ListView和RecyclerView

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.

RecyclerView

首先使用ListView,在layout中创建每一条消息的布局item.xml,代码如下:

<?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">

    <LinearLayout
        android:id="@+id/layout_left"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginRight="20dp"
        android:layout_gravity="left|center_vertical"
        android:orientation="horizontal">
        <ImageView
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:layout_gravity="left|center_vertical"
            android:id="@+id/imageView_itemLeft"/>
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/textView_itemLeft"
            android:gravity="left|center_vertical"/>
    </LinearLayout>

    <LinearLayout
        android:id="@+id/layout_right"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:layout_marginLeft="20dp"
        android:gravity="right|center_vertical">
        <TextView
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:gravity="right|center_vertical"
            android:id="@+id/textView_itemRight"/>
        <ImageView
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:id="@+id/imageView_itemRight" />
    </LinearLayout>

</LinearLayout>

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.

<?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="match_parent">

    <ListView
        android:id="@+id/listView_msg"
        android:layout_width="match_parent"
        android:layout_height="400dp"
        android:background="#ffffff"
        android:layout_weight="1">

    </ListView>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:orientation="horizontal"
        android:layout_gravity="bottom">

        <EditText
            android:layout_width="wrap_content"
            android:layout_height="40dp"
            android:layout_marginBottom="10dp"
            android:layout_weight="1"
            android:id="@+id/editText_msg"
            android:hint="write your title here"/>
        <Button
            android:id="@+id/send_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:text="Add"/>

    </LinearLayout>
</LinearLayout>

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 List<T> objects)

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<Message> {

    private int resourceId;
    public MsgAdapter(Context context, int resouceId, List<Message> 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<Message> msgList = new ArrayList<Message>();
    @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.

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

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycle_view"
        android:layout_width="match_parent"
        android:layout_weight="1"
        android:layout_height="100dp" />
    <!--android:background="#ffddff"-->
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="50dp">
        <!--android:background="#00ffff"-->
        <EditText
            android:id="@+id/input_text"
            android:layout_width="wrap_content"
            android:layout_height="40dp"
            android:layout_marginBottom="10dp"
            android:layout_marginRight="10dp"
            android:layout_weight="1"
            android:hint="yourMsg"/>
        <Button
            android:id="@+id/send_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Send"/>
    </LinearLayout>


</LinearLayout>

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<MsgAdapter.ViewHolder> {

    private List<Msg> 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<Msg> 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<Msg> 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.