Android Studio-UI线程未使用异步任务进行更新

Question

我有以下问题：我有一个移动应用程序，点击按钮("是按钮"(，它在异步任务中从服务器获取数据(工作正常(。当该任务运行时，我希望我的应用程序显示一个Layout(framelayout(，我在创建时将其设置为不可见，其中包含一个文本视图和一个Circle Progressbar。因此，我在Asnyc任务的onPreExecute方法中放入了我设置为可见的行，因为我读到这个方法将在UI线程上执行。

但这个框架布局只在我的整个代码运行后显示，而不是在执行时显示。

这是我的代码，我从中删除了一块我认为没有问题的地方，让它对你来说更可读。

public class KeepImageActivity extends AppCompatActivity {
private static final int PORT_NO = 1234;
private String pathname;
private File imgFile;
private Button yesbutton;
private Button nobutton;
private ImageView imageView;
private FrameLayout frameLayout;

@SuppressLint("SourceLockedOrientationActivity")
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
setContentView(R.layout.activity_keep_image);
Intent receivedIntent = getIntent();
pathname = receivedIntent.getStringExtra(CameraActivity.EXTRA_MESSAGE);
imgFile = new File(pathname);
imageView = findViewById(R.id.imageViewID);
yesbutton = findViewById(R.id.yesbuttonID);
nobutton = findViewById(R.id.discardbuttonID);
nobutton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
onDiscard(pathname);
}
});
yesbutton.setOnClickListener(new View.OnClickListener() {
@RequiresApi(api = Build.VERSION_CODES.O)
@Override
public void onClick(View view) {
onYes();
}
});
frameLayout = findViewById(R.id.framelayout);
frameLayout.setVisibility(View.GONE);
showImage();
}
//method to show a picture in an ImageView
private void showImage() {
imgFile = new File(pathname);
Bitmap bitmap = BitmapFactory.decodeFile(imgFile.getAbsolutePath());
imageView.setRotation(90);
imageView.setImageBitmap(bitmap);
}
private void onDiscard(String pathname) {
//....not relevant
}

@RequiresApi(api = Build.VERSION_CODES.O)
private void onYes() {
//doInBackground just fetches data and writes to jpg
class ConnectTask extends AsyncTask<Void, Void, Void> {
@Override
protected Void doInBackground(Void... voids) {
//doing stuff (connection to server and downloading image ("image_from_server"))
}
@Override
//here my frameLayout is not being set to Visible, at least not DURING execution.
protected void onPreExecute() {
frameLayout.setVisibility(View.VISIBLE);
}
@Override
protected void onPostExecute(Void aVoid) {
}
@Override
protected void onProgressUpdate(Void... values) {
}
}

ConnectTask connect =  new ConnectTask();
Void[] param = null;
//execute Async task
connect.execute(param);

//wait until picture on Phone
File xaifile = new File(Environment.getExternalStorageDirectory() + "/image_from_server.jpg");
while (true){
if (xaifile.exists()){
break;
}
}
replacePicture(xaifile);
}

//here i replace the picture/bitmap in the ImageView with another one (that just came from server)
private void replacePicture(File xaifile) {
Bitmap bitmap = BitmapFactory.decodeFile(Environment.getExternalStorageDirectory() + "/image_from_server.jpg");
imageView.setRotation(90);
imageView.setImageBitmap(bitmap);
//finished loading, I commented this line to see if my Layout would show afterwards. It does.
//frameLayout.setVisibility(View.INVISIBLE);
//delete both files, not needed anymore
xaifile.delete();
imgFile.delete();
}
//AND IT SEEMS LIKE HERE IS THE MOMENT THAT THE UPDATE TO THE UI IS SHOWN. WHY??
}

Answer 1

编写此部分：

//wait until picture on Phone
File xaifile = new File(Environment.getExternalStorageDirectory() + "/image_from_server.jpg");
while (true){
if (xaifile.exists()){
break;
}
}
replacePicture(xaifile);

在onPostExecute 中

当前您正在阻塞主线程。