StackOverflow2013

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POCoreGraphics - Blending only *part* of a view
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copied!<p>I recently came across this brilliant article about improving scroll performance with UITableViewCells: <a href="http://engineering.twitter.com/2012/02/simple-strategies-for-smooth-animation.html" rel="noreferrer">http://engineering.twitter.com/2012/02/simple-strategies-for-smooth-animation.html</a> -- While many great tips can be found in this article, there is one in particular that has me intrigued:</p> <blockquote> <p>Tweets in Twitter for iPhone 4.0 have a drop shadow on top of a subtle textured background. This presented a challenge, as blending is expensive. <strong>We solved this by reducing the area Core Animation has to consider non-opaque, by splitting the shadow areas from content area of the cell.</strong></p> </blockquote> <p>Using the iOS Simulator, clicking <em>Debug</em> - <em>Color Blended Layers</em> would reveal something like this:</p> <p><img src="https://i.imgur.com/RX5C2.png" alt="TwitterTableViewCell"></p> <p>The areas marked in red are blended, and the green area is opaque. Great. What the article fails to mention is: How do I implement this? It is my understanding that a UIView is either opaque or it's not. It seems to me that the only way to accomplish this would be with subviews, but the article explicitly states that as being <em>a naive implementation</em>: </p> <blockquote> <p>Instead, our Tweet cells contain a single view with no subviews; a single drawRect: draws everything.</p> </blockquote> <p>So how do I section off what is opaque, and what is not in my single drawRect: method?</p>

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