CODEWITHSUNDEEP
uiimagensdatawritetofileuiimagepngrepresentation
Shawn Throop
Shawn Throop

Reputation: 1291

Why does saving UIImages to disk increase memory usage

I am downloading cover images uploaded by App.net users. App.net requires these cover images to be at least 960 pixels wide. I fetch them with a simple AFImageRequestOperation:

NSURLRequest *urlRequest = [NSURLRequest requestWithURL:URL];
AFImageRequestOperation *imageRequestOperation = [AFImageRequestOperation imageRequestOperationWithRequest:urlRequest success:^(UIImage *image) {
    if (completionHandler) {
        completionHandler(image); // Load image into UI...
    }
}];
[self.fetchQueue addOperation:imageRequestOperation];

This is working, no memory spikes.

I want to cache the authenticated users' images so users don't have to download them each time the app opens. As soon as I archive the downloaded image to disk, I get huge spikes in memory. For example, my cover image is currently 3264 x 2448 pixels. When downloaded on my Mac it comes to around 1,3 MB. However, as soon as I create a NSData object with either UIImagePNGRepresentation(image) or via TMCache's setObject:forKey: method, the app's used memory spikes to around 60,0 MB.

For clarity, This is all I'm doing to write the file to disk:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
    NSURL *fileURL = ... //  URL of file in "/Application Support" 
    NSData *imageData = UIImagePNGRepresentation(imageToSave);
    [imageData writeToURL:fileURL atomically:YES];
});

Can anyone tell me what is going on? Why is a 1,3 MB being extrapolated into almost sixty times that. How can I avoid this massive and potentially crippling inflation. This is one image, what if the user opens several profiles, each with a cached image?

Upvotes: 1

Views: 796

Answers (1)

azsromej
azsromej

Reputation: 1629

The image dimensions are what have the greatest bearing on memory usage. For a given image size (regardless of PNG, JPG), the memory usage is pretty much the same and is given by: width x height x 4 bytes.

A cover image of 3264x2448 would decode to roughly 32MB. Perhaps the atomic write explains the doubling you see.

Spikes like this may be unavoidable if that's the size of the image you need to work with. The important thing is to make sure the memory usage isn't growing without bound. When you run the app and look at the memory instrument gauge, does it eventually go down as your app does its work? You can also try wrapping the image-writing code in an @autoreleasepool.

Upvotes: 1

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