Android-遗留无头片段 [英] Android -Retained headless fragment

问题描述

我正在阅读如何在UI和后台线程之间进行交互此处。

I am reading about how to interact between UI and background thread here.

本文有以下注释：

AsyncTask不处理配置更改自动，
即重新创建活动。程序员必须在编码中处理
。一个常见的解决方案是在保留的无头片段中声明AsyncTask
。

The AsyncTask does not handle configuration changes automatically, i.e. if the activity is recreated. The programmer has to handle that in his coding. A common solution to this is to declare the AsyncTask in a retained headless fragment.

我不明白什么是无头的分段。

I dont understand what is retained headless fragment.

例如，通过这种方式我可以添加片段：

For example, in this way I can add fragment:

FragmentManager manager = getSupportFragmentManager();
FragmentTransaction transaction = manager.beginTransaction();
transaction.add(R.id.frame, new MyFragment());
transaction.commit();

在片段中我可以像这样执行AsyncTask：

And in fragment I can execute AsyncTask like this:

MyTask myTask = new MyTask();
String[] args = {"Hello"};
myTask.execute(args);

这是否称为在保留的无头片段中声明AsyncTask？

Is this called "to declare the AsyncTask in a retained headless fragment"?

推荐答案

无头片段只是一个没有视图的片段。在片段lifeCycle的 onCreate（）中，使用 setRetainInstance（true）; 。即使活动重新创建，这也不会破坏片段。因此，如果AsyncTask在片段中运行，则在重新创建活动时，您将不会丢失AsyncTask。

Headless fragment is nothing but a fragment which does not have a view. In onCreate() of the fragment lifeCycle, use setRetainInstance(true);. This will not destroy the fragment even if the activity recreates. So if an AsyncTask is running in fragment, on recreation of the activity, you wont lose the AsyncTask.

在 onCreate 对于活动，您必须添加带有标记的片段。在添加之前，使用 getFragmentManager（）。findFragmentByTag（TAG）检查片段是否存在，如果片段为null，则创建片段的新实例并添加它。
在Fragment中不会有任何视图膨胀，因此无需覆盖 onCreateView（）。

In onCreate of the activity, you have to add the fragment with a tag. Before adding, check if the fragment exist using getFragmentManager().findFragmentByTag(TAG), if the fragment is null then create a new instance of the fragment and add it. In Fragment there will not be any view inflated, so no need to override onCreateView().

headlessFragment的一个例子：

An example of headlessFragment :

public class HeadlessProgressFragment extends Fragment {

private ProgressListener mProgressListener;
private AsyncTask<Void, Integer, Void> mProgressTask;

public interface ProgressListener {
    void updateProgress(int progress);
}

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setRetainInstance(true);
}

public void setProgressListener(Context context) {
    mProgressListener = (ProgressListener) context;
}

public void startProgress(final int size) {
    if (mProgressTask == null || mProgressTask.getStatus() != AsyncTask.Status.RUNNING || mProgressTask.getStatus() == AsyncTask.Status.FINISHED) {
            mProgressTask = new AsyncTask<Void, Integer, Void>() {
                @Override
                protected Void doInBackground(Void... params) {
                    for (int index = 0; index < size; index++) {
                        try {
                            Thread.sleep(5000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        } finally {
                            publishProgress(index + 1);
                        }
                    }
                    return null;
                }

                @Override
                protected void onProgressUpdate(Integer... values) {
                    super.onProgressUpdate(values);
                    if (mProgressListener != null) {
                        mProgressListener.updateProgress(values[0]);
                    }
                }
            };
            mProgressTask.execute();
        }
    }
}

在活动中这样的事情：

In Activity Something like this :

public class MainActivity extends FragmentActivity implements HeadlessProgressFragment.ProgressListener {

private static final String TAG = "progress_fragment";
private ProgressBar mProgressBar;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.dummy_view);
    mHeadlessProgressFragment = (HeadlessProgressFragment) getSupportFragmentManager().findFragmentByTag(TAG);
    if (mHeadlessProgressFragment == null) {
        mHeadlessProgressFragment = new HeadlessProgressFragment();
        getSupportFragmentManager().beginTransaction().add(mHeadlessProgressFragment,TAG).commit();
    }
    mHeadlessProgressFragment.setProgressListener(this);
    mProgressBar = (ProgressBar) findViewById(R.id.progress_bar);
    final Button startFillBtn = (Button) findViewById(R.id.btn_start_filling);
    startFillBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            mHeadlessProgressFragment.startProgress(100);
        }
    });
}

    @Override
    public void updateProgress(int progress) {
        mProgressBar.setProgress(progress);
    }
}

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