ios - UIGraphicsGetImageFromCurrentImageContext crash when using a queue -

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i'm trying understand way code crash when using queue, working when using main thread. code works fine:

- (void)penmoved:(nsset *)touches {     uitouch *touch = [touches anyobject];     cgpoint currentpoint = [touch locationinview:self];     cgpoint midpoint = midpoint(previouspoint, currentpoint);      cgrect bounds = self.bounds;     uigraphicsbeginimagecontextwithoptions(bounds.size, yes, [uiscreen mainscreen].scale);      [path addquadcurvetopoint:midpoint controlpoint:previouspoint];     [incrementalimage drawatpoint:cgpointzero];     [path stroke];      previouspoint = currentpoint;      incrementalimage = uigraphicsgetimagefromcurrentimagecontext();      uigraphicsendimagecontext();      [self setneedsdisplay]; }

but version of code crashes after while when incrementalimage = uigraphicsgetimagefromcurrentimagecontext(); called.

- (void)penmoved:(nsset *)touches {     dispatch_async(drawingqueue, ^{         uitouch *touch = [touches anyobject];         cgpoint currentpoint = [touch locationinview:self];         cgpoint midpoint = midpoint(previouspoint, currentpoint);          cgrect bounds = self.bounds;         uigraphicsbeginimagecontextwithoptions(bounds.size, yes, [uiscreen mainscreen].scale);          [path addquadcurvetopoint:midpoint controlpoint:previouspoint];         [incrementalimage drawatpoint:cgpointzero];         [path stroke];          previouspoint = currentpoint;          incrementalimage = uigraphicsgetimagefromcurrentimagecontext();          uigraphicsendimagecontext();          dispatch_async(dispatch_get_main_queue(), ^{             [self setneedsdisplay];         });     }); }

drawingqueue defined ivar this:

dispatch_queue_t drawingqueue;

and initialized this:

drawingqueue = dispatch_queue_create("drawingqueue", null);

you talking touches. talking view. things not thread safe. must not talk them on background thread. , talking incrementalimage property on 2 different threads. , that's obvious dangers; there other properties in there too.


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