Android Loader使用详解

1、CursorLoader使用Demo

public class MainActivity extends Activity implements  LoaderManager.LoaderCallbacks<Cursor> {

String TITLE = MediaStore.Audio.Media.TITLE;
String SINGER = MediaStore.Audio.Media.ARTIST;
String SONGURL = MediaStore.Audio.Media.DATA;
Uri uri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;

private LoaderManager loaderManager;

private Cursor myCursor;

private ListView lv;
private SimpleCursorAdapter adapter;
private String songUrl;

/**
* 创建loader之后, 会在子线程执行loadInBackground()方法进行加载数据
*/

@Override
public Loader<Cursor> onCreateLoader(int i, Bundle bundle) {

CursorLoader loader = new CursorLoader(this, uri, null, null, null,null);

return loader;
}

/**
* loader加载数据完成,返回一个cursor对象,供我们查询数据
*/

@Override
public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
myCursor = cursor;
Cursor oldCursor = adapter.swapCursor(cursor);
Log.e("tag", "onLoadFinished: " + oldCursor);
}

/**
* loader重置时回调,需要释放对这个loader数据有引用的cursor
*/

@Override
public void onLoaderReset(Loader<Cursor> loader) {
adapter.swapCursor(null);
}

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

setContentView(R.layout.activity_main);
intiData();
loaderManager = getLoaderManager();

//调用该方法后会回调onCreateLoader来创建loader对象
loaderManager.initLoader(1, null, this);

}

private void intiData() {
lv = (ListView) findViewById(R.id.lv);
adapter = new SimpleCursorAdapter(this, android.R.layout.simple_list_item_2, null,

　　　　　　　　　　　　　　　　　　new String[] {TITLE, SINGER }, new int[] { android.R.id.text1, android.R.id.text2 },

lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {

myCursor.moveToPosition(position);
songUrl = myCursor.getString(myCursor.getColumnIndex(SONGURL));

Log.e("tag", "clickStart: " + songUrl);

}
});

}

public void clickStart(View view) {
Intent i = new Intent(MainActivity.this, MusicService.class);
i.setAction("pause");
// i.putExtra("id", songUrl);
startService(i);
Log.e("tag", "clickStart: ");
// Log.e("tag", "clickStart: " );
}

public void clickStop(View view) {
Intent i = new Intent(MainActivity.this, MusicService.class);
i.setAction("stop");
startService(i);
}
}

2、AsyncTaskLoader详细使用DEMO

基于上篇的Fragments 代码，添加Loaders功能，实现加载应用程序数据到界面上，DEMO的运行效果如下：

核心代码为

应用程序的数据对象类：

public static class AppEntry {
        private final AppListLoader mLoader;
        private final ApplicationInfo mInfo;
        private final File mApkFile;
        private String mLable;
        private Drawable mIcon;
        private boolean mMounted;

        public AppEntry(AppListLoader loader, ApplicationInfo info) {
            mLoader = loader;
            mInfo = info;
            mApkFile = new File(info.sourceDir);
        }

        public ApplicationInfo getApplicationInfo() {
            return mInfo;
        }

        public String getLable() {
            return mLable;
        }

        public Drawable getIcon() {
            if (mIcon == null) {
                if (mApkFile.exists()) {
                    mIcon = mInfo.loadIcon(mLoader.mPm);
                    return mIcon;
                } else {
                    mMounted = false;
                }
            } else if (!mMounted) {
                if (mApkFile.exists()) {
                    mMounted = true;
                    mIcon = mInfo.loadIcon(mLoader.mPm);
                    return mIcon;
                }
            } else {
                return mIcon;
            }
            return mLoader.getContext().getResources().getDrawable(
                    android.R.drawable.sym_def_app_icon);
        }

        @Override
        public String toString() {
            // TODO Auto-generated method stub
            return mLable.toString();
        }

        void loadLable(Context mContext) {
            if (mLable == null || !mMounted) {
                if (!mApkFile.exists()) {
                    mMounted = false;
                    mLable = mInfo.packageName;
                } else {
                    mMounted = true;
                    CharSequence lable = mInfo.loadLabel(mContext
                            .getPackageManager());
                    mLable = lable != null ? lable.toString()
                            : mInfo.packageName;
                }
            }
        }

}

实现AsyncTaskLoader 加载数据

public static class AppListLoader extends AsyncTaskLoader<List<AppEntry>> {

        final InterestingConfigChanges mLastConfig = new InterestingConfigChanges();
        final PackageManager mPm;

        List<AppEntry> mApps;
        packageIntentReceiver mPackageObserver;

        public AppListLoader(Context context) {
            super(context);
            // TODO Auto-generated constructor stub
            mPm = getContext().getPackageManager();
        }

