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POData out of sync between a custom CursorLoader and a CursorAdapter backing a ListView
 

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  1. POData out of sync between a custom CursorLoader and a CursorAdapter backing a ListView
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    <h2>Background:</h2> <p>I have a custom <code>CursorLoader</code> that works directly with SQLite Database instead of using a <code>ContentProvider</code>. This loader works with a <code>ListFragment</code> backed by a <code>CursorAdapter</code>. So far so good.</p> <p>To simplify things, lets assume there is a Delete button on the UI. When user clicks this, I delete a row from the DB, and also call <code>onContentChanged()</code> on my loader. Also, on <code>onLoadFinished()</code> callback, I call <code>notifyDatasetChanged()</code> on my adapter so as to refresh the UI.</p> <h2>Problem:</h2> <p>When the delete commands happen in rapid succession, meaning the <code>onContentChanged()</code> is called in rapid succession, <strong><code>bindView()</code> ends up to be working with stale data</strong>. What this means is a row has been deleted, but the ListView is still attempting to display that row. This leads to Cursor exceptions.</p> <p>What am I doing wrong?</p> <h2>Code:</h2> <p>This is a custom CursorLoader (based on <a href="https://groups.google.com/d/msg/android-developers/J-Uql3Mn73Y/3haYPQ-pR7sJ" rel="noreferrer">this advice</a> by Ms. Diane Hackborn)</p> <pre><code>/** * An implementation of CursorLoader that works directly with SQLite database * cursors, and does not require a ContentProvider. * */ public class VideoSqliteCursorLoader extends CursorLoader { /* * This field is private in the parent class. Hence, redefining it here. */ ForceLoadContentObserver mObserver; public VideoSqliteCursorLoader(Context context) { super(context); mObserver = new ForceLoadContentObserver(); } public VideoSqliteCursorLoader(Context context, Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) { super(context, uri, projection, selection, selectionArgs, sortOrder); mObserver = new ForceLoadContentObserver(); } /* * Main logic to load data in the background. Parent class uses a * ContentProvider to do this. We use DbManager instead. * * (non-Javadoc) * * @see android.support.v4.content.CursorLoader#loadInBackground() */ @Override public Cursor loadInBackground() { Cursor cursor = AppGlobals.INSTANCE.getDbManager().getAllCameras(); if (cursor != null) { // Ensure the cursor window is filled int count = cursor.getCount(); registerObserver(cursor, mObserver); } return cursor; } /* * This mirrors the registerContentObserver method from the parent class. We * cannot use that method directly since it is not visible here. * * Hence we just copy over the implementation from the parent class and * rename the method. */ void registerObserver(Cursor cursor, ContentObserver observer) { cursor.registerContentObserver(mObserver); } } </code></pre> <p>A snippet from my <code>ListFragment</code> class that shows the <code>LoaderManager</code> callbacks; as well as a <code>refresh()</code> method that I call whenever user adds/deletes a record.</p> <pre><code>@Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); mListView = getListView(); /* * Initialize the Loader */ mLoader = getLoaderManager().initLoader(LOADER_ID, null, this); } @Override public Loader&lt;Cursor&gt; onCreateLoader(int id, Bundle args) { return new VideoSqliteCursorLoader(getActivity()); } @Override public void onLoadFinished(Loader&lt;Cursor&gt; loader, Cursor data) { mAdapter.swapCursor(data); mAdapter.notifyDataSetChanged(); } @Override public void onLoaderReset(Loader&lt;Cursor&gt; loader) { mAdapter.swapCursor(null); } public void refresh() { mLoader.onContentChanged(); } </code></pre> <p>My <code>CursorAdapter</code> is just a regular one with <code>newView()</code> being over-ridden to return newly inflated row layout XML and <code>bindView()</code> using the <code>Cursor</code> to bind columns to <code>View</code>s in the row layout.</p> <hr> <h2>EDIT 1</h2> <p>After digging into this a bit, I think the fundamental issue here is the way the <code>CursorAdapter</code> handles the underlying <code>Cursor</code>. I'm trying to understand how that works.</p> <p>Take the following scenario for better understanding.</p> <ol> <li>Suppose the <code>CursorLoader</code> has finished loading and it returns a <code>Cursor</code> that now has 5 rows.</li> <li>The <code>Adapter</code> starts displaying these rows. It moves the <code>Cursor</code> to the next position and calls <code>getView()</code></li> <li>At this point, even as the list view is in the process of being rendered, a row (say, with _id = 2) is deleted from the database.</li> <li><strong>This is where the issue is</strong> - The <code>CursorAdapter</code> has moved the <code>Cursor</code> to a position which corresponds to a deleted row. The <code>bindView()</code> method still tries to access the columns for this row using this <code>Cursor</code>, which is invalid and we get exceptions.