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PONSManagedObject's KVO method calls for external binary data attributes make table view scrolling sluggish, why?
 

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  1. PONSManagedObject's KVO method calls for external binary data attributes make table view scrolling sluggish, why?
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    <p>After weeks of debugging, I finally found the culprits of the sporadic lags during my table view scrolling. They are <code>-willChangeValueForKey:</code> and <code>-didChangeValueForKey:</code> calls for an external binary data attribute.</p> <p>For convenience, you can think of my app as a Twitter client. So the major entity is <code>Tweet</code> which has a <code>thumbnail_pic_data</code> attribute that I set to <code>allowsExternalBinaryDataStorage</code>. A corresponding non-persistent property called <code>thumbnail_picture</code> is used as its convenient accessor.</p> <p>The table view is a tweet list view — the timeline, with every tweet's <code>thumbnail_picture</code> displayed inline of its corresponding cell. Pictures are downloaded lazily. After successful downloading, I set <code>thumbnail_pic_data</code> with a custom setter as below:</p> <pre><code>- (void)setThumbnail_pic_data:(NSData *)thumbnail_pic_data { [self willChangeValueForKey:@"thumbnail_pic_data"]; // culprit [self setPrimitiveThumbnail_pic_data:thumbnail_pic_data]; [self didChangeValueForKey:@"thumbnail_pic_data"]; // culprit UIImage *picture; if (thumbnail_pic_data) { picture = [UIImage imageWithData:thumbnail_pic_data]; } self.thumbnail_picture = picture; } </code></pre> <p>With the code above, I see sporadic lags during my table view scrolling after each picture downloading. After I comment out <code>willChangeValueForKey:</code> and <code>didChangeValueForKey:</code> calls, the lags are gone. So I know they are the culprits.</p> <p>However, the timing results I got with the follow code shows that they are not <em>directly</em> using the CPU for a long time:</p> <pre><code>CFAbsoluteTime t1 = CFAbsoluteTimeGetCurrent(); [self willChangeValueForKey:@"thumbnail_pic_data"]; CFAbsoluteTime t2 = CFAbsoluteTimeGetCurrent(); NSLog(@"willChangeValueForKey time: %f", t2 - t1); [self setPrimitiveThumbnail_pic_data:thumbnail_pic_data]; CFAbsoluteTime t3 = CFAbsoluteTimeGetCurrent(); NSLog(@"setPrimitiveThumbnail_pic_data time: %f", t3 - t2); [self didChangeValueForKey:@"thumbnail_pic_data"]; CFAbsoluteTime t4 = CFAbsoluteTimeGetCurrent(); NSLog(@"didChangeValueForKey time: %f", t4 - t3); </code></pre> <p>Timing results:</p> <blockquote> <p>willChangeValueForKey time: 0.000145<br> setPrimitiveThumbnail_pic_data time: 0.001512<br> didChangeValueForKey time: 0.001810 </p> <p>willChangeValueForKey time: 0.000138<br> setPrimitiveThumbnail_pic_data time: 0.001418<br> didChangeValueForKey time: 0.002211 </p> <p>willChangeValueForKey time: 0.000302<br> setPrimitiveThumbnail_pic_data time: 0.001891<br> didChangeValueForKey time: 0.003349 </p> <p>willChangeValueForKey time: 0.000162<br> setPrimitiveThumbnail_pic_data time: 0.001462<br> didChangeValueForKey time: 0.002114</p> </blockquote> <p>As a temporary workaround, I'll leave those two KVO calls commented out. I'm not sure whether any bad thing would happen for <code>Core Data Programming Guide</code> says:</p> <blockquote> <p>You must ensure that you invoke the relevant access and change notification methods (willAccessValueForKey:, didAccessValueForKey:, willChangeValueForKey:, didChangeValueForKey:, willChangeValueForKey:withSetMutation:usingObjects:, and didChangeValueForKey:withSetMutation:usingObjects:).</p> </blockquote> <p>Most importantly, I want to know why those two KVO calls makes table view scrolling sluggish so that hopefully I can find a better workaround.</p>
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    1. COSluggish behaviour means that CPU intensive process are running in the main thread. What if `willChangeValueForKey` and `didChangeValueForKey` are creating processes in the current (main) thread and that's why you're getting those results? Have you tried running setThumbnail_pic_data: in a background thread?
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    2. CO@rvil but Core Data is not thread safe.
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