使用Data Binding构建MVVM应用程序

Data Binding是Google I/O大会上官方推介的，官方原生支持 MVVM 模型可以让我们更快速更好的构建代码框架。

Data Binding 性能

在Data Binding前已经有像RoboGuice、ButterKnife 这样的依赖注入框架，那么为什么要使用Data Binding？
1、Data Binding可以使代码更加干净优雅，使你的工作更加简单
首先再也不用findViewById，一些依赖注入框架像ButterKnife可以很漂亮的帮你绑定资源像这样@Bind(R.id.title) TextView title;，但是控件一多还是有非常多的绑定，
而Data Binding可以更进一步的帮你减少不必要的代码像这样mBinding.setUser(user);
2、Data Binding性能
若是findViewById，实际上是对所有子view都遍历了一遍，而每次findViewById都去遍历一遍。而Data Binding可以做到编译时就知道对应的view，只需遍历布局层次一次，就可以收集到所有的 View。
若是依赖注入的框架，我们知道数据绑定这类库是缓慢的，而Data Binding在性能方面是零反射，即一切都发生在编译期。

Data Binding 使用

确保 Android 的 Gradle 插件版本不低于 1.5.0
修改对应模块（Module）的 build.gradle：

android {
    ....
    dataBinding {
        enabled = true
    }
}

Data Binding 创建

默认会根据xml文件名生成Data Binding类（会转换为首字母大写并添加Binding后缀）
若为Activity

@Override
protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   MainActivityBinding binding = DataBindingUtil.setContentView(this, R.layout.main_activity);
   //or
   //MainActivityBinding binding = MainActivityBinding.inflate(getLayoutInflater());
   User user = new User("Test", "User");
   binding.setUser(user);
}

若为fragment

private FragmentNotificationBinding mBinding;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    mBinding = DataBindingUtil.inflate(inflater, R.layout.fragment_notification, container, false);
    mBinding.setNotification(notification);
    return mBinding.getRoot();
}

若为Custom View

private LayoutCameraStatusBinding mBinding;
public CameraStatusView(Context context) {
    super(context);
    init(context);
}
private void init(Context context) {
    mContext = context;
    mBinding = DataBindingUtil.inflate(LayoutInflater.from(context), R.layout.layout_camera_status, this, true);
}
public void initCameraStatus(CameraStatus cameraStatus) {
    mBinding.setStatus(cameraStatus);
}

若为Recyclerview/ListView

//in ChatItemLeftViewHolder class
public class ChatItemLeftViewHolder extends RecyclerView.ViewHolder {
    public final RecyclerviewItemChatLeftBinding binding;

    public ChatItemLeftViewHolder(View rootView) {
        super(rootView);
        binding = DataBindingUtil.bind(rootView);
    }
}
//in adapter
@Override
public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    if (viewType == ITEM_TYPE_SENDER) {
        RecyclerviewItemChatLeftBinding binding = RecyclerviewItemChatLeftBinding.inflate(LayoutInflater.from(parent.getContext()), parent, false);
        return new ChatItemLeftViewHolder(binding.getRoot());
    } else {
        RecyclerviewItemChatRightBinding binding = RecyclerviewItemChatRightBinding.inflate(LayoutInflater.from(parent.getContext()), parent, false);
        return new ChatItemRightViewHolder(binding.getRoot());
    }
    //or
    //ListItemBinding binding = DataBindingUtil.inflate(layoutInflater, R.layout.list_item, viewGroup, false);
}
@Override
public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
    Message message = messages.get(position);
    if (holder instanceof ChatItemLeftViewHolder) {
        ((ChatItemLeftViewHolder) holder).binding.setMessage(message);
    } else if (holder instanceof ChatItemRightViewHolder) {
        ((ChatItemRightViewHolder) holder).binding.setMessage(message);
    }
}

其他用法参见官方文档

Data Binding 构建MVVM应用程序

MVVM(Model-View-ViewModel)

Model

//bean
public class CameraStatus {
    //此处省略
}
public class CameraStatusModel {
}

View

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">

    <data>

        <variable
            name="status"
            type="com.royole.focuslib.live.CameraStatus" />

        <import type="android.view.View" />
    </data>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@android:color/black"
        android:gravity="center_vertical"
        android:orientation="horizontal">
        //此处省略
        <TextView
            android:id="@+id/tv_sd_remain_count"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{String.valueOf(status.remainCount)}"
            android:textAppearance="?android:attr/textAppearanceSmall"
            android:textColor="@android:color/white" />

        <ImageView
            android:id="@+id/iv_self_timer"
            android:layout_width="20dp"
            android:layout_height="wrap_content"
            android:src="@mipmap/camera_status_self_timer"
            android:visibility="@{status.isSelfTimer ? View.VISIBLE : View.GONE}" />

    </LinearLayout>
</layout>

ViewModel

public class CameraStatusView extends LinearLayout {

    private final String TAG = CameraStatusView.this.getClass().getSimpleName();
    private Context mContext;
    private LayoutCameraStatusBinding mBinding;

    public CameraStatusView(Context context) {
        super(context);
        init(context);
    }

    public CameraStatusView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context);
    }

    public CameraStatusView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init(context);
    }

    private void init(Context context) {
        mContext = context;
        mBinding = DataBindingUtil.inflate(LayoutInflater.from(context), R.layout.layout_camera_status, this, true);
    }

    public void initCameraStatus(CameraStatus cameraStatus) {
        mBinding.setStatus(cameraStatus);
    }
}

Data Binding 高级使用

后续进行说明…