Question

I have been working on an object class for my iPhone app that lazy loads images when they are first requested by another part of the app. I decided to make the loading of the image thread safe so the same image doesn’t get loaded twice by accident, however I am curious about the overhead of making an @synchronized(self) call every time the accessor is run, like so:

- (UIImage *)image
{
    @synchronized(self)
    {
        if (_image == nil)
        {
            _image = [UIImage imageWithContentsOfFile:self.imageUrl];
        }
    }
    return _image;
}

Would it be better to first check if the property is nil and then use the @synchronized directive?

- (UIImage *)image
{
    if (_image == nil)
    {
        @synchronized(self)
        {
            if (_image == nil)
            {
                _image = [UIImage imageWithContentsOfFile:self.imageUrl];
            }
        }
    }
    return _image;
}

Any thoughts?

Answer 1

The overhead is minimal, but the ovhead of loading an image may be huge and, thus, that you surround it with a synchronization primitive means that anything blocking on that could pay the price….