        @Override
        public List<AppEntry> loadInBackground() {
            // TODO Auto-generated method stub
            List<ApplicationInfo> apps = mPm
                    .getInstalledApplications(PackageManager.GET_UNINSTALLED_PACKAGES
                            | PackageManager.GET_DISABLED_COMPONENTS);
            if (apps == null)
                apps = new ArrayList<ApplicationInfo>();
            final Context mContext = getContext();
            List<AppEntry> entries = new ArrayList<AppEntry>(apps.size());
            for (ApplicationInfo info : apps) {
                AppEntry entry = new AppEntry(this, info);
                entry.loadLable(mContext);
                entries.add(entry);
            }
            Collections.sort(entries, ALPHA_COMPARATOR);
            return entries;
        }

        @Override
        public void deliverResult(
                List<com.xuzhi.fragment.FragmentDemoActivity.AppEntry> data) {
            // TODO Auto-generated method stub
            if (isReset()) {
                if (data != null) {
                    //释放资源处理
                }
            }
            
            List<AppEntry> oladApps=data;
            mApps=data;
            if(isStarted()){
                super.deliverResult(data);
            }
            
            
            if(oladApps!=null){
                //释放资源
            }
        }
        
        
        protected void onStartLoading() {
            if(mApps!=null)
                deliverResult(mApps);
            
            if(mPackageObserver==null)
                mPackageObserver=new packageIntentReceiver(this);
            
            boolean configChange=mLastConfig.applyNewConfig(getContext().getResources());
            
            if(takeContentChanged()|| mApps== null || configChange){
                forceLoad();
            }
        };
        
        @Override
        public void onCanceled(
                List<com.xuzhi.fragment.FragmentDemoActivity.AppEntry> data) {
            // TODO Auto-generated method stub
            super.onCanceled(data);
            cancelLoad();
        }
        
        @Override
        protected void onReset() {
            // TODO Auto-generated method stub
            super.onReset();
            onStopLoading();
            
            if(mApps!=null){
                //释放资源
                mApps=null;
            }
            
            if(mPackageObserver!=null){
                getContext().unregisterReceiver(mPackageObserver);
                mPackageObserver=null;
            }
        }

}

实现数据源：

public static class AppListAdapter extends ArrayAdapter<AppEntry>{

        
        private LayoutInflater mInflater;
        public AppListAdapter(Context context) { 
            // TODO Auto-generated constructor stub
            super(context, android.R.layout.simple_list_item_2);
            mInflater=(LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            
        }
        
        
        public void setData(List<AppEntry> data){
            clear();
            if(data!=null){
                addAll(data);
            }
        }
        
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            // TODO Auto-generated method stub
            View view;
            if(convertView==null){
                view=mInflater.inflate(R.layout.list_item_icon_text, parent,false);
            }else{
                view=convertView;
            }
            
            AppEntry item=getItem(position);
            ((ImageView)view.findViewById(R.id.icon)).setImageDrawable(item.getIcon());
            ((TextView)view.findViewById(R.id.text)).setText(item.getLable());
            return view;
        }
        

}

在Activity 或者Fragments 里面实现接口：
public static class DetailsFragment extends ListFragment implements OnQueryTextListener,LoaderCallbacks<List<AppEntry>>

实现接口里面的函数：

public boolean onQueryTextChange(String newText) {
            // TODO Auto-generated method stub
            return false;
        }

        public boolean onQueryTextSubmit(String query) {
            // TODO Auto-generated method stub
            return false;
        }

         
        public Loader<List<com.xuzhi.fragment.FragmentDemoActivity.AppEntry>> onCreateLoader(
                int id, Bundle args) {
            // TODO Auto-generated method stub
            return new AppListLoader(getActivity());
        }

        /**
         * Load 完成后
         */
        public void onLoadFinished(
                Loader<List<com.xuzhi.fragment.FragmentDemoActivity.AppEntry>> arg0,
                List<com.xuzhi.fragment.FragmentDemoActivity.AppEntry> arg1) {
            // TODO Auto-generated method stub
            mAdapter.setData(arg1);
              if (isResumed()) {
                    setListShown(true);
                } else {
                    setListShownNoAnimation(true);
                }
        }

        /**
         * Loader 重置时
         */
        public void onLoaderReset(
                Loader<List<com.xuzhi.fragment.FragmentDemoActivity.AppEntry>> arg0) {
            // TODO Auto-generated method stub
             mAdapter.setData(null);

}

得到LoaderManager初始化Loader，启动加载在Fragment 的onActivityCreated回调方法上添加，本DEMO的DetailsFragment类添加：

@Override
        public void onActivityCreated(Bundle savedInstanceState) {
            // TODO Auto-generated method stub
            super.onActivityCreated(savedInstanceState);
            mAdapter = new AppListAdapter(getActivity());
            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);

}

参考资料：

Android Loaders（一）概述

Android Loaders（二）Loader的使用

Android Loaders（三）实现一个Base Loader