Android LiveData 可以被简单地描述为“在组件之间共享数据”，它是一种用于构建响应式、以数据驱动的 Android 应用程序的库
在本教程中，我们将讨论Android应用程序中的LiveData架构组件。为了更好地理解本教程，请先快速了解一下Android ViewModel。
安卓实时数据流
LiveData是架构模式的一部分。它基本上是一个包含原始/集合类型的数据持有者。它用于观察视图的变化，并在其处于活动状态时更新视图。因此，LiveData是生命周期感知的。我们知道ViewModel用于将数据传递给View。仅使用ViewModel可能是一项繁琐且昂贵的操作，因为每次数据需要改变视图时，我们都需要进行多个调用。此外，我们还需要在不同的位置存储数据模型。LiveData基于观察者模式，使得ViewModel和View之间的通信变得简单。它会观察数据的变化，并自动更新数据，而不是我们在多个位置进行多次添加和删除数据引用的调用（例如SQLite，ArrayList，ViewModel）。
Android的LiveData与RxJava的对比。
Android LiveData与RxJava相似，不过LiveData具有生命周期感知能力。如果视图处于后台，它不会在视图中更新数据，从而帮助我们避免了IllegalStateException等异常。在ViewModel中，我们如何更新Activity中的LiveData？当我们在Activity中注册Observer时，需要覆写onChanged()方法。当LiveData发生变化时，onChanged()方法会被触发。因此，在onChanged()方法中，我们可以将变化的LiveData更新到视图中。
LiveData是一种数据类型，当数据发生变化时会通知其观察者。LiveData就像一个数据变化的通知器。
LiveData通过调用setValue()和postValue()来通知观察者。setValue()在主线程上运行，而postValue()在后台线程上运行。调用LiveData类型实例的getValue()会返回当前数据。
可变数据
MutableLiveData只是一个扩展了LiveData类型的类。MutableLiveData常用，因为它公开提供了postValue()和setValue()方法，而LiveData类并不提供。LiveData/MutableLiveData在从集合类型（List、ArrayList等）更新RecyclerView中的数据中常用。在接下来的部分，我们将创建一个应用程序，从SQLite数据库中添加/删除RecyclerView中的行。我们将使用MutableLiveData，以便在LiveData更改时更新RecyclerView记录。我们将使用DiffUtil通过比较旧的和新的ArrayList来更新最少数量的RecyclerView行。
Android LiveData示例项目结构
在你的build.gradle文件中添加以下内容：
implementation 'com.android.support:design:27.1.1'
implementation 'com.android.support:cardview-v7:27.1.1'
implementation 'android.arch.lifecycle:extensions:1.1.1'
安卓LiveData代码
以下是activity_main.xml布局的代码。
<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="https://schemas.android.com/apk/res/android"
xmlns:app="https://schemas.android.com/apk/res-auto"
xmlns:tools="https://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<android.support.design.widget.AppBarLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="@style/AppTheme.AppBarOverlay">
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary"
app:popupTheme="@style/AppTheme.PopupOverlay" />
</android.support.design.widget.AppBarLayout>
<RelativeLayout xmlns:android="https://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:layout_behavior="@string/appbar_scrolling_view_behavior">
<android.support.v7.widget.RecyclerView
android:id="@+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</RelativeLayout>
<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="bottom|end"
android:layout_margin="@dimen/fab_margin"
app:srcCompat="@android:drawable/ic_input_add" />
</android.support.design.widget.CoordinatorLayout>
以下是list_item_row.xml布局的代码:
<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView xmlns:android="https://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="8dp"
android:gravity="center_vertical">
<TextView
android:id="@+id/tvUrl"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentStart="true"
android:autoLink="web"
android:padding="8dp"
android:textColor="@android:color/black"
android:textSize="20sp" />
<TextView
android:id="@+id/tvDate"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_below="@+id/tvUrl" />
<ImageButton
android:id="@+id/btnDelete"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentEnd="true"
android:layout_centerVertical="true"
android:src="@android:drawable/ic_menu_delete" />
</RelativeLayout>
</android.support.v7.widget.CardView>
下面提供了DbSettings.java类的代码。
package com.Olivia.androidlivedata.db;
import android.provider.BaseColumns;
public class DbSettings {
public static final String DB_NAME = "favourites.db";
public static final int DB_VERSION = 1;
public class DBEntry implements BaseColumns {
public static final String TABLE = "fav";
public static final String COL_FAV_URL = "url";
public static final String COL_FAV_DATE = "date";
}
}
以下是FavoritesDbHelper.java类的代码：
package com.Olivia.androidlivedata.db;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class FavouritesDBHelper extends SQLiteOpenHelper {
