重新加载View Controller并以编程方式更改UIImageView约束

Question

I have a UIImageView which its height is affected by the aspect ratio of a photo that the user either takes or selects from their library. I am having trouble getting the UIImageView to resize appropriately (or at all) after the user selects a photo.

Code:

func imagePickerController(picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : AnyObject]) {
        //let mediaType2 : UIImage = info[UIImagePickerControllerOriginalImage] as! UIImage
        let mediaType : CFString = info[UIImagePickerControllerMediaType] as! CFString

        if mediaType == kUTTypeMovie {
            let path = (info[UIImagePickerControllerMediaURL] as! NSURL).path
            if UIVideoAtPathIsCompatibleWithSavedPhotosAlbum(path!) {
                UISaveVideoAtPathToSavedPhotosAlbum(path!, self, "video:didFinishSavingWithError:contextInfo:", nil)
            }
        }
        else{
            let img : UIImage = info[UIImagePickerControllerOriginalImage] as! UIImage
            let screenSize: CGRect = UIScreen.mainScreen().bounds
            let multiplyNum = screenSize.width / img.size.width
            imageView.frame = CGRectMake(0, 0, screenSize.width, multiplyNum*img.size.height)
            imageView.image = img

        }

        self.dismissViewControllerAnimated(true, completion: {})

    }

The constraints are currently set up inside the storyboard but I want to programatically override imageView's constraint.

Answer 1

You should update the constraints of the UIImageView rather than setting the frame. Set a height constraint for the image view then create an IBOutlet for that constraint (control drag from the constraint into it's view controller), call it something like "imageViewHeightConstraint." Then in your code say something like:

imageViewHeightConstraint.constant = multiplyNum*img.size.height

Answer 2

@beyowulf's answer will work and is simple to achieve

But if you don't want to setup a constraint outlet, you can specify the ratio attribute in code, and witch is also very simple:

//remove the last image ration constraint
if let ratioConstrain = self.ratioConstrain {
    self.imageView.removeConstraint(ratioConstraint)
}

self.ratioConstraint = NSLayoutConstraint(
    item: self.imageView, attribute: NSLayoutAttribute.Width,
    relatedBy: NSLayoutRelation.Equal,
    toItem: self.imageView, 
    attribute: NSLayoutAttribute.Height,
    multiplier: image!.size.width / image!.size.height, 
    constant: 0)

self.imageView.addConstraint(self.ratioConstraint!)

Don't user frame that will not work In this case, you need to remove the height constraint of your imageView, and set the width equal to the super view, you will get what you want: equal width of the screen, and height change with the ratio of the image itself.

There's one issue, if the image you get from your roll is too small, you may want the width to be smaller than the screen, in this case you should set the imageView width constraint to lessThanOrEqual(<=), and add some position constraint(like center vertical, center horizontal), that will looks better.