【Android学习笔记】Activity的小弟--Fragment以及异步加载数据的Loader

一、What is a fragment？

首先，fragment总是被嵌入到一个activity中，你可以把它看做是activity中的一个模块，也是用来呈现数据的，但它不需要在manifest中配置，因此可以看做一个轻量级的activity。它有着自己的生命周期，但又依赖与attach的activity。Fragment的出现是为了解决大屏适配的问题，但现在你可以利用fragment来实现更多的功能，比如微信的主界面、QQ的主界面（fragment+framelayout+其他）等。

二、How to create and use it ？

通常的步骤如下：

1.编写xml布局文件（类似activity对应的xml）

2.继承Fragment（或者DialogFragment、ListFragment），并在生命周期方法的onCreateView方法进行inflate

3.在activity的xml中使用，通过<fragment android:name="完整的包名">

4.在attach的activity中通过FragmentManager来findFragmentById或者findFragmentByTag（没有界面的fragment），然后通过FragmentTransaction来进行remove、replace、add等动态的操作，最后commit

三、Lifecycle

Fragment的生命周期往往依赖与attach的activity，比如activity如果resumed则fragment resumed，activity如果paused，则fragment paused，如果activity stopped 则 fragment stopped

如图所示

至少应该实现如下的几个回调方法：

onCreate()

在创建fragment时系统会调用此方法。在实现代码中，你可以初始化想要在fragment中保持的那些必要组件，当fragment处于暂停或者停止状态之后可重新启用它们。

onCreateView()

在第一次为fragment绘制用户界面时系统会调用此方法。为fragment绘制用户界面，这个函数必须要返回所绘出的fragment的根View。如果fragment没有用户界面可以返回空。

onPause()

系统回调用该函数作为用户离开fragment的第一个预兆（尽管这并不总意味着fragment被销毁）。在当前用户会话结束之前，通常要在这里提交任何应该持久化的变化（因为用户可能不再返回）。

大部分应用程序都应该至少为每个fragment实现这三个方法，但是还有许多其他用以操纵fragment生命周期中各个阶段的回调函数。

四、Communicating

1.Fragment访问Activity，通过getActivity()或者采用回调接口的方式（fragment内部定义个接口，activity实现它）

2.Activity访问Fragment，通过FragmentManager

3.Fragment或者Activity访问持久化的数据参看文章http://item.congci.com/item/android-loaders-yibu-jiazai-shuju

五、官方Doc中的典型例子

具体的场景一看代码就明白了，主要是横屏显示（左边title右边detail）和竖屏显示的处理逻辑

public static class TitlesFragment extends ListFragment {
    boolean mDualPane;
    int mCurCheckPosition = 0;

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

        // Populate list with our static array of titles.
        setListAdapter(new ArrayAdapter<String>(getActivity(),
                android.R.layout.simple_list_item_activated_1, Shakespeare.TITLES));

        // Check to see if we have a frame in which to embed the details
        // fragment directly in the containing UI.
        View detailsFrame = getActivity().findViewById(R.id.details);
        mDualPane = detailsFrame != null && detailsFrame.getVisibility() == View.VISIBLE;

        if (savedInstanceState != null) {
            // Restore last state for checked position.
            mCurCheckPosition = savedInstanceState.getInt("curChoice", 0);
        }

        if (mDualPane) {
            // In dual-pane mode, the list view highlights the selected item.
            getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
            // Make sure our UI is in the correct state.
            showDetails(mCurCheckPosition);
        }
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("curChoice", mCurCheckPosition);
    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        showDetails(position);
    }

    /**
     * Helper function to show the details of a selected item, either by
     * displaying a fragment in-place in the current UI, or starting a
     * whole new activity in which it is displayed.
     */
    void showDetails(int index) {
        mCurCheckPosition = index;

        if (mDualPane) {
            // We can display everything in-place with fragments, so update
            // the list to highlight the selected item and show the data.
            getListView().setItemChecked(index, true);

            // Check what fragment is currently shown, replace if needed.
            DetailsFragment details = (DetailsFragment)
                    getFragmentManager().findFragmentById(R.id.details);
            if (details == null || details.getShownIndex() != index) {
                // Make new fragment to show this selection.
                details = DetailsFragment.newInstance(index);

                // Execute a transaction, replacing any existing fragment
                // with this one inside the frame.
                FragmentTransaction ft = getFragmentManager().beginTransaction();
                if (index == 0) {
                    ft.replace(R.id.details, details);
                } else {
                    ft.replace(R.id.a_item, details);
                }
                ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                ft.commit();
            }

        } else {
            // Otherwise we need to launch a new activity to display
            // the dialog fragment with selected text.
            Intent intent = new Intent();
            intent.setClass(getActivity(), DetailsActivity.class);
            intent.putExtra("index", index);
            startActivity(intent);
        }
    }
}

public static class DetailsFragment extends Fragment {
    /**
     * Create a new instance of DetailsFragment, initialized to
     * show the text at 'index'.
     */
    public static DetailsFragment newInstance(int index) {
        DetailsFragment f = new DetailsFragment();

        // Supply index input as an argument.
        Bundle args = new Bundle();
        args.putInt("index", index);
        f.setArguments(args);

        return f;
    }

    public int getShownIndex() {
        return getArguments().getInt("index", 0);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        if (container == null) {
            // We have different layouts, and in one of them this
            // fragment's containing frame doesn't exist.  The fragment
            // may still be created from its saved state, but there is
            // no reason to try to create its view hierarchy because it
            // won't be displayed.  Note this is not needed -- we could
            // just run the code below, where we would create and return
            // the view hierarchy; it would just never be used.
            return null;
        }

        ScrollView scroller = new ScrollView(getActivity());
        TextView text = new TextView(getActivity());
        int padding = (int)TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,
                4, getActivity().getResources().getDisplayMetrics());
        text.setPadding(padding, padding, padding, padding);
        scroller.addView(text);
        text.setText(Shakespeare.DIALOGUE[getShownIndex()]);
        return scroller;
    }
}

public static class DetailsActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if (getResources().getConfiguration().orientation
                == Configuration.ORIENTATION_LANDSCAPE) {
            // If the screen is now in landscape mode, we can show the
            // dialog in-line with the list so we don't need this activity.
            finish();
            return;
        }

        if (savedInstanceState == null) {
            // During initial setup, plug in the details fragment.
            DetailsFragment details = new DetailsFragment();
            details.setArguments(getIntent().getExtras());
            getFragmentManager().beginTransaction().add(android.R.id.content, details).commit();
        }
    }
}

六、参考文献

1.官方文档