public FavouritesDBHelper(Context context) {
super(context, DbSettings.DB_NAME, null, DbSettings.DB_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
String createTable = "CREATE TABLE " + DbSettings.DBEntry.TABLE + " ( " +
DbSettings.DBEntry._ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
DbSettings.DBEntry.COL_FAV_URL + " TEXT NOT NULL, " +
DbSettings.DBEntry.COL_FAV_DATE + " INTEGER NOT NULL);";
db.execSQL(createTable);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS " + DbSettings.DBEntry.TABLE);
onCreate(db);
}
}
以下是Favourites.java模型类的代码。
package com.Olivia.androidlivedata;
public class Favourites {
public long mId;
public String mUrl;
public long mDate;
public Favourites(long id, String name, long date) {
mId = id;
mUrl = name;
mDate = date;
}
public Favourites(Favourites favourites) {
mId = favourites.mId;
mUrl = favourites.mUrl;
mDate = favourites.mDate;
}
}
因此，在我们的SQLite数据库中，我们创建了一个包含三条记录的表：ID、URL和DATE。下面是FavouritesViewModel.java类的代码。
package com.Olivia.androidlivedata;
import android.app.Application;
import android.arch.lifecycle.AndroidViewModel;
import android.arch.lifecycle.MutableLiveData;
import android.content.ContentValues;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import com.Olivia.androidlivedata.db.DbSettings;
import com.Olivia.androidlivedata.db.FavouritesDBHelper;
import java.util.ArrayList;
import java.util.List;
public class FavouritesViewModel extends AndroidViewModel {
private FavouritesDBHelper mFavHelper;
private MutableLiveData<List<Favourites>> mFavs;
FavouritesViewModel(Application application) {
super(application);
mFavHelper = new FavouritesDBHelper(application);
}
public MutableLiveData<List<Favourites>> getFavs() {
if (mFavs == null) {
mFavs = new MutableLiveData<>();
loadFavs();
}
return mFavs;
}
private void loadFavs() {
List<Favourites> newFavs = new ArrayList<>();
SQLiteDatabase db = mFavHelper.getReadableDatabase();
Cursor cursor = db.query(DbSettings.DBEntry.TABLE,
new String[]{
DbSettings.DBEntry._ID,
DbSettings.DBEntry.COL_FAV_URL,
DbSettings.DBEntry.COL_FAV_DATE
},
null, null, null, null, null);
while (cursor.moveToNext()) {
int idxId = cursor.getColumnIndex(DbSettings.DBEntry._ID);
int idxUrl = cursor.getColumnIndex(DbSettings.DBEntry.COL_FAV_URL);
int idxDate = cursor.getColumnIndex(DbSettings.DBEntry.COL_FAV_DATE);
newFavs.add(new Favourites(cursor.getLong(idxId), cursor.getString(idxUrl), cursor.getLong(idxDate)));
}
cursor.close();
db.close();
mFavs.setValue(newFavs);
}
public void addFav(String url, long date) {
SQLiteDatabase db = mFavHelper.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(DbSettings.DBEntry.COL_FAV_URL, url);
values.put(DbSettings.DBEntry.COL_FAV_DATE, date);
long id = db.insertWithOnConflict(DbSettings.DBEntry.TABLE,
null,
values,
SQLiteDatabase.CONFLICT_REPLACE);
db.close();
List<Favourites> favourites = mFavs.getValue();
ArrayList<Favourites> clonedFavs;
if (favourites == null) {
clonedFavs = new ArrayList<>();
} else {
clonedFavs = new ArrayList<>(favourites.size());
for (int i = 0; i < favourites.size(); i++) {
clonedFavs.add(new Favourites(favourites.get(i)));
}
}
Favourites fav = new Favourites(id, url, date);
clonedFavs.add(fav);
mFavs.setValue(clonedFavs);
}
public void removeFav(long id) {
SQLiteDatabase db = mFavHelper.getWritableDatabase();
db.delete(
DbSettings.DBEntry.TABLE,
DbSettings.DBEntry._ID + " = ?",
new String[]{Long.toString(id)}
);
db.close();
List<Favourites> favs = mFavs.getValue();
ArrayList<Favourites> clonedFavs = new ArrayList<>(favs.size());
for (int i = 0; i < favs.size(); i++) {
clonedFavs.add(new Favourites(favs.get(i)));
}
int index = -1;
for (int i = 0; i < clonedFavs.size(); i++) {
Favourites favourites = clonedFavs.get(i);
if (favourites.mId == id) {
index = i;
}
}
if (index != -1) {
clonedFavs.remove(index);
