Question

I am using the following code to allow a user of my application to take/choose a photo, which will then be saved to the documents directory and set as the image of a UIImageView:

    -(void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex {

        if (actionSheet.tag == 0){
            if (buttonIndex == 0) {
                NSLog(@"Take Picture Button Clicked");
                // Create image picker controller
                UIImagePickerController *imagePicker = [[UIImagePickerController alloc] init];

                // Set source to the camera
                imagePicker.sourceType =  UIImagePickerControllerSourceTypeCamera;

                // Delegate is self
                imagePicker.delegate = self;

                // Show image picker
                [self presentModalViewController:imagePicker animated:YES];
            } 
            else if (buttonIndex == 1) {
                NSLog(@"Choose From Library Button Clicked");
                // Create image picker controller
                UIImagePickerController *imagePicker = [[UIImagePickerController alloc] init];

                // Set source to the camera
                imagePicker.sourceType =  UIImagePickerControllerSourceTypePhotoLibrary;

                // Delegate is self
                imagePicker.delegate = self;

                // Show image picker
                [self presentModalViewController:imagePicker animated:YES];
            } 
            else if (buttonIndex == 2) {
                NSLog(@"Cancel Button Clicked");
            } 
        }
......

- (void)saveImage:(UIImage*)image:(NSString*)imageName 
{ 
    NSData *imageData = UIImagePNGRepresentation(image); //convert image into .png format.

    NSFileManager *fileManager = [NSFileManager defaultManager];//create instance of NSFileManager

    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES); //create an array and store result of our search for the documents directory in it

    NSString *documentsDirectory = [paths objectAtIndex:0]; //create NSString object, that holds our exact path to the documents directory

    NSString *fullPath = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.png", imageName]]; //add our image to the path 

    [fileManager createFileAtPath:fullPath contents:imageData attributes:nil]; //finally save the path (image)

    receiptImageView1.image = [UIImage imageWithContentsOfFile:fullPath];
    self.receiptImage1 = fullPath;

    NSLog(@"image saved");
}

//Receive the image the user picks from the image picker controller
-(void)imagePickerController:(UIImagePickerController*)picker
didFinishPickingMediaWithInfo:(NSDictionary*)info {
    UIImage* image = [info objectForKey: UIImagePickerControllerOriginalImage];
    NSString* imageName = @"Receipt1Image1";
    [self saveImage:image :imageName];
}

Basically my problem is that this code seems to be executing very slowly, for example when I select an image from the camera roll it does eventually save and bring me back to the calling view, but only after a long delay..

Can anyone shed any light on this?

Answer 1

Saving large images (like the ones taken by the camera in an iPhone 4/4S) takes a long time. If you profile the process, you’ll find that UIImagePNGRepresentation() takes a while to generate your PNG image, but the primary bottleneck in my experience has been the writing to disk of a 1+ MB image.

There’s little you can do to speed up this process aside from using JPEG compression, which I’ve found to be a little faster in my benchmarks, or using a faster third-party image compression routine. Therefore, if you don’t want to block your user interface while this is happening, dispatch this saving process on a background thread or queue. You could do something like the following:

-(void)imagePickerController:(UIImagePickerController*)picker
didFinishPickingMediaWithInfo:(NSDictionary*)info {

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        UIImage* image = [info objectForKey: UIImagePickerControllerOriginalImage];
        NSString* imageName = @"Receipt1Image1";
        [self saveImage:image :imageName];
    });
}

However, watch out for the fact that each of these UIImages use up quite a bit of memory to keep around, so you may want to use a dispatch semaphore to prevent more than one such image saving operation from occurring at the same time.

Also, as a stylistic note, defining an Objective-C method like

- (void)saveImage:(UIImage*)image:(NSString*)imageName 

while allowed, is highly discouraged. Give each parameter a name, like in the following:

- (void)saveImage:(UIImage*)image fileName:(NSString*)imageName 

It will make your code a lot more descriptive.