</li> </ol> <h3>Question:</h3> <ul> <li>Is this understanding correct? I am particularly interested in point 4 above where I am making the assumption that when a row gets deleted, the <code>Cursor</code> doesn't get refreshed unless I ask for it to be.</li> <li>Assuming this is right, how do I ask my <code>CursorAdapter</code> to discard/abort its rendering of the <code>ListView</code> <strong>even as it is in progress</strong> and ask it to use the fresh <code>Cursor</code> (returned through <code>Loader#onContentChanged()</code> and <code>Adapter#notifyDatasetChanged()</code>) instead?</li> </ul> <p><strong>P.S.</strong> Question to moderators: Should this edit be moved to a separate question?</p> <hr> <h2>EDIT 2</h2> <p>Based on suggestion from various answers, it looks like there was a fundamental mistake in my understanding of how <code>Loader</code>s work. It turns out that:</p> <ol> <li>The <code>Fragment</code> or <code>Adapter</code> should not be directly operating on the <code>Loader</code> at all.</li> <li>The <code>Loader</code> should monitor for all changes in data and should just give the <code>Adapter</code> the new <code>Cursor</code> in <code>onLoadFinished()</code> whenever data changes.</li> </ol> <p>Armed with this understanding, I attempted the following changes. - No operation on the <code>Loader</code> whatsoever. The refresh method does nothing now.</p> <p>Also, to debug what's going on inside the <code>Loader</code> and the <code>ContentObserver</code>, I came up with this:</p> <pre><code>public class VideoSqliteCursorLoader extends CursorLoader { private static final String LOG_TAG = "CursorLoader"; //protected Cursor mCursor; public final class CustomForceLoadContentObserver extends ContentObserver { private final String LOG_TAG = "ContentObserver"; public CustomForceLoadContentObserver() { super(new Handler()); } @Override public boolean deliverSelfNotifications() { return true; } @Override public void onChange(boolean selfChange) { Utils.logDebug(LOG_TAG, "onChange called; selfChange = "+selfChange); onContentChanged(); } } /* * This field is private in the parent class. Hence, redefining it here. */ CustomForceLoadContentObserver mObserver; public VideoSqliteCursorLoader(Context context) { super(context); mObserver = new CustomForceLoadContentObserver(); } /* * Main logic to load data in the background. Parent class uses a * ContentProvider to do this. We use DbManager instead. * * (non-Javadoc) * * @see android.support.v4.content.CursorLoader#loadInBackground() */ @Override public Cursor loadInBackground() { Utils.logDebug(LOG_TAG, "loadInBackground called"); Cursor cursor = AppGlobals.INSTANCE.getDbManager().getAllCameras(); //mCursor = AppGlobals.INSTANCE.getDbManager().getAllCameras(); if (cursor != null) { // Ensure the cursor window is filled int count = cursor.getCount(); Utils.logDebug(LOG_TAG, "Count = " + count); registerObserver(cursor, mObserver); } return cursor; } /* * This mirrors the registerContentObserver method from the parent class. We * cannot use that method directly since it is not visible here. * * Hence we just copy over the implementation from the parent class and * rename the method. */ void registerObserver(Cursor cursor, ContentObserver observer) { cursor.registerContentObserver(mObserver); } /* * A bunch of methods being overridden just for debugging purpose. * We simply include a logging statement and call through to super implementation * */ @Override public void forceLoad() { Utils.logDebug(LOG_TAG, "forceLoad called"); super.forceLoad(); } @Override protected void onForceLoad() { Utils.logDebug(LOG_TAG, "onForceLoad called"); super.onForceLoad(); } @Override public void onContentChanged() { Utils.logDebug(LOG_TAG, "onContentChanged called"); super.onContentChanged(); } } </code></pre> <p>And here are snippets of my <code>Fragment</code> and <code>LoaderCallback</code></p> <pre><code>@Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); mListView = getListView(); /* * Initialize the Loader */ getLoaderManager().initLoader(LOADER_ID, null, this); } @Override public Loader&lt;Cursor&gt; onCreateLoader(int id, Bundle args) { return new VideoSqliteCursorLoader(getActivity()); } @Override public void onLoadFinished(Loader&lt;Cursor&gt; loader, Cursor data) { Utils.logDebug(LOG_TAG, "onLoadFinished()"); mAdapter.swapCursor(data); } @Override public void onLoaderReset(Loader&lt;Cursor&gt; loader) { mAdapter.swapCursor(null); } public void refresh() { Utils.logDebug(LOG_TAG, "CamerasListFragment.refresh() called"); //mLoader.onContentChanged(); } </code></pre> <p>Now, whenever there is a change in the DB (row added/deleted), the <code>onChange()</code> method of the <code>ContentObserver</code> should be called - correct? I don't see this happening. My <code>ListView</code> never shows any change. The only time I see any change is if I explicitly call <code>onContentChanged()</code> on the <code>Loader</code>.