Android 异步任务状态

Question

我在Fragment 中使用AsyncTask 来做一些后台操作。我可以使用setRetainInstance(true) 处理片段中异步任务的状态，即在方向更改期间。

问题是，当我从片段调用活动时，我想保存异步任务的状态（无论是待处理还是已完成）。因为当我从活动中返回碎片时，异步任务没有被保留。

注意：

setRetainInstance(true) 在方向更改期间有效，onSaveInstanceState(Bundle bundle) 在写入 setRetainInstance(true) 时无效。 onSaveIntanceState(Bundle bundle) 在调用另一个片段或活动时起作用。

Answer 1

这是一个直接来自 API Demos 的示例，它保留了方向更改的进度。请使用它并根据您的需要进行修改。这段代码非常简单，一旦你理解了它，处理方向变化的进度将是小菜一碟。你基本上可以忽略mThread 并在那里实现你的AsyncTask，看看：

import com.example.android.apis.R;

import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ProgressBar;

/**
 * This example shows how you can use a Fragment to easily propagate state
 * (such as threads) across activity instances when an activity needs to be
 * restarted due to, for example, a configuration change.  This is a lot
 * easier than using the raw Activity.onRetainNonConfiguratinInstance() API.
 */

public class FragmentRetainInstance extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // First time init, create the UI.
    if (savedInstanceState == null) {
        getFragmentManager().beginTransaction().add(android.R.id.content,
                new UiFragment()).commit();
    }
}

/**
 * This is a fragment showing UI that will be updated from work done
 * in the retained fragment.
 */
public static class UiFragment extends Fragment {
    RetainedFragment mWorkFragment;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_retain_instance, container, false);

        // Watch for button clicks.
        Button button = (Button)v.findViewById(R.id.restart);
        button.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                mWorkFragment.restart();
            }
        });

        return v;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        FragmentManager fm = getFragmentManager();

        // Check to see if we have retained the worker fragment.
        mWorkFragment = (RetainedFragment)fm.findFragmentByTag("work");

        // If not retained (or first time running), we need to create it.
        if (mWorkFragment == null) {
            mWorkFragment = new RetainedFragment();
            // Tell it who it is working with.
            mWorkFragment.setTargetFragment(this, 0);
            fm.beginTransaction().add(mWorkFragment, "work").commit();
        }
    }

}

/**
 * This is the Fragment implementation that will be retained across
 * activity instances.  It represents some ongoing work, here a thread
 * we have that sits around incrementing a progress indicator.
 */
public static class RetainedFragment extends Fragment {
    ProgressBar mProgressBar;
    int mPosition;
    boolean mReady = false;
    boolean mQuiting = false;

    /**
     * This is the thread that will do our work.  It sits in a loop running
     * the progress up until it has reached the top, then stops and waits.
     */
    final Thread mThread = new Thread() {
        @Override
        public void run() {
            // We'll figure the real value out later.
            int max = 10000;

            // This thread runs almost forever.
            while (true) {

                // Update our shared state with the UI.
                synchronized (this) {
                    // Our thread is stopped if the UI is not ready
                    // or it has completed its work.
                    while (!mReady || mPosition >= max) {
                        if (mQuiting) {
                            return;
                        }
                        try {
                            wait();
                        } catch (InterruptedException e) {
                        }
                    }

                    // Now update the progress.  Note it is important that
                    // we touch the progress bar with the lock held, so it
                    // doesn't disappear on us.
                    mPosition++;
                    max = mProgressBar.getMax();
                    mProgressBar.setProgress(mPosition);
                }

                // Normally we would be doing some work, but put a kludge
                // here to pretend like we are.
                synchronized (this) {
                    try {
                        wait(50);
                    } catch (InterruptedException e) {
                    }
                }
            }
        }
    };

    /**
     * Fragment initialization.  We way we want to be retained and
     * start our thread.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Tell the framework to try to keep this fragment around
        // during a configuration change.
        setRetainInstance(true);

        // Start up the worker thread.
        mThread.start();
    }

    /**
     * This is called when the Fragment's Activity is ready to go, after
     * its content view has been installed; it is called both after
     * the initial fragment creation and after the fragment is re-attached
     * to a new activity.
     */
    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        // Retrieve the progress bar from the target's view hierarchy.
        mProgressBar = (ProgressBar)getTargetFragment().getView().findViewById(
                R.id.progress_horizontal);

        // We are ready for our thread to go.
        synchronized (mThread) {
            mReady = true;
            mThread.notify();
        }
    }

    /**
     * This is called when the fragment is going away.  It is NOT called
     * when the fragment is being propagated between activity instances.
     */
    @Override
    public void onDestroy() {
        // Make the thread go away.
        synchronized (mThread) {
            mReady = false;
            mQuiting = true;
            mThread.notify();
        }

        super.onDestroy();
    }

    /**
     * This is called right before the fragment is detached from its
     * current activity instance.
     */
    @Override
    public void onDetach() {
        // This fragment is being detached from its activity.  We need
        // to make sure its thread is not going to touch any activity
        // state after returning from this function.
        synchronized (mThread) {
            mProgressBar = null;
            mReady = false;
            mThread.notify();
        }

        super.onDetach();
    }

    /**
     * API for our UI to restart the progress thread.
     */
    public void restart() {
        synchronized (mThread) {
            mPosition = 0;
            mThread.notify();
        }
    }
}
}