Android - 开发实例(9)：使用Loader进行数据加载

Android3.0 开始提出Loader和LoaderManager的概念，通过LoaderManager类可以很轻松的从Fragment或Activity 中进行异步加载数据。

这和AsyncTask很类似，但是AsyncTask没有LoaderManager的统一管理，并且不方便实现MVC的开发模式，对于和UI相关的数据加载就不要使用AsyncTask使用Loader最佳！

官方示例：

http://developer.android.com/reference/android/app/LoaderManager.html

http://developer.android.com/reference/android/content/AsyncTaskLoader.html

当然我们实际开发中，需要自己去实现一个更加酷的Loader，来满足我们的开发需要。

BaseDataLoader.java  -- 一个基础Loader，提取于AsyncTaskLoader.html的示例代码，并做了自己的处理。

package cn.gavinliu.loader;

import android.content.AsyncTaskLoader;
import android.content.Context;

/**
 * 这是一个基础的数据加载Loader
 * 
 * @author Gavinliu
 * @param <T>
 * 
 */
public abstract class BaseDataLoader<T> extends AsyncTaskLoader<T> {

	private T mData;

	public BaseDataLoader(Context context) {
		super(context);
	}

	/**
	 * Handles a request to start the Loader.
	 */
	@Override
	protected void onStartLoading() {
		if (mData != null) {
			// If we currently have a result available, deliver it
			// immediately.
			deliverResult(mData);
		}

		registerObserver();

		if (takeContentChanged() || mData == null || isConfigChanged()) {
			// If the data has changed since the last time it was loaded
			// or is not currently available, start a load.
			forceLoad();
		}
	}

	/**
	 * Called when there is new data to deliver to the client. The super class
	 * will take care of delivering it; the implementation here just adds a
	 * little more logic.
	 */
	@Override
	public void deliverResult(T data) {
		if (isReset()) {
			// An async query came in while the loader is stopped. We
			// don't need the result.
			if (mData != null) {
				onReleaseResources(mData);
			}
		}
		T oldApps = mData;
		mData = data;

		if (isStarted()) {
			// If the Loader is currently started, we can immediately
			// deliver its results.
			super.deliverResult(data);
		}

		// At this point we can release the resources associated with
		// 'oldApps' if needed; now that the new result is delivered we
		// know that it is no longer in use.
		if (oldApps != null) {
			onReleaseResources(oldApps);
		}
	}

	/**
	 * Handles a request to stop the Loader.
	 */
	@Override
	protected void onStopLoading() {
		// Attempt to cancel the current load task if possible.
		cancelLoad();
	}

	/**
	 * Handles a request to cancel a load.
	 */
	@Override
	public void onCanceled(T data) {
		super.onCanceled(data);

		// At this point we can release the resources associated with 'apps'
		// if needed.
		onReleaseResources(data);
	}

	/**
	 * Handles a request to completely reset the Loader.
	 */
	@Override
	protected void onReset() {
		super.onReset();

		// Ensure the loader is stopped
		onStopLoading();

		// At this point we can release the resources associated with 'apps'
		// if needed.
		if (mData != null) {
			onReleaseResources(mData);
			mData = null;
		}

		unregisterObserver();
	}

	protected boolean isConfigChanged() {
		return false;
	}

	protected void registerObserver() {
	}

	protected void unregisterObserver() {
	}

	protected void onReleaseResources(T mData) {
	}

}

MainActivity.java -- 使用示例

package cn.gavinliu.loader;

import android.app.Activity;
import android.app.LoaderManager.LoaderCallbacks;
import android.content.Context;
import android.content.Loader;
import android.os.Bundle;

public class MainActivity extends Activity implements LoaderCallbacks<String> {

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

		getLoaderManager().initLoader(0, null, this);

	}

	@Override
	public Loader<String> onCreateLoader(int id, Bundle args) {
		System.out.println("onCreateLoader");
		mLoader = new MyLoader(getApplicationContext());
		return mLoader;
	}

	@Override
	public void onLoadFinished(Loader<String> loader, String data) {
		System.out.println("onLoadFinished");
		System.out.println("----->" + data);
	}

	@Override
	public void onLoaderReset(Loader<String> loader) {
		System.out.println("onLoaderReset");
	}

	private MyLoader mLoader;

	static class MyLoader extends BaseDataLoader<String> {
		String mData;

		public MyLoader(Context context) {
			super(context);
		}

		@Override
		public String loadInBackground() {
			System.out.println("LoadInBackground");
			return "Load Success";
		}

	}

}