How do I draw text on a background thread in iOS?

Question

I need to draw text on a background thread to save it as an image.

I'm doing

UIGraphicsPushContext()
[NSString drawInRect:]
UIGraphicsPopContext()

The code works fine, but sometimes it crashes in drawInRect when I'm also drawing on the main thread at the same time.

I tried to use NSAttributedString as suggested here: UIStringDrawing methods don't seem to be thread safe in iOS 6. But [NSAttributedString drawInRect:] doesn't seem to render anything on my background thread for some reason. Main thread seems to work fine though.

I've been thinking of using Core Text, but looks like Core Text also has similar problem: CoreText crashes when run in multiple threads

Is there a thread safe way to draw text?

UPDATE: If I run this code, it almost immediately crashes in drawInRect with EXC_BAD_ACCESS:

   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{

      UIGraphicsBeginImageContextWithOptions(CGSizeMake(100, 100), NO, 0);
      UIFont* font = [UIFont systemFontOfSize:14.0f];

      for (int i = 0; i < 100000000; i++) {
         [@"hello" drawInRect:CGRectMake(0, 0, 100, 100) withFont:font];
      }

      UIGraphicsEndImageContext();
   });

   UIGraphicsBeginImageContextWithOptions(CGSizeMake(100, 100), NO, 0);
   UIFont* font = [UIFont systemFontOfSize:12.0f];

   for (int i = 0; i < 100000000; i++) {
      [@"hello" drawInRect:CGRectMake(0, 0, 100, 100) withFont:font];
   }

   UIGraphicsEndImageContext();

If I remove UIFont and just draw text without font it works fine.

UPDATE: This only seems to crash on iOS 6.1, but seems to work fine on iOS 7.1.

I you want help solving a problem, post the actual code giving you the problem and provide details about the crash. — rmaddy, May 19 '14 at 02:50

SomeGuy · Accepted Answer · 2014-05-19T06:17:48.027

6

Since iOS6 (might have been earlier) you can use those methods on a different thread, as long as you have created a new context using UIGraphicsBeginImageContext... on that same thread.

The drawRect: method defaults to the current context of their own thread.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{

    UIGraphicsBeginImageContextWithOptions(CGSizeMake(100, 100), NO, 0);

    UIFont* font = [UIFont systemFontOfSize:26];
    NSString* string = @"hello";
    NSAttributedString* attributedString = [[NSAttributedString alloc] initWithString:string attributes:@{NSFontAttributeName:font}];

    [attributedString drawInRect:CGRectMake(0, 0, 100, 100)];

    UIImage* image = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    [UIImagePNGRepresentation(image) writeToFile:@"/testImage.png" atomically:YES];

});

Run that on the simulator and it will output the result to the root directory of your hard drive.

edited May 19 '14 at 06:17

answered May 19 '14 at 03:11

SomeGuy

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I modified your code to continuously call drawInRect on a background and on the main thread in a loop. Everything is fine, but if I use [@"hello" drawInRect:CGRectMake(0, 0, 100, 100) withFont:font] it crashes. Please see my updated post. – Lev May 19 '14 at 03:42
@Lev I looked at your loop example, it was crashing because of the UIFont though, not the drawInRect: method. I've updated my example to use a UIFont with an NSAttributedString instead, which I have tested with iOS6 inside the big loop and it works fine :) – SomeGuy May 19 '14 at 06:19
I found that it only crashes on iOS 6.1 when using [@"hello" drawInRect:CGRectMake(0, 0, 100, 100) withFont:font] – Lev May 20 '14 at 07:42
@Lev did you try my updated code? I'm no longer using drawInRect:withFont: and instead I'm using an NSAttributedString – SomeGuy May 20 '14 at 07:59

score 0 · Answer 2 · edited May 23 '17 at 11:54

According to Apple's NSString UIKit Additions Reference, [NSString drawInRect:...] is one of those methods that MUST be called from your app's main thread (look at the "Overview" section of that document). It says:

The methods described in this class extension must be used from your app’s main thread.

Then again, anything that updates the UI should always be on the main thread... certainly there are some things that could potentially be on background threads (e.g. including string and image drawing -- see this related question and answers).

And lastly, other people have reported problems trying to draw on background threads using "UIGraphicsPushContext" (at least with iOS 5.X), so that still may be a problem with iOS 6.

So, is there a way to render text into an image on a background thread? If I can't use NSString drawing method on a background thread, what does this statement mean?: "String and image drawing is now thread-safe." — Lev, May 19 '14 at 03:12

How do I draw text on a background thread in iOS?

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