ios - 使用队列时 UIGraphicsGetImageFromCurrentImageContext 崩溃

Question

我试图了解我的代码在使用队列时崩溃的方式，但在使用主线程时却能正常工作。此代码运行良好:

- (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];
}

但是，当调用 incrementalImage = UIGraphicsGetImageFromCurrentImageContext(); 时，此版本的代码会在一段时间后崩溃。

- (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 定义为这样的 ivar:

dispatch_queue_t drawingQueue;

并像这样初始化:

drawingQueue = dispatch_queue_create("drawingQueue", NULL);

Answer 1

你说的是触摸。你在谈论一个观点。这些东西不是线程安全的。您不得在后台线程中谈论它们。您正在谈论两个不同线程上的 incrementalImage 属性。这就是明显的危险；那里也有其他属性。