R – Cocoa Touch Question. Using KVO in a touch sequence context

cocoacocoa-touchkey-value-observingobjective cuikit

I would like to use KVO in the following context:

1) In touchesBegan:withEvent: I alloc/init an instance of an object that I then observe via KVO

My intent is to observe varous behaviors of the object throughout its life time.

2) In touchesEnded:withEvent: I assign this instance to an NSMutableArray and release the instance reference since NSMutableArray now retains it. I also must remove the oberver of the instance via removeObserver:forKeyPath:

This is problematic because I now have lost all observation unless I add the observe back again to the array element which smells bad.

Is there a way to have the observer remain attached to the object regardless of who owns it?

Thanks,
Doug

Best Answer

In Objective-C, you don't "own" an object, you merely have a claim on it. You don't need to release the instance just because the NSMutableArray retains it -- you can both have a claim on it. When you've finished with the object, remove yourself as an observer and release the object. When you've finished with the NSMutableArray, release that. This way, everything takes care of itself.

Related Solutions

Cocoa – How to get pixel data from a UIImage (Cocoa Touch) or CGImage (Core Graphics)

FYI, I combined Keremk's answer with my original outline, cleaned-up the typos, generalized it to return an array of colors and got the whole thing to compile. Here is the result:

+ (NSArray*)getRGBAsFromImage:(UIImage*)image atX:(int)x andY:(int)y count:(int)count
{
    NSMutableArray *result = [NSMutableArray arrayWithCapacity:count];

    // First get the image into your data buffer
    CGImageRef imageRef = [image CGImage];
    NSUInteger width = CGImageGetWidth(imageRef);
    NSUInteger height = CGImageGetHeight(imageRef);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = (unsigned char*) calloc(height * width * 4, sizeof(unsigned char));
    NSUInteger bytesPerPixel = 4;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(rawData, width, height,
                    bitsPerComponent, bytesPerRow, colorSpace,
                    kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    CGColorSpaceRelease(colorSpace);

    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGContextRelease(context);

    // Now your rawData contains the image data in the RGBA8888 pixel format.
    NSUInteger byteIndex = (bytesPerRow * y) + x * bytesPerPixel;
    for (int i = 0 ; i < count ; ++i)
    {
        CGFloat alpha = ((CGFloat) rawData[byteIndex + 3] ) / 255.0f;
        CGFloat red   = ((CGFloat) rawData[byteIndex]     ) / alpha;
        CGFloat green = ((CGFloat) rawData[byteIndex + 1] ) / alpha;
        CGFloat blue  = ((CGFloat) rawData[byteIndex + 2] ) / alpha;
        byteIndex += bytesPerPixel;

        UIColor *acolor = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
        [result addObject:acolor];
    }

  free(rawData);

  return result;
}

R – How to observe the creation/destruction of an object instance

Doug - there really isn't enough information in your description to know what it is you are doing and how to best (or if it is appropriate at all) apply KVO to the problem.

KVO is all about observing properties on objects. You typically shouldn't care when they are created or destroyed except insofar as you must stop observing them before they are destroyed.

You should instead start and stop observing objects when those objects become interesting to you. Consider a graphics drawing package where a document has an ordered array of shapes, and you are interested in observing the backgroundColor property of each shape.

We wouldn't try to observe the instantiation and deallocation of the Shape instance, but instead we observe the "shapes" property on the document. Through that observer, we can determine when a shape is added to, or removed from, the document. When a shape is added to the document, we start observing it. When it is removed from the document, we stop observing it. (Note that it may be removed from the document but not deallocated, if it is on the undo stack, etc.)

In the object graph for your model, to use KVO you'll want to add and remove the objects from your object graph in a KVO compliant way so you can observe the relationship mutations, and in that observer, start and stop property observers on the related objects.

Best Answer

Related Solutions

Cocoa – How to get pixel data from a UIImage (Cocoa Touch) or CGImage (Core Graphics)

R – How to observe the creation/destruction of an object instance

Related Topic