iphone - UIImageView only displays when I call initWithImage

Question

For some reason, I can only display a UIImageView when I alloc/init it each iteration with a different image. What's strange is I know the image data is being loaded because I am running processing on the image and the processing is working as expected. In short, here are the two methods I was trying:

// interface
@interface ViewController : UIViewController <UIAlertViewDelegate>
{

    UIImageView *imageView;

}

@property (nonatomic, retain) UIImageView *imageView;

@end

// implementation
@implementation ViewController

@synthesize imageView;

//...

- (void) loadAndDisplayImage {

    // Load testing image
    UIImage *testImg;
    testImg = [UIImage imageNamed:@"Test.png"];

    self.imageView = [[UIImageView alloc] initWithImage:testImg];

    //size of imageView rect
    CGRect frame = self.imageView.frame;
    int ivw = frame.size.width;
    int ivh = frame.size.height;

    //...

}

@end

When I use this method self.imageView = [[UIImageView alloc] initWithImage:testImg]; the ivw and ivh have valid values and the image is displayed. However, if I change the implementation to this:

// implementation
@implementation ViewController

@synthesize imageView;

//...

- (void) viewDidLoad {

    self.imageView = [[UIImageView alloc] init];
    [self loadAndDisplayImage];

}

- (void) loadAndDisplayImage {

    // Load testing image
    UIImage *testImg;
    testImg = [UIImage imageNamed:@"Test.png"];

    self.imageView.image = testImg;

    //size of imageView rect
    CGRect frame = self.imageView.frame;
    int ivw = frame.size.width;
    int ivh = frame.size.height;

    //...

}

@end

Where I am setting the image using self.imageView.image = testImg;, the values ivw and ivh are both zero and no image is displayed but the subsequent processing on the image is still accurate. In both cases, I am sending the image to processing using [self doRecognizeImage:self.imageView.image];. I can't figure out how this is possible. It would make a lot more sense to me if the processing failed when the image could not be shown.

Ideas? Thanks.

Answer 1

The problem is that when you set the image property on an already-init'd UIImageView, the frame size isn't updated to match the new image size (unlike initWithImage:).

Whenever you have a problem like this, it's always worth checking out the docs in case you missed something:

Setting the image property does not change the size of a UIImageView. Call sizeToFit to adjust the size of the view to match the image.

So, add a call to sizeToFit after you've set the image property:

self.imageView.image = testImg;
[self.imageView sizeToFit];

As a side note, I'd only use the self. dot notation when you're writing to a property, not reading it, or calling a method. In other words, you can get away with just writing:

// we are not setting imageView itself here, only a property on it
imageView.image = testImg; 

// this is a method call, so "self." not needed
[imageView sizeToFit];