UIImage from NSInputStream

Question

I'm downloading a 2400x1600 image from Parse and I don't want it to hold all that data in memory at once. PFFile object from Parse has a convenient method to get NSData as NSInputStream so when the data is finally downloaded I end up with a NSInputStream. So now I want to use that NSInputStream to get my UIImage. It should work like creating a UIImage with contents of file method i.e. not the whole image is loaded into memory at once. I think that writing to a file from NSInputStream and then use the UIImage's contents of file method should work fine in my case, but I have found no way to write to a file from a NSInputStream.

Any code example or some guideline would be really appreciated.

Thanks in advance.

Answer 1

To accomplish this you can set up an NSOutputStream to then stream the received data to a file. Create your output stream using initToFileAtPath:append: with YES for append. In your input stream call back, pass the data to your output stream by calling write:maxLength: (read more in the docs). Once the stream is complete, you then have the full image on file without ever having it fully in memory.

Henri's answer above is more appropriate since you're using Parse, but this is the general solution.

Answer 2

In the documentation on iOS/OS X, Parse brings this an example.

Retrieving the image back involves calling one of the getData variants on the PFFile. Here we retrieve the image file off another UserPhoto named anotherPhoto:

PFFile *userImageFile = anotherPhoto[@"imageFile"];
[userImageFile getDataInBackgroundWithBlock:^(NSData *imageData, NSError *error) {
    if (!error) {
        UIImage *image = [UIImage imageWithData:imageData];
    }
}];

Now, I don't quite see the reason for you to use NSInputStream, mainly for two reasons:

1. NSInputStream is supposedly meant for INPUTTING data, not taking it from somewhere
2. NSInputStream is meant for streaming, so for scenarios in which you want to do something with the data as it is coming in, from your description it seems as if you only ever care about the data once it has completed the download.

In short, you should be using the aforementioned way, unless you truly care about the way the data is loaded in, for example wanting to manipulate it as it comes in (highly unlikely in the case you describe).

As to having it all in memory at once, the dimensions you give are not that large, yes you could stream it into a file, but assuming you want to show it full-size in the app, the problem of memory would appear at some point nevertheless, i.e you would just be postponing the inevitable. If that is not the case (not showing full-size), then it might be a good idea to chop the source image up into tiles and use those instead, far quicker to download specific tiles and easier on memory.