Loaders

 

Loaders，获取数据的东西。

 

总体流程是：通过getLoaderManager().initLoader(0,null,this)获得Loader，如果没有，那么就会调用接口函数获取，注意：有ID，所以要在获取的接口函数里进行相应的选择。

 

 会自动更新数据；

一个主要依赖Activity或者Fragment存在的。

通过getLoaderManager().initLoader(0,null,this)方法初始化LOADER。

要继承LoaderManager.LoaderCallbacks，包括：

LoaderManager.LoaderCallbacks includes these methods:

• onCreateLoader() — Instantiate and return a new Loader for the given ID.

• onLoadFinished() — Called when a previously created loader has finished its load.

• onLoaderReset() — Called when a previously created loader is being reset, thus making its data unavailable.

These methods are described in more detail in the following sections.

　　第一个用于创建LOADER，一般来说，只有init的时候ID对应的LOADER不在的时候才会运行该方法。

　　第二个一般用来获取数据后的填充适配器。

　　第三个用于重置。

 

　　Loader可以自定义，一般使用继承AsyncTaskLoader的类。

　　

/**
 * A custom Loader that loads all of the installed applications.
 */
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);

        // Retrieve the package manager for later use; note we don't
        // use 'context' directly but instead the save global application
        // context returned by getContext().
        mPm = getContext().getPackageManager();
    }

    /**
     * This is where the bulk of our work is done.  This function is
     * called in a background thread and should generate a new set of
     * data to be published by the loader.
     */
    @Override public List<AppEntry> loadInBackground() {
        // Retrieve all known applications.
        List<ApplicationInfo> apps = mPm.getInstalledApplications(
                PackageManager.GET_UNINSTALLED_PACKAGES |
                PackageManager.GET_DISABLED_COMPONENTS);
        if (apps == null) {
            apps = new ArrayList<ApplicationInfo>();
        }

        final Context context = getContext();

        // Create corresponding array of entries and load their labels.
        List<AppEntry> entries = new ArrayList<AppEntry>(apps.size());
        for (int i=0; i<apps.size(); i++) {
            AppEntry entry = new AppEntry(this, apps.get(i));
            entry.loadLabel(context);
            entries.add(entry);
        }

        // Sort the list.
        Collections.sort(entries, ALPHA_COMPARATOR);

        // Done!
        return entries;
    }

    /**
     * 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(List<AppEntry> apps) {
        if (isReset()) {
            // An async query came in while the loader is stopped.  We
            // don't need the result.
            if (apps != null) {
                onReleaseResources(apps);
            }
        }
        List<AppEntry> oldApps = mApps;
        mApps = apps;

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

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

        // Start watching for changes in the app data.
        if (mPackageObserver == null) {
            mPackageObserver = new PackageIntentReceiver(this);
        }

        // Has something interesting in the configuration changed since we
        // last built the app list?
        boolean configChange = mLastConfig.applyNewConfig(getContext().getResources());

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

    /**
     * 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(List<AppEntry> apps) {
        super.onCanceled(apps);

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

    /**
     * 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 (mApps != null) {
            onReleaseResources(mApps);
            mApps = null;
        }

        // Stop monitoring for changes.
        if (mPackageObserver != null) {
            getContext().unregisterReceiver(mPackageObserver);
            mPackageObserver = null;
        }
    }

    /**
     * Helper function to take care of releasing resources associated
     * with an actively loaded data set.
     */
    protected void onReleaseResources(List<AppEntry> apps) {
        // For a simple List<> there is nothing to do.  For something
        // like a Cursor, we would close it here.
    }
}

上面是官方的代码。

构造方法传入context。

loadInBackground是后台真正获取数据的代码。

deliverResult提交数据给客户端，可以直接返回。

onStartLoading处理要开始的请求，官方在这里进行了更新的监听。

onStopLoading要求停止，官方直接使用cancelLoad()。

onCanceled()取消的操作，官方建议对资源进行释放。

onReset，重置，详细看代码。

 

这样的话，就能从头到尾自定义和使用一个Loader了。