Loader之二：CursorLoader基本实例

参考APIDEMO：sdk\samples\android-19\content\LoaderCursor

1、创建主布局文件，里面只包含一个Fragment。

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/FrameLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <fragment
        android:id="@+id/fragment_list"
        android:name="com.example.android.content.loadercursor.CursorLoaderListFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</FrameLayout>

2、创建主Activity文件中的android:name加载相应的Fragment

package com.example.android.content.loadercursor;

import android.app.Activity;
import android.app.ListFragment;
import android.app.LoaderManager;
import android.database.Cursor;
import android.os.Bundle;
import android.widget.SearchView;

/**
 * The entry point to the CursorLoader sample. This sample demonstrates the use
 * of the {@link LoaderManager} to retrieve data from a {@link Cursor}. Here, a
 * list of contacts is displayed in a {@link ListFragment} and filtered using a
 * {@link SearchView} ActionBar item.
 */
public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

}

3、创建相应的Fragment

package com.example.android.content.loadercursor;

import android.app.ListFragment;
import android.app.LoaderManager;
import android.content.CursorLoader;
import android.content.Loader;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.provider.ContactsContract.Contacts;
import android.view.View;
import android.widget.ListView;
import android.widget.SearchView;
import android.widget.SimpleCursorAdapter;
import android.widget.Toast;

/**
 * A {@link ListFragment} that shows the use of a {@link LoaderManager} to
 * display a list of contacts accessed through a {@link Cursor}.
 */
/*
 * (1)继承Fragment或者其子类，用于创建一个Fragment。实现LoaderManager.LoaderCallbacks接口，用于与Loader的交互
 * 。 官方文档： A callback interface for a client to interact with the LoaderManager.
 * For example, you use the onCreateLoader() callback method to create a new
 * loader.
 */
public class CursorLoaderListFragment extends ListFragment implements
		LoaderManager.LoaderCallbacks<Cursor> {

	// This is the Adapter being used to display the list's data.
	SimpleCursorAdapter mAdapter;

	// The SearchView for doing filtering.
	SearchView mSearchView;

	/*
	 * （2）在Activity被创建时调用此方法。Called when the fragment's activity has been
	 * created and this fragment's view hierarchy instantiated. You typically
	 * initialize a Loader within the activity's onCreate() method, or within
	 * the fragment's onActivityCreated() method.
	 */@Override
	public void onActivityCreated(Bundle savedInstanceState) {
		super.onActivityCreated(savedInstanceState);

		// Give some text to display if there is no data. In a real
		// application this would come from a resource.
		setEmptyText("No phone numbers");

		/*
		 * Create an empty adapter we will use to display the loaded data. The
		 * simple_list_item_2 layout contains two rows on top of each other
		 * (text1 and text2) that will show the contact's name and status.
		 */
		// （3）设置Fragment的顯示內容
		mAdapter = new SimpleCursorAdapter(getActivity(),
				android.R.layout.simple_list_item_2, null, new String[] {
						Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS },
				new int[] { android.R.id.text1, android.R.id.text2 }, 0);
		setListAdapter(mAdapter);

		// Start out with a progress indicator.
		setListShown(false);

		// Prepare the loader. Either re-connect with an existing one,
		// or start a new one.
		/*
		 * (4)創建一個Loader，此Loader用于為Fragment載入內容。You typically initialize a
		 * Loader within the activity's onCreate() method, or within the
		 * fragment's onActivityCreated() method.
		 * 此方法將自動調用LoaderManager.LoaderCallbacks接口的onCreateLoader方法。
		 */
		getLoaderManager().initLoader(0, null, this);

	}

	/**
	 * An item has been clicked in the {@link ListView}. Display a toast with
	 * the tapped item's id.
	 */
	@Override
	public void onListItemClick(ListView l, View v, int position, long id) {
		Toast.makeText(getActivity(), "Item clicked: " + id, Toast.LENGTH_LONG)
				.show();
	}

	// These are the Contacts rows that we will retrieve.
	static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {
			Contacts._ID, Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS,
			Contacts.LOOKUP_KEY, };

	/*
	 * （5）Loader被創建時的操作，一般用于加載內容。In this example, the onCreateLoader() callback
	 * method creates a CursorLoader. You must build the CursorLoader using its
	 * constructor method, which requires the complete set of information needed
	 * to perform a query to the ContentProvider.
	 */public Loader<Cursor> onCreateLoader(int id, Bundle args) {
		// This is called when a new Loader needs to be created. This
		// sample only has one Loader, so we don't care about the ID.
		// First, pick the base URI to use depending on whether we are
		// currently filtering.
		Uri baseUri;
		baseUri = Contacts.CONTENT_URI;

		// Now create and return a CursorLoader that will take care of
		// creating a Cursor for the data being displayed.
		String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
				+ Contacts.HAS_PHONE_NUMBER + "=1) AND ("
				+ Contacts.DISPLAY_NAME + " != '' ))";
		String order = Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC";

		return new CursorLoader(getActivity(), baseUri,
				CONTACTS_SUMMARY_PROJECTION, select, null, order);
	}

	/*
	 * （6）内容被加载完成后的操作。The loader will release the data once it knows the
	 * application is no longer using it. For example, if the data is a cursor
	 * from a CursorLoader, you should not call close() on it yourself. If the
	 * cursor is being placed in a CursorAdapter, you should use the
	 * swapCursor() method so that the old Cursor is not closed.
	 */public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
		// Swap the new cursor in. (The framework will take care of closing the
		// old cursor once we return.)
		// Swap in a new Cursor, returning the old Cursor. Unlike
		// changeCursor(Cursor), the returned old Cursor is not closed.
		mAdapter.swapCursor(data);

		// The list should now be shown.
		if (isResumed()) {
			setListShown(true);
		} else {
			setListShownNoAnimation(true);
		}
	}

	// (7)Loader被重新加载时的操作。
	public void onLoaderReset(Loader<Cursor> loader) {
		// This is called when the last Cursor provided to onLoadFinished()
		// above is about to be closed. We need to make sure we are no
		// longer using it.
		mAdapter.swapCursor(null);
	}

}