Android Performance Patterns - s5ep8 Threading And Loaders

Let’s say you’ve got some 20 bitmaps being decoded on a worker thread. Well, what happens if your activity is destroyed before that work completes?
相比于本期的大叔，笔者更喜欢之前的光头哥
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No One Writes to the Colonel

五十六年了，上校唯一做过的事情就是等待。

我们知道，Worker Thread是在UI Thread之外用来处理耗时任务的线程，存在这样一种情况，当任务由Worker Thread执行完毕时，引发这项任务的Activity已经走完了onDestroy，这会产生什么样的后果呢？

第一，Activity虽然结束了它的生命周期，可这个实例并没有从内存中释放——因为Worker Thread还拥有着Activity的reference，这会导致内存泄漏，如图1；第二，已经onDestroy过的Activity早已从屏幕上消失，无法响应任务执行后的返回，我们必须重新创建Activity2来处理任务返回后界面变化，如图2。

图1

图2

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You Should Use Loaders

针对上面的场景，建议使用Loader。设计合理的Loader可以避免内存泄漏、正确处理UI事件、并且不需要重复执行。

LoaderManager是用来管理Loader的接口，借助前面链接中例子不难理解Loader运行的机制：获取LoaderManager实例－初始化Loader－实现LoaderCallbacks接口处理回调。

LoaderManager可以缓存任务执行结果，这样当Activity销毁并重建时，就不需要重复执行前面已经执行过的任务了。

如果一个Activity被终止后不会重建，我们就应该在LoaderManager的onLoaderReset中处理这种情形，释放引用。

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The Downside of Loaders

集成较为复杂，代码量大。

参考下面Reference给出的例子，实现了Fragment的ListView从联系人ContentProvider中通过CursorLoader获取数据的过程。

public static class CursorLoaderListFragment extends ListFragment
        implements OnQueryTextListener, OnCloseListener,
        LoaderManager.LoaderCallbacks<Cursor> {

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

    // The SearchView for doing filtering.
    SearchView mSearchView;

    // If non-null, this is the current filter the user has provided.
    String mCurFilter;

    @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);
    }

    public static class MySearchView extends SearchView {
        public MySearchView(Context context) {
            super(context);
        }

        // The normal SearchView doesn't clear its search text when
        // collapsed, so we will do this for it.
        @Override
        public void onActionViewCollapsed() {
            setQuery("", false);
            super.onActionViewCollapsed();
        }
    }

    @Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        // Place an action bar item for searching.
        MenuItem item = menu.add("Search");
        item.setIcon(android.R.drawable.ic_menu_search);
        item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM
                | MenuItem.SHOW_AS_ACTION_COLLAPSE_ACTION_VIEW);
        mSearchView = new MySearchView(getActivity());
        mSearchView.setOnQueryTextListener(this);
        mSearchView.setOnCloseListener(this);
        mSearchView.setIconifiedByDefault(true);
        item.setActionView(mSearchView);
    }

    public boolean onQueryTextChange(String newText) {
        // Called when the action bar search text has changed.  Update
        // the search filter, and restart the loader to do a new query
        // with this filter.
        String newFilter = !TextUtils.isEmpty(newText) ? newText : null;
        // Don't do anything if the filter hasn't actually changed.
        // Prevents restarting the loader when restoring state.
        if (mCurFilter == null && newFilter == null) {
            return true;
        }
        if (mCurFilter != null && mCurFilter.equals(newFilter)) {
            return true;
        }
        mCurFilter = newFilter;
        getLoaderManager().restartLoader(0, null, this);
        return true;
    }

    @Override public boolean onQueryTextSubmit(String query) {
        // Don't care about this.
        return true;
    }

    @Override
    public boolean onClose() {
        if (!TextUtils.isEmpty(mSearchView.getQuery())) {
            mSearchView.setQuery(null, true);
        }
        return true;
    }

    @Override public void onListItemClick(ListView l, View v, int position, long id) {
        // Insert desired behavior here.
        Log.i("FragmentComplexList", "Item clicked: " + id);
    }

    // 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.CONTACT_PRESENCE,
        Contacts.PHOTO_ID,
        Contacts.LOOKUP_KEY,
    };

    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;
        if (mCurFilter != null) {
            baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,
                    Uri.encode(mCurFilter));
        } else {
            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 + " != '' ))";
        return new CursorLoader(getActivity(), baseUri,
                CONTACTS_SUMMARY_PROJECTION, select, null,
                Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
    }

    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.)
        mAdapter.swapCursor(data);

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

    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);
    }
}

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