【框架】Loader监听数据源的变化

步骤：

1.新建一个继承SQLiteOpenHelper的帮助类

2.在MainActivity中定义LoaderManager和SimpleCursorAdapter

3.按顺序重写如下方法：initLoader来启动

onCreateLoader->onStartLoading(forceLoad启动时强制加载)->loadInBackground->onLoadFinished(adapter.swapCursor(data))->onLoaderReset(adapter.swapCursor(null))

*manager.restartLoader()数据改变时立刻加载

public class DBHelper extends SQLiteOpenHelper {
public static final String USERTABLE = "usertable";
private static final String DBNAME = "QF.DB";
private static final int CURRENTVERSION = 1;

public DBHelper(Context context) {
super(context, DBNAME, null, CURRENTVERSION);
}

@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("CREATE TABLE IF NOT EXISTS " + USERTABLE + "(_id INTEGER PRIMARY KEY AUTOINCREMENT" +
",USERNAME,NICKNAME);");
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

}
}

/**
* Loader:自动监听数据源是否发生变化
*/
public class MainActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks<Cursor> {

private static SQLiteDatabase db;
private DBHelper dbHelper;
private SimpleCursorAdapter adapter;
private LoaderManager manager;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
dbHelper = new DBHelper(this);
db = dbHelper.getReadableDatabase();
ListView listView = (ListView) findViewById(R.id.lv);
// 1.获得一个Loader管理器
manager = getSupportLoaderManager();
//2.初始化Loader
//1.Loader的唯一标识符
//2.初始化Loader时传递的参数
//3.Loader的回调
manager.initLoader(1, null, this);
//最后一个参数表示当数据源发生改变时，Cursor能够立马感知到数据源发生变化
adapter = new SimpleCursorAdapter(this,
R.layout.listview_item, null, new String[]{"USERNAME"}, new int[]{R.id.username},
CursorAdapter.FLAG_REGISTER_CONTENT_OBSERVER);
listView.setAdapter(adapter);
}

//当Loader创建时回调
//1.Loader的id
//2.初始化Loader的参数
@Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
Log.d("lenve", "onCreateLoader: " + Thread.currentThread());
return new MyLoader(this);
}

//当异步加载完成时调用该方法
@Override
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
Log.d("lenve", "onLoadFinished: " + Thread.currentThread());
//交换Cursor
adapter.swapCursor(data);
}

//当由于Loader重置导致Cursor失效时，调用该方法
@Override
public void onLoaderReset(Loader<Cursor> loader) {
Log.d("lenve", "onLoaderReset: " + Thread.currentThread());
adapter.swapCursor(null);
}

public void addData(View view) {
ContentValues values = new ContentValues();
values.put("USERNAME", "李四i");
db.insert(DBHelper.USERTABLE, null, values);
//重启Loader
manager.restartLoader(1, null, this);
}

static class MyLoader extends AsyncTaskLoader<Cursor> {

public MyLoader(Context context) {
super(context);
}

//在后台线程执行
@Override
public Cursor loadInBackground() {
Log.d("lenve", "loadInBackground: " + Thread.currentThread());
Cursor cursor = db.rawQuery("SELECT * FROM " + DBHelper.USERTABLE, null);
//在这里放回一个Cursor，该Cursor最终被onLoadFinished()方法接收到
return cursor;
}

//开始加载时调用
@Override
protected void onStartLoading() {
super.onStartLoading();
Log.d("lenve", "onStartLoading: " + Thread.currentThread());
//第一次需要强制加载
forceLoad();
}
}
}

其中，适配器可以定义一个继承CursorAdapter的类，和baseAdapter的道理类似；且cursorAdapter的两个方法newView和bindView相当于BaseAdapter的getView方法的拆分

/**
* newView和bindView相当于BaseAdapter中的getView()方法
*/
public class MyAdapter extends CursorAdapter {
private LayoutInflater inflater;

public MyAdapter(Context context, Cursor c, int flags) {
super(context, c, flags);
inflater = LayoutInflater.from(context);
}

//初始化View
@Override
public View newView(Context context, Cursor cursor, ViewGroup parent) {
View view = inflater.inflate(R.layout.listview_item, null);
ViewHolder holder = new ViewHolder();
holder.username = (TextView) view.findViewById(R.id.username);
view.setTag(holder);
return view;
}

//给View上的控件绑定事件
//cursor.getColumnIndex("USERNAME")获取USERNAME字段的下标
@Override
public void bindView(View view, Context context, Cursor cursor) {
ViewHolder holder = (ViewHolder) view.getTag();
holder.username.setText(cursor.getString(cursor.getColumnIndex("USERNAME")));
}

class ViewHolder {
TextView username;
}
}