Question

I am going through the following tutorial. I need to get thumbnail images of a large image. According to the tutorial the following method does it.

CGImageRef MyCreateThumbnailImageFromData (NSData * data, int imageSize)

note: look at the tutorial if you need the full code for this method

My questions are;

1.) i have a URL http://www.myimage.com/smile.jpg , and i need to resize it to a thumbnail. I don’t undestand what is the NSData * data parameter. All what i have is a String URL. So how can i pass my URL to this method programatically ?

2.) The above method returns a CGImageRef But i need a UIImage so i could add it to a UIIMageVIew and then display it in my project. So how can i use CGImageRef to display images in my project ?

3.) The images i am downloading are very big, 2MB or more. By making it to appear in a thumbnail size, will it reduce the time taken to load the image to the view ?

Answer 1

Lets go through this, step by step:

1) An NSData object is a wrapper for an array of bytes, or char *. This is the raw bytes of the image you need.

2) A CGImageRef is CoreGraphics‘s way of representing an image, which can be converted to a UIImage with the selector +imageWithCGImage:. Generally speaking, you have more fine control over the image with a CGImageRef.

3) Converting these images to a thumbnail will not reduce the time it takes to download. The file must first be downloaded to memory before it is converted.

Example of how to use your function:

int myImageSize = .... // do what you need to to figure out the size of the image
UIImage *myImage = [UIImage imageWithCGImage:MyCreateThumbnailImageFromData([NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://www.myimage.com/smile.jpg"]], myImageSize)];

This, however will block the user interface, consider using GCD or NSURLConnection instead of -dataWithContentsOfURL:.

EDIT: Example with GCD:

dispatch_async(dispatch_get_global_queue(0, 0), ^{
    __block UIImage *myImage = [UIImage imageWithCGImage:MyCreateThumbnailImageFromData([NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://www.myimage.com/smile.jpg"]], myImageSize)];

    [myImageView performSelectorOnMainThread:@selector(setImage:) withObject:myImage waitUntilDone:NO];
});