}
mFavs.setValue(clonedFavs);
}
}
MutableLiveData保存了一个Favorite实例对象列表。在addFav（）和removeFav（）方法中，我们会通知数据变化给在MainActivity中定义的Observer。为了比较旧的列表和新的列表，我们创建了一个ArrayList的副本。MainActivity.java类的代码如下所示：
package com.Olivia.androidlivedata;
import android.arch.lifecycle.Observer;
import android.arch.lifecycle.ViewModelProviders;
import android.content.DialogInterface;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.util.DiffUtil;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.TextView;
import java.util.Date;
import java.util.List;
public class MainActivity extends AppCompatActivity {
private FavAdapter mFavAdapter;
private FavouritesViewModel mFavViewModel;
private List<Favourites> mFav;
FloatingActionButton fab;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
fab = findViewById(R.id.fab);
final RecyclerView recyclerView = findViewById(R.id.recyclerView);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
mFavViewModel = ViewModelProviders.of(this).get(FavouritesViewModel.class);
final Observer<List<Favourites>> favsObserver = new Observer<List<Favourites>>() {
@Override
public void onChanged(@Nullable final List<Favourites> updatedList) {
if (mFav == null) {
mFav = updatedList;
mFavAdapter = new FavAdapter();
recyclerView.setAdapter(mFavAdapter);
} else {
DiffUtil.DiffResult result = DiffUtil.calculateDiff(new DiffUtil.Callback() {
@Override
public int getOldListSize() {
return mFav.size();
}
@Override
public int getNewListSize() {
return updatedList.size();
}
@Override
public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
return mFav.get(oldItemPosition).mId ==
updatedList.get(newItemPosition).mId;
}
@Override
public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
Favourites oldFav = mFav.get(oldItemPosition);
Favourites newFav = updatedList.get(newItemPosition);
return oldFav.equals(newFav);
}
});
result.dispatchUpdatesTo(mFavAdapter);
mFav = updatedList;
}
}
};
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
final EditText inUrl = new EditText(MainActivity.this);
AlertDialog dialog = new AlertDialog.Builder(MainActivity.this)
.setTitle("New favourite")
.setMessage("Add a url link below")
.setView(inUrl)
.setPositiveButton("Add", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
String url = String.valueOf(inUrl.getText());
long date = (new Date()).getTime();
mFavViewModel.addFav(url, date);
}
})
.setNegativeButton("Cancel", null)
.create();
dialog.show();
}
});
mFavViewModel.getFavs().observe(this, favsObserver);
}
public class FavAdapter extends RecyclerView.Adapter<FavAdapter.FavViewHolder> {
@Override
public FavViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.list_item_row, parent, false);
return new FavViewHolder(itemView);
}
@Override
public void onBindViewHolder(FavViewHolder holder, int position) {
Favourites favourites = mFav.get(position);
holder.mTxtUrl.setText(favourites.mUrl);
holder.mTxtDate.setText((new Date(favourites.mDate).toString()));
}
@Override
public int getItemCount() {
return mFav.size();
}
class FavViewHolder extends RecyclerView.ViewHolder {
TextView mTxtUrl;
TextView mTxtDate;
FavViewHolder(View itemView) {
super(itemView);
mTxtUrl = itemView.findViewById(R.id.tvUrl);
mTxtDate = itemView.findViewById(R.id.tvDate);
ImageButton btnDelete = itemView.findViewById(R.id.btnDelete);
btnDelete.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
int pos = getAdapterPosition();
Favourites favourites = mFav.get(pos);
mFavViewModel.removeFav(favourites.mId);
}
});
}
}
}
}
在上面的代码中，我们已经在Activity本身中定义了ReyclerView Adapter类。mFavViewModel.getFavs().observe(this, favsObserver);用于在MainActivity中设置一个观察器，每当ViewModel类更新LiveData时，该观察器将被通知。 favsObserver匿名类由onChanged()方法组成，该方法提供最新的数据，然后将其更新到RecyclerView中。上述应用程序的输出如下所示。这标志着Android LiveData教程的结束。您可以从下面的链接下载项目。
下载 AndroidLiveData 项目。
你也可以从我们的GitHub代码库中检查Android Studio项目代码。
