我想从一个单独的类中检索一个位图。单独的类具有使用数据回调从在线服务器下载位图的方法。默认情况下,该方法返回null,直到下载位图为止。当我从主类中调用该方法时,将得到null。在继续执行任何主类操作之前,我如何等待此检索为非null(即位图已下载)?
主班
profileBitmap = myParse.getImageBitmap(getContext(), tagId);
单独的班级
public Bitmap getImageBitmap (final Context context, String tagId) {
// give us the user's photo
ParseQuery<ParseObject> parseQuery = new ParseQuery<>("Photo");
parseQuery.whereEqualTo("username", tagId);
parseQuery.getFirstInBackground(new GetCallback<ParseObject>() {
@Override
public void done(ParseObject object, ParseException e) {
if (object != null && e == null) {
ParseFile profilePic = (ParseFile) object.get("profilePicture");
//download process
profilePic.getDataInBackground(new GetDataCallback() {
@Override
public void done(byte[] data, ParseException e) {
if (data != null && e == null) {
// converts file to image
selectedImageBitmap = BitmapFactory.decodeByteArray(data, 0, data.length);
} else {
// returns error user bitmap
selectedImageBitmap = BitmapFactory.decodeResource(context.getResources(), android.R.drawable.ic_dialog_alert);
FancyToast.makeText(context, "Error: " + e.getMessage(), FancyToast.LENGTH_SHORT, FancyToast.ERROR, false).show();
}
}
});
} else {
// returns empty user bitmap
selectedImageBitmap = BitmapFactory.decodeResource(context.getResources(), R.drawable.empty_user);
FancyToast.makeText(context, "Error: " + e.getMessage(), FancyToast.LENGTH_SHORT, FancyToast.ERROR, false).show();
}
}
});
return selectedImageBitmap;
}
答案 0 :(得分:1)
我会使用listener
:
在新文件中创建interface
:
public interface onBitmapDownloadedListener{
void onDownloadFinished(Bitmap bitmap);
}
在您的主类中实现interface
,并将主类的引用传递给MyParse
类
class MainClass implements onBitmapDownloadedListener{
//now when you create an instance of myParse, pass the listener
......
myParse = new MyParse(this);
@Override
public void onDownloadFinished(Bitmap bitmap){
//here you get the bitmap
profileBitmap = bitmap
}
在MyParse
类中构造一个接受接口的构造函数:
class MyParse {
private onBitmapDownloadedListener listener;
//constrcutor
public MyParse(onBitmapDownloadedListener listener){
this.listener = listener
}
//your method
public Bitmap getImageBitmap (final Context context, String tagId) {
........
........
........
@Override
public void done(byte[] data, ParseException e) {
if (data != null && e == null) {
// converts file to image
selectedImageBitmap = BitmapFactory.decodeByteArray(data, 0, data.length);
//notify here
listener.onDownloadFinished(selectedImageBitmap);
}
....
....
....
}
}