Question

I am trying to save a list of users (with UIImage property) to NSUserdefault whenever the user clicks on a certain tab in my app.

In my ‘user’ object, I implemented the encoding as shown below:

- (void)encodeWithCoder:(NSCoder *)encoder
{
    NSURL *url = [NSURL URLWithString:self.avatar_url];
    NSData *data = [NSData dataWithContentsOfURL:url];
    self.avatar = [[[UIImage alloc] initWithData:data] autorelease];

    [encoder encodeObject:UIImagePNGRepresentation(self.avatar) forKey:@"avatar"];
}

However, I realize that by doing this, my app hangs after clicking on the tab that triggers the saving to NSUserdefault. I attribute the slow performance to the conversion (and encoding) of a UIImage from a URL.

Is there a way to speed up this process of encoding and saving an image to NSUserdefault (that does not cause the app to lag).

Answer 1

The reason it is slow is because you are blocking the main thread until the image downloads. Whenever performing operations that rely on network activity you should move that to a background thread. And as @Richard J. Rosss III pointed out you can save the data directly, there is little reason to convert to an image then back to data unless you absolutely need a png and the original format was not png.

Concurrency Programming Guide