我正在使用Firebase开发我的第一个Android应用程序，我喜欢它。我的问题是，我应该如何正确地从Firebase数据库中获取数据？我对获取数据有点困惑，然后使用该数据获取更多数据。
我基本上将'OnCompleteListeners'嵌套在另一个内部，这种模式让我感到不安。是否有更好，更正确的方式？
这是我目前的做法：
//This is a weather app, users can monitor different weather stations
//We first get the weather stations that the current user owns:
// I used two collections in this approach, "ownership" permission is read only to the user
DocumentReference docRef = mDatabase.collection("ownership").document( mUser.getUid() );
docRef.get().addOnCompleteListener( new OnCompleteListener<DocumentSnapshot>() {
@Override
public void onComplete(@NonNull Task<DocumentSnapshot> task) {
if ( task.isSuccessful() ) {
DocumentSnapshot document = task.getResult();
if ( document.exists() ) {
Map<String, Object> data = document.getData();
if ( data != null && data.containsKey("stations") ) {
// data is an string array, here is my ugly solution:
stationIds = data.get("stations").toString().replaceAll("[\\[\\]]", "").split(",");
if ( stationIds.length != 0 ) {
for ( int i = 0; i < stationIds.length; i++ ) {
// Now that I have the stations IDs I can get its data:
// !! the collection "stations" have read and write permission
// THIS PATTERN SEEMS VERY REDUNDANT!
DocumentReference docRef = mDatabase.collection("stations").document(stationIds[i]);
docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
@Override
public void onComplete(@NonNull Task<DocumentSnapshot> task) {
if (task.isSuccessful()) {
DocumentSnapshot document = task.getResult();
if (document.exists()) {
Map<String, Object> data = document.getData();
Log.d(TAG, "Document Snapshot: " + document.getData());
if ( data != null && data.containsKey("name") ) {
LinearLayout ll_root = MainActivity.this.findViewById(R.id.ll_station_list);
LinearLayout ll_new = new LinearLayout(MainActivity.this);
ll_new.setLayoutParams(new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT));
ll_new.setPadding(20,50,20,20);
TextView tv_station_title = new TextView(MainActivity.this);
tv_station_title.setText(data.get("name").toString());
ll_new.addView( tv_station_title);
ll_root.addView( ll_new );
ll_new.setOnClickListener( new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, LiveStation.class);
startActivity(intent);
}
});
}
} else {
Log.w(TAG, "Station not found");
}
} else {
Log.w(TAG, "Unable to get station");
}
}
});
}
}
}
Log.d( TAG, "Document Snapshot: " + document.getData() );
} else {
Log.w(TAG, "No document found");
}
} else {
Log.w(TAG, "Get user stations failed: " + task.getException() );
}
}
});
我对Android中的异步调用不是很熟悉。通常我创建一个与数据库对话的模型，它可以从应用程序结构中的任何地方调用，但使用Firebase似乎我不能这样做。似乎每个视图都需要自己对数据库的唯一调用。或许我把整个想法弄错了。