility(View.VISIBLE);
rightLayout.setVisibility(View.GONE);
imageView = (ImageView)view.findViewById(R.id.imageView_itemLeft);
textView = (TextView)view.findViewById(R.id.textView_itemLeft);
imageView.setImageResource(R.drawable.left);
}else {
leftLayout.setVisibility(View.GONE);
rightLayout.setVisibility(View.VISIBLE);
imageView = (ImageView)view.findViewById(R.id.imageView_itemRight);
textView = (TextView)view.findViewById(R.id.textView_itemRight);
imageView.setImageResource(R.drawable.right);
}
textView.setText(msg.getContent());
return view;
}
}
最后在MainActivity中渲染ListView.
public class MainActivity extends AppCompatActivity {
private List msgList = new ArrayList();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initMessage();
final MsgAdapter msgAdapter = new MsgAdapter(this, R.layout.item, msgList);
final ListView listView = (ListView)findViewById(R.id.listView_msg);
listView.setAdapter(msgAdapter);
final EditText editText = (EditText) findViewById(R.id.editText_msg);
Button btn = (Button) findViewById(R.id.send_button);
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String content = editText.getText().toString();
if (!"".equals(content)) {
Message msg = new Message(0, content);
msgList.add(msg);
msgAdapter.notifyDataSetChanged();
listView.smoothScrollToPosition(listView.getCount()-1);//移动到尾部
editText.setText("");
}
}
});
}
private void initMessage() {
for (int i = 0; i < 3; i++) {
Message msg0 = new Message(1, "How are you?");
Message msg1 = new Message(0, "I am fine, thank you. And you?");
Message msg2 = new Message(1, "I am fine, too");
msgList.add(msg0);
msgList.add(msg1);
msgList.add(msg2);
}
}
}
下面用RecyclerView实现上述效果.
首先App的build.gradle文件中,添加RecyclerView的支持包(版本号和Android Studio保持一致).
compile 'com.android.support:recyclerview-v7:25.3.1'
在activity_main.xml文件中使用RecyclerView,替换ListView.
其中新增的item.xml和Message文件不变.新增继承至RecyclerView.Adapter的类MsgAdapter.首先创建ViewHolder,在创建方法中,记录需要具体设置的控件属性.在onCreateViewHolder方法中创建ViewHolder,在onBindViewHolder方法中为控件负值,在getItemCount方法中,设置item的个数.
public class MsgAdapter extends RecyclerView.Adapter {
private List msgList;
static class ViewHolder extends RecyclerView.ViewHolder {
LinearLayout left_layout;
ImageView left_imageView;
TextView left_textView;
LinearLayout right_layout;
ImageView right_imageView;
TextView right_textView;
public ViewHolder(View view) {
super(view);
left_layout = (LinearLayout)view.findViewById(R.id.left_layout);
left_imageView = (ImageView)view.findViewById(R.id.left_image);
left_textView = (TextView)view.findViewById(R.id.left_text);
right_layout = (LinearLayout)view.findViewById(R.id.right_layout);
right_imageView = (ImageView)view.findViewById(R.id.right_image);
right_textView = (TextView)view.findViewById(R.id.right_text);
}
}
public MsgAdapter(List msgList) {
this.msgList = msgList;
}
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_msg, parent, false);
return new ViewHolder(view);
}
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
Msg msg = msgList.get(position);
if (msg.getType() == msg.TYPE_RECEIVED) {
holder.right_layout.setVisibility(View.GONE);
holder.left_layout.setVisibility(View.VISIBLE);
holder.left_imageView.setImageResource(R.drawable.left);
holder.left_textView.setText(msg.getText());
}else {
holder.right_layout.setVisibility(View.VISIBLE);
holder.left_layout.setVisibility(View.GONE);
holder.right_imageView.setImageResource(R.drawable.right);
holder.right_textView.setText(msg.getText());
}
}
@Override
public int getItemCount() {
return msgList.size();
}
}
最后在MainActivity中使用MsgAdapter.
public class MainActivity extends AppCompatActivity {
private List msgList = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initMsg();
final RecyclerView recyclerView = (RecyclerView)findViewById(R.id.recycle_view);
LinearLayoutManager layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
final MsgAdapter adapter = new MsgAdapter(msgList);
recyclerView.setAdapter(adapter);
final EditText input_text = (EditText)findViewById(R.id.input_text);
Button send_button = (Button)findViewById(R.id.send_button);
send_button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String content = input_text.getText().toString();
if (!"".equals(content)) {
Msg msg = new Msg(content, Msg.SENT);
msgList.add(msg);
adapter.notifyItemInserted(msgList.size() + 1);
recyclerView.scrollToPosition(msgList.size() + 1);
input_text.setText("");
}
}
});
}
private void initMsg() {
for (int i = 0; i < 3; i++) {
Msg msg0 = new Msg("How are you?", 0);
Msg msg1 = new Msg("I am fine, thank you. And you?", 1);
Msg msg2 = new Msg("I am fine, too.", 0);
msgList.add(msg0);
msgList.add(msg1);
msgList.add(msg2);
}
}
}
由上面的例子可以看出,ListView和RecyclerView的区别,主要在MsgAdapter的创建上.ListView的Adapter创建方式比较简单,只需要重写getView一个方法,而RecyclerView则需要重写onCreateViewHolder,onBindViewHoler和getItemsCount的3个方法.RecyerView实现方法的增加,方便了设计者对渲染过程的控制.其中的getView方法,被拆分成onCreateViewHolder和onBindViewHoler两个实现过程,在getItemsCount方法中,可以方便的控制渲染条数,比如,让最后一条不显示.
对比学习能够在加深理解的同时,掌握各自的不同,从而方便我们面对对具体问题的解决方式.
具体Demo如下:ListView和RecyclerView.