Android学习笔记——UI基础之编写界面最佳实践

参考书籍：Android第一行代码（第二版）.郭霖著

1、制作Nine-Patch图片
一种被特殊处理过的png图片，能够指定那些区域可以被拉伸、哪些不可以。在Android sdk目录下有一个tools文件夹，找到draw9patch.bat文件来制作Nine-Patch图片（要打开此文件需将JDK的bin目录配置到环境变量中，如使用的AS内置jdk,则要配置的路径是Android Studio安装目录/jre/bin）.
由于Android Studio已内置此功能，所以只需将图片导入项目后点击鼠标右键->Create 9-Patch file…然后确认路径名称即可。

2、编写精美聊天界面
需要准备接收和发送消息背景图（nine-patch），使用RecyclerView控件，所以要在build.gradle中添加相应依赖库
compile ‘com.android.support:recyclerview-v7:24.2.1’
编写主界面，修改布局文件：

<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/msg_recycler_view"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"/>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
        <EditText
            android:id="@+id/input_text"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:hint="Type something here"
            android:maxLines="2"/>
        <Button
            android:id="@+id/send"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" 
            android:text="Send"/>
    LinearLayout>
LinearLayout>

中间显示聊天消息内容，底部为文本输入框和发送按钮。
然后定义消息实体类：

public class Msg {
    public static final int TYPE_RECEIVED = 0;
    public static final int TYPE_SENT = 1;
    private String content;
    private int type;
    public Msg(String content, int type){
        this.content = content;
        this.type = type;
    }

    public String getContent() {
        return content;
    }

    public int getType() {
        return type;
    }
}

content表示消息内容，type表示消息类型（两个值可选：TYPE_RECEIVED/TYPE_SENT）.
编写RecyclerVeiw子项布局，新建msg_item.xml如下：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="10dp">

    <LinearLayout
        android:id="@+id/left_layout"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="left"
        android:background="@drawable/message_left">

        <TextView
            android:id="@+id/left_msg"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:layout_margin="10dp"
            android:textColor="#fff"/>

    LinearLayout>

    <LinearLayout
        android:id="@+id/right_layout"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="right"
        android:background="@drawable/message_right">

        <TextView
            android:id="@+id/right_msg"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:layout_margin="10dp" />

    LinearLayout>

LinearLayout>

接收消息居左对齐，发送消息居右对齐，分别设置了背景图。后面通过代码来显示隐藏发送/接受消息。
接下来创建RecyclerView的适配器类，新建类MsgAdapter:

public class MsgAdapter extends RecyclerView.Adapter<MsgAdapter.ViewHolder> {
    private List mMsgList;
    static class ViewHolder extends RecyclerView.ViewHolder{
        LinearLayout leftLayout;
        LinearLayout rightLayout;
        TextView leftMsg;
        TextView rightMsg;

        public ViewHolder(View itemView) {
            super(itemView);
            leftLayout = (LinearLayout) itemView.findViewById(R.id.left_layout);
            rightLayout = (LinearLayout) itemView.findViewById(R.id.right_layout);
            leftMsg = (TextView) itemView.findViewById(R.id.left_msg);
            rightMsg = (TextView) itemView.findViewById(R.id.right_msg);
        }
    }
    public  MsgAdapter(List msgList){
        mMsgList = msgList;
    }
    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.msg_item,parent,false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        Msg msg = mMsgList.get(position);
        if (msg.getType() == Msg.TYPE_RECEIVED){
            //如果是收到的消息，则显示左边的消息布局，将右边的消息布局隐藏
            holder.leftLayout.setVisibility(View.VISIBLE);
            holder.rightLayout.setVisibility(View.GONE);
            holder.leftMsg.setText(msg.getContent());
        }else if(msg.getType() == Msg.TYPE_SENT){
            //如果是发出的消息，则相反的处理
            holder.rightLayout.setVisibility(View.VISIBLE);
            holder.leftLayout.setVisibility(View.GONE);
            holder.rightMsg.setText(msg.getContent());
        }

    }

    @Override
    public int getItemCount() {
        return mMsgList.size();
    }
}

最后修改MainActivity中的代码，初始化数据并给发送按钮加入事件响应：

public class MainActivity extends AppCompatActivity {
    private List msgList = new ArrayList<>();
    private EditText inputText;
    private Button send;
    private RecyclerView msgRecyclerView;
    private MsgAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initMsgs();//初始化消息数据
        inputText = (EditText) findViewById(R.id.input_text);
        send = (Button) findViewById(R.id.send);
        msgRecyclerView = (RecyclerView) findViewById(R.id.msg_recycler_view);
        LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        msgRecyclerView.setLayoutManager(layoutManager);
        adapter = new MsgAdapter(msgList);
        msgRecyclerView.setAdapter(adapter);
        send.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String cotent = inputText.getText().toString();
                if (!"".equals(cotent)){
                    Msg msg = new Msg(cotent,Msg.TYPE_SENT);
                    msgList.add(msg);
                    adapter.notifyItemInserted(msgList.size() - 1);//当有新消息时，刷新RecyclerView中的显示
                    msgRecyclerView.scrollToPosition(msgList.size() - 1);//将RecyclerView定位到最后一行
                    inputText.setText("");//清空输入框
                }
            }
        });

    }
    private void initMsgs(){
        Msg msg1 = new Msg("Hello guy." , Msg.TYPE_RECEIVED);
        msgList.add(msg1);
        Msg msg2 = new Msg("Hello.Who is tha?" , Msg.TYPE_SENT);
        msgList.add(msg2);
        Msg msg3 = new Msg("This is Tom.Nice talking to you." , Msg.TYPE_RECEIVED);
        msgList.add(msg3);

    }
}

效果如下：