LoaderManager的原理和示例

   LoaderManager是对异步查询的一种封装，常用于对cursor的操作，我们知道，cursor查询时比较耗时的，如果cursor在主线程上去做查询，就阻塞主线程，我们考虑异步操作，android提供LoaderManager,它有个接口LoaderCallbacks，里面有三个回调方法，其中onCreateLoader是在子线程中运行，主要做查询使用，onLoadFinished处理onCreateLoader得到的结果，onLoaderReset是重置loader，将cursor设置为空，我们通过一个方法启动它，getLoaderManager().initLoader();以下是一个demo:

public class MainActivity extends Activity {

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		FragmentManager fragmentManager = getFragmentManager();
		if (fragmentManager.findFragmentById(android.R.id.content) == null) {
			CursorLoaderListFragment list = CursorLoaderListFragment
					.getInstance();
			fragmentManager.beginTransaction().add(android.R.id.content, list)
					.commit();
		}
	}

	public static class CursorLoaderListFragment extends ListFragment implements
			LoaderCallbacks<Cursor> {

		SimpleCursorAdapter mAdapter;
		static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {
				Contacts._ID, Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS,
				Contacts.CONTACT_PRESENCE, Contacts.PHOTO_ID,
				Contacts.LOOKUP_KEY, };

		@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");

			// We have a menu item to show in action bar.
			setHasOptionsMenu(true);

			// Create an empty adapter we will use to display the loaded data.
			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.
			getLoaderManager().initLoader(0, null, this);
		}

		private static CursorLoaderListFragment getInstance() {
			CursorLoaderListFragment fragment = new CursorLoaderListFragment();
			return fragment;
		}

		@Override
		public Loader<Cursor> onCreateLoader(int id, Bundle args) {
			// TODO Auto-generated method stub
			Uri baseUri = Contacts.CONTENT_URI;
			String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
					+ Contacts.HAS_PHONE_NUMBER + "=1) AND ("
					+ Contacts.DISPLAY_NAME + " != '' ))";
			return new CursorLoader(getActivity(), baseUri,
					CONTACTS_SUMMARY_PROJECTION, select, null,
					Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
		}

		@Override
		public void onLoadFinished(Loader<Cursor> arg0, Cursor arg1) {
			// TODO Auto-generated method stub
			mAdapter.swapCursor(arg1);

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

		@Override
		public void onLoaderReset(Loader<Cursor> arg0) {
			// TODO Auto-generated method stub
			mAdapter.swapCursor(null);
		}

	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

我也做了一个查询，是在主线程运行，如果数据量很大，就会看到差别非常明显，在主线程运行的程序，反应比较慢。

public class MainActivity extends Activity {

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		FragmentManager fragmentManager = getFragmentManager();
		 if (fragmentManager.findFragmentById(android.R.id.content) == null) {
	            CursorLoaderListFragment list = CursorLoaderListFragment.getInstance();
	            fragmentManager.beginTransaction().add(android.R.id.content, list).commit();
	        }
	}
	
	public static class CursorLoaderListFragment extends ListFragment{
		
		SimpleCursorAdapter mAdapter;
		static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {
            Contacts._ID,
            Contacts.DISPLAY_NAME,
            Contacts.CONTACT_STATUS,
            Contacts.CONTACT_PRESENCE,
            Contacts.PHOTO_ID,
            Contacts.LOOKUP_KEY,
        };
		
		 @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");

	            // We have a menu item to show in action bar.
	            setHasOptionsMenu(true);

	            String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
	                    + Contacts.HAS_PHONE_NUMBER + "=1) AND ("
	                    + Contacts.DISPLAY_NAME + " != '' ))";
	            Cursor cursor = getActivity().getContentResolver()
	            		.query(Contacts.CONTENT_URI, CONTACTS_SUMMARY_PROJECTION, select, 
	            				null, Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
	            
	            // Create an empty adapter we will use to display the loaded data.
	            mAdapter = new SimpleCursorAdapter(getActivity(),
	                    android.R.layout.simple_list_item_2, cursor,
	                    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(true);

	            // Prepare the loader.  Either re-connect with an existing one,
	            // or start a new one.
	        }
		 
		 private static CursorLoaderListFragment getInstance() {
				CursorLoaderListFragment fragment = new CursorLoaderListFragment();
				return fragment;
			}
		
	}

有兴趣的人可以对比一下。

源代码下载