java - 当我按下激活 fragment 中的异步任务的按钮时，应用程序停止工作

Question

我在应用程序中打开一个新 fragment ，我想计算价格并每秒打印它(通过 asyncTask)，但是当我运行应用程序时停止工作。 你能帮我找出问题所在吗？

public class RandomDrive extends Fragment {
private Chronometer chronometer;
private Button start;
private Button stop;
private TextView price;
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_random_drive, container, false);
    chronometer = (Chronometer) v.findViewById(R.id.chronometer);
    price = v.findViewById(R.id.see_price);
    class SyncTaskCounter extends AsyncTask<Void, Void, Void> {
        @Override
        protected Void doInBackground(Void... voids) {
            double price = 0;

            while (!isCancelled()) {
                price = price + 0.05;
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                printPrice(price);
            }
            return null;
        }
        @Override
        protected void onCancelled() {
            super.onCancelled();
        }
    }
    final SyncTaskCounter asyncTaskCount;
    asyncTaskCount = new SyncTaskCounter();
    start = (Button) v.findViewById(R.id.start);
    start.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            chronometer.setBase(SystemClock.elapsedRealtime());
            chronometer.start();

            asyncTaskCount.execute();
        }
    });
    stop = (Button) v.findViewById(R.id.stop);
    stop.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            chronometer.stop();
            asyncTaskCount.cancel(true);

        }
    });
    return v;
}

public void printPrice(double price1) {
    price.setText("the price is: " + price1 + " shekel");

}
}

问题出在 asyncTask 中，因为当我在没有 asyncTask 的情况下运行该函数并仅在最后计算价格时 - 应用程序运行。

Answer 1

在 Android 上，您必须在 UI 线程上运行所有 UI 代码。 AsyncTask 中的代码在其他线程中执行，因此无法调用 UI 方法。

为此，您应该使用 Handler(代码将在 UI 线程中调用): https://developer.android.com/reference/android/os/Handler

final Handler h=new Handler();
h.postDelayed(new Runnable() {
   public void run() {
      printPrice(price);
      price = price + 0.05;
      h.postDelayed(this, 1000); // call for next update
   }
}, 1000);

我必须使用AsyncTask，那么你应该从方法更新UI:onProgressUpdate https://developer.android.com/reference/android/os/AsyncTask

class SyncTaskCounter extends AsyncTask<Void, Double, Void> {
        @Override
        protected Void doInBackground(Void... voids) {
            double price = 0;

            while (!isCancelled()) {
                price = price + 0.05;
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                publishProgress(price); // this instructs to call onProgressUpdate from UI thread.
            }
            return null;
        }
        @Override
        protected void onProgressUpdate(Double... price) {
                printPrice(price[0]);  // this is called on UI thread
        }

        @Override
        protected void onCancelled() {
            super.onCancelled();
        }
    }