</p> <p>What's going wrong here?</p> <hr> <h2>EDIT 3</h2> <p>Ok, so I re-wrote my <code>Loader</code> to extend directly from <code>AsyncTaskLoader</code>. I still don't see my DB changes being refreshed, nor the <code>onContentChanged()</code> method of my <code>Loader</code> being called when I insert/delete a row in the DB :-(</p> <p>Just to clarify a few things:</p> <ol> <li><p>I used the code for <code>CursorLoader</code> and just modified one single line that returns the <code>Cursor</code>. Here, I replaced the call to <code>ContentProvider</code> with my <code>DbManager</code> code (which in turn uses <code>DatabaseHelper</code> to perform a query and return the <code>Cursor</code>).</p> <p><code>Cursor cursor = AppGlobals.INSTANCE.getDbManager().getAllCameras();</code></p></li> <li><p>My inserts/updates/deletes on the database happen from elsewhere and not through the <code>Loader</code>. In most cases the DB operations are happening in a background <code>Service</code>, and in a couple of cases, from an <code>Activity</code>. I directly use my <code>DbManager</code> class to perform these operations.</p></li> </ol> <p>What I still don't get is - <strong>who tells my <code>Loader</code> that a row has been added/deleted/modified?</strong> In other words, where is <code>ForceLoadContentObserver#onChange()</code> called? In my Loader, I register my observer on the <code>Cursor</code>:</p> <pre><code>void registerContentObserver(Cursor cursor, ContentObserver observer) { cursor.registerContentObserver(mObserver); } </code></pre> <p>This would imply that the onus is on the <code>Cursor</code> to notify <code>mObserver</code> when it has changed. But, then AFAIK, a 'Cursor' is not a "live" object that updates the data it is pointing to as and when data is modified in the DB.</p> <p>Here's the latest iteration of my Loader:</p> <pre><code>import android.content.Context; import android.database.ContentObserver; import android.database.Cursor; import android.support.v4.content.AsyncTaskLoader; public class VideoSqliteCursorLoader extends AsyncTaskLoader&lt;Cursor&gt; { private static final String LOG_TAG = "CursorLoader"; final ForceLoadContentObserver mObserver; Cursor mCursor; /* Runs on a worker thread */ @Override public Cursor loadInBackground() { Utils.logDebug(LOG_TAG , "loadInBackground()"); Cursor cursor = AppGlobals.INSTANCE.getDbManager().getAllCameras(); if (cursor != null) { // Ensure the cursor window is filled int count = cursor.getCount(); Utils.logDebug(LOG_TAG , "Cursor count = "+count); registerContentObserver(cursor, mObserver); } return cursor; } void registerContentObserver(Cursor cursor, ContentObserver observer) { cursor.registerContentObserver(mObserver); } /* Runs on the UI thread */ @Override public void deliverResult(Cursor cursor) { Utils.logDebug(LOG_TAG, "deliverResult()"); if (isReset()) { // An async query came in while the loader is stopped if (cursor != null) { cursor.close(); } return; } Cursor oldCursor = mCursor; mCursor = cursor; if (isStarted()) { super.deliverResult(cursor); } if (oldCursor != null &amp;&amp; oldCursor != cursor &amp;&amp; !oldCursor.isClosed()) { oldCursor.close(); } } /** * Creates an empty CursorLoader. */ public VideoSqliteCursorLoader(Context context) { super(context); mObserver = new ForceLoadContentObserver(); } @Override protected void onStartLoading() { Utils.logDebug(LOG_TAG, "onStartLoading()"); if (mCursor != null) { deliverResult(mCursor); } if (takeContentChanged() || mCursor == null) { forceLoad(); } } /** * Must be called from the UI thread */ @Override protected void onStopLoading() { Utils.logDebug(LOG_TAG, "onStopLoading()"); // Attempt to cancel the current load task if possible. cancelLoad(); } @Override public void onCanceled(Cursor cursor) { Utils.logDebug(LOG_TAG, "onCanceled()"); if (cursor != null &amp;&amp; !cursor.isClosed()) { cursor.close(); } } @Override protected void onReset() { Utils.logDebug(LOG_TAG, "onReset()"); super.onReset(); // Ensure the loader is stopped onStopLoading(); if (mCursor != null &amp;&amp; !mCursor.isClosed()) { mCursor.close(); } mCursor = null; } @Override public void onContentChanged() { Utils.logDebug(LOG_TAG, "onContentChanged()"); super.onContentChanged(); } } </code></pre>
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    1. COWhy don't you keep it simple: delete the UI element and db row manually instead of loading the cursor on every delete..
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    2. COActually the scenario I presented here is a very simplified form of what my app actually does. Having said that, how can I selectively delete a single row in a `ListView` without going through the Adapter?
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    3. COI think synchronization is a better solution. try my answer..
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