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I know it's safe to draw on any thread so long as I call
UIGraphicsBeginImageContextWithOptions(view.bounds.size, view.opaque, 0.0);
UIGraphicsEndImageContext();
on the same thread.
Taking a screenshot of a view via this method takes about 300 ms, which is not bad, but I'm in a tight situation, so I want to do it in a background thread.
Here's what I'm doing:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
UIGraphicsBeginImageContextWithOptions(view.bounds.size, view.opaque, 0.0);
[view.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage * screenshot = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
});
The only thing here in question is the view, which lies on the main thread. Is it safe to call renderInContext on a view.layer from a background thread? Or generally, is it safe to read-only of a UIKit object from another thread?
(And please don't give me the default "UIKit is not thread safe" answer. I already know that. This here is a special case (and don't tell me there are no special cases).)
(the code above works fine, but I'm not sure if that's just a coincidence.)
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Core Data is designed to work in a multithreaded environment. However, not every object under the Core Data framework is thread safe. To use Core Data in a multithreaded environment, ensure that: Managed object contexts are bound to the thread (queue) that they are associated with upon initialization.
In general, immutable classes like NSArray are thread-safe, while their mutable variants like NSMutableArray are not. In fact, it's fine to use them from different threads, as long as access is serialized within a queue.
In general, the collection classes (for example, NSMutableArray , NSMutableDictionary ) are not thread-safe when mutations are concerned. That is, if one or more threads are changing the same array, problems can occur.
Thread Safety APIs in iOSBy creating a queue of tasks where only one task can be processed at any given time, you are indirectly introducing thread safety to the component that is using the queue.
Core Graphics and Core Animation being low-level APIs, are generally thread safe. However, the same rules about access still apply: Any work must not be accessed by more than one thread at the same time, else drawing will fail and your app will crash. I would be wary (but not afraid) of UIImage, as UIKit objects aren't just not thread safe, they're basically ticking time bombs in background threads, and will happily dive straight off a cliff into Exception Land for no good reason. However, because UIImage is just a CGImage wrapper, again, most drawing is thread safe.
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