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    <p><a href="https://developer.apple.com/library/ios/#releasenotes/General/WhatsNewIniPhoneOS/Articles/iPhoneOS4.html#//apple_ref/doc/uid/TP40009559-SW29" rel="noreferrer"><em>What's New in iOS: iOS 4.0</em></a> says this:</p> <blockquote> <ul> <li>Drawing to a graphics context in UIKit is now thread-safe. Specifically: <ul> <li>The routines used to access and manipulate the graphics context can now correctly handle contexts residing on different threads.</li> <li>String and image drawing is now thread-safe.</li> <li>Using color and font objects in multiple threads is now safe to do.</li> </ul></li> </ul> </blockquote> <p>It sounds like you tried something like this:</p> <pre><code>UIGraphicsBeginImageContextWithOptions(size,false,0); dispatch_async(someQueue, ^{ [UIColor.whiteColor setFill]; UIRectFill(0, 0, 20, 20); dispatch_async(dispatch_get_main_queue(), ^{ UIImage *image = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); self.imageView.image = image; }); }; </code></pre> <p>That won't work because each thread has its own stack of graphics contexts (starting in iOS 4.0). You need to do it like this:</p> <pre><code>dispatch_async(someQueue, ^{ UIGraphicsBeginImageContextWithOptions(size,false,0); [UIColor.whiteColor setFill]; UIRectFill(0, 0, 20, 20); UIImage *image = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); dispatch_async(dispatch_get_main_queue(), ^{ self.imageView.image = image; }); }; </code></pre> <h3>UPDATE</h3> <p>The <a href="https://developer.apple.com/library/ios/documentation/uikit/reference/UIKitFunctionReference/Reference/reference.html" rel="noreferrer">documentation</a> for <code>UIGraphicsBeginImageContextWithOptions</code> and other UIKit graphics functions now says</p> <blockquote> <p>In iOS 4 and later, you may call this function from any thread of your app.</p> </blockquote> <p>The <a href="https://developer.apple.com/library/ios/documentation/uikit/reference/UIColor_Class/Reference/Reference.html" rel="noreferrer">documentation</a> for <code>UIColor</code> says</p> <blockquote> <p>Color objects are immutable and so it is safe to use them from multiple threads in your app.</p> </blockquote> <p>The <a href="https://developer.apple.com/library/ios/documentation/uikit/reference/UIFont_Class/Reference/Reference.html" rel="noreferrer">documentation</a> for <code>UIFont</code> says</p> <blockquote> <p>Font objects are immutable and so it is safe to use them from multiple threads in your app.</p> </blockquote> <p><strong>However</strong>, the <a href="https://developer.apple.com/library/iOs/documentation/UIKit/Reference/NSString_UIKit_Additions/Reference/Reference.html" rel="noreferrer">documentation</a> for the UIKit <code>NSString</code>-drawing additions says</p> <blockquote> <p>The methods described in this class extension must be used from your app’s main thread.</p> </blockquote> <p>So you must <strong>not</strong> try something like <code>[@"hello" drawAtPoint:CGPointZero withAttributes:attrs]</code> from a background thread.</p>
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    1. POIs it possible to use Quartz 2D to make a UIImage on another thread?
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    1. POIs it possible to use Quartz 2D to make a UIImage on another thread?
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    1. COha thats exactly what i tried. Thanks for the great answer.
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    2. COThe docs says that UIGraphicsBeginImageContextWithOptions should be called on the main thread only, but I use it in background thread without having any problems.. Do you have any idea why the docs says it should be called on the main thread?
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    3. CO@graver I don't know for sure. I suppose it does something like `[[UIScreen mainScreen] scale]` if you pass 0 for the scale, and maybe that's not thread-safe.
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