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    <p>Do you load images from local files by any chance?</p> <p>By using Instruments I've found that there is some lazy loading mechanism in <code>UIImage</code> - the real image data was decompressed from PNG only at the stage of rendering it on the main thread which caused the lag during scrolling.</p> <p>So, after the loading of <code>UIImage</code> with <code>-initWithContentsOfFile:</code> method, I've added code to render the contents of this image to offscreen context, saved that context as new <code>UIImage</code> and used it for <code>UIImageView</code> in <code>UITableViewCell</code>, this made the scroll to be smooth and pleasant to the eye again. </p> <p>In case of reference there is the simple code I'm using to force reading of image contents in separate thread (using ARC):</p> <pre><code>UIImage *productImage = [[UIImage alloc] initWithContentsOfFile:path]; CGSize imageSize = productImage.size; UIGraphicsBeginImageContext(imageSize); [productImage drawInRect:CGRectMake(0, 0, imageSize.width, imageSize.height)]; productImage = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); </code></pre> <p>And I think the surface of <code>UIImage</code> created in such a way will be in the supported format suited for rendering, which will also offload work needed to render it on main thread.</p> <p>EDIT: The docs for <code>UIGraphicsGetImageFromCurrentImageContext()</code> say that it should be only used from main thread, but searching on the net or SO shows that starting from iOS 4 the <code>UIGraphics..</code> methods became thread safe. </p>
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    1. POCustom UITableViewCell with large UIImageView causing scroll lag
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    1. COThis was absolutely invaluable to me. I had been suffering from choppy scrolling on a UICollectionView with UIImageViews in each cell to display a photo. I tried *a lot* of different solutions but the only way I get close to 60FPS was extremely hacky. Most of the solutions I tried involved preloading a NSCache of UIImages using imageWithContentsOfFile but whatever I did there was always a little jump as new cells came on screen until I implemented your code above. As you say, there must be some sort of lazy loading mechanism built into imageWithContentsOfFile which isn't mentioned in the docs.
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      1. USAlistair Holt
    2. COThis work for me, the much smoother after store the UIImage instead of downloaded data then convert to UIImage each time it will be used. Thank you a lot.
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      1. USEnvil
    3. COBeen searching so long for the slow/choppy cause. You sir are a hero
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      1. USuser1838169
 

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