Using NSImageView to display multiple images in quick sucession

I have an application where, in one window, there is an NSImageView. The user should be able to drag and drop ANY FILE/FOLDER (not only images) into the image view, so I subclassed NSImageView class to add support for those types.

The reason why I chose an NSImageView instead of a normal view is because I also wanted to display an animation (say an arrow pointing downwards and going up and down) when the user hovers over with files ready to drop. My question is this: what would be the best way (most efficient, quickest, least CPU usage, etc) to do this?

In fact, I have already done it, but what made me ask this question is the fact that when I set the images to change at a rate below 0.02 sec it starts to lag. Here is how I did it:

In the NSImageView subclass:

• have an ivar: NSTimer* animTimer;
• override awakeFromNib, calling [super awakeFromNib] and loading the images into an array (about 45 images) using NSImage
• whenever user enters with files, start animTimer with frequency = 0.025 (less and it lags), and a selector that sets the next image in the array (called drawNextImage)
• whenever the user exits or ends the drag and drop, call [animTimer invalidate] to stop updating images

Here is how I set the image in the subclass:

- (void)drawNextImage
{
    currentImageIndex++; // ivar / kNumberDNDImages is a constant defined as 46
    if (currentImageIndex >= kNumberDNDImages) { currentImageIndex = 0;}
    [super setImage: [imagesArray objectAtIndex: currentImageIndex]]; // imagesArray is ivar
}

So, how would I do this quick enough? I'd like the frequency to be about 0.01 secs, but less than 0.025 lags, so that is what I have set for the moment. Oh, and my images are the correct size (+ or - one pixel or something) and they are in .png (I need the transparency 开发者_如何学编程- jpegs, for example, won't do it).

EDIT:

I have tried to follow NSResponder's suggestion, and have updated my method to this:

- (void)drawNextImage
{
    currentImageIndex++;
    if (currentImageIndex >= kNumberDNDImages) { currentImageIndex = 0;}
    NSRect smallImgRect;
    smallImgRect.origin = NSMakePoint(kSmallImageWidth * currentImageIndex, [self.bigDNDImage size].height); // Up left corner - ??
    smallImgRect.size = NSMakeSize(kSmallImageWidth, [self.bigDNDImage size].height);

    // Bottom left corner - ??
    NSPoint imgPoint = NSMakePoint(([self bounds].size.width - kSmallImageWidth) / 2, 0);

    [bigDNDImage drawAtPoint: imgPoint fromRect: smallImgRect operation: NSCompositeCopy fraction: 1];
}

I have also moved this method and the other drag and drop methods from the NSImageView subclass to an NSView subclass I already had. Everything is exactly the same, except for the superclass and this method. I also modified some constants.

In my early testing of this, I got some error/warning messages that didn't stop execution talking abou NSGraphicsContext or something. These have disappeared now, but just so you know. I have absolutely no idea why they were appearing and what they mean. If they ever appear again I'll worry about them, not now :)

EDIT 2:

This is what I'm doing now:

- (void)drawNextImage
{
    currentImageIndex++;
    if (currentImageIndex >= kNumberDNDImages) { currentImageIndex = 0;}
    [self drawCurrentImage];
}
- (void)drawCurrentImage
{
    NSRect smallImgRect;
    smallImgRect.origin = NSMakePoint(kSmallImageWidth * currentImageIndex, 0); // Bottom left, for sure
    smallImgRect.size = NSMakeSize(kSmallImageWidth, [self.bigDNDImage size].height);

    // Bottom left as well
    NSPoint imgPoint = NSMakePoint(([self bounds].size.width - kSmallImageWidth) / 2, 0);

    [bigDNDImage drawAtPoint: imgPoint fromRect: smallImgRect operation: NSCompositeCopy fraction: 1];
}

And the catch here is to call drawCurrentImage when drawRect is called (see, it actually was easier to solve than I thought).

Now, I must say I haven't tried this with the composite image, because I couldn't find a good and quick way to merge 40+ images the way I wanted (one next to the other). But for the ones ineterested, I modified this to do the same thing as my NSImageView subclass (reading 40+ images from an array and displaying them) and I found no speed bump: NSView is as laggy below 0.025 as NSImageView. Also I found some problems when using core animation (the image is drawn in weird places instead of the place I tell her to) and some warnings talking about NSGraphicsContext, which I don't know how to solve at all (I'm a complete noob when it comes to drawing and such with Objective-C's tools). So for the time being I'm using NSImageView, unless I find a way to merge all those images and try it with NSView.

Core Animation would probably be quickest, since it'll do everything on the GPU. Create a layer for each image, setting each layer's contents to the CGImage you can make from each image, add them all as sublayers of a single top-level layer, host the top-level layer in a plain NSView, and then just toggle each image layer's hidden property in turn.

I'd probably draw all of the component images into one long image, and draw segments into a view using -drawAtPoint:fromRect:operation:fraction:. I'm sure you could make it faster than that by resorting to OpenGL, though.