How to set UIScrollView contensize when content is load asynchronously - ios

Question

My view hierarchy is this
PhotoDetailViewController.swift
View
UIScrollView
UIImageView
I set this up using storyboard, and add four constraints(top=0, bottom=0, leading=0, tailing=0) to UIScrollView, four constraints(top=0, bottom=0, leading=0, tailing=0) to UIImageView, but there are two error says
"ScrollView has ambiguous scrollable content width"
"ScrollView has ambiguous scrollable content height"
I understand that this is because I haven't set UIScrollView contentSize, but What I trying to do is load photo from PHAsset asynchronously, so I can only get the photo size at run time. So the question is:
1:Given that photo size can only be get at run time, how to solve the "ambiguous scrollable content" error?
2:In which View's life cycle method should I call PHImageManager.requestImageForAsset? because I think I should set UIScrollView contentSize programmatically, but when?
update with PhotoDetailViewController.swift
import UIKit
import Photos
class PhotoDetailViewController : UIViewController {
#IBOutlet weak var scrollView: UIScrollView!
#IBOutlet weak var imageViewBottomConstraint: NSLayoutConstraint!
#IBOutlet weak var imageViewLeadingConstraint: NSLayoutConstraint!
#IBOutlet weak var imageViewTopConstraint: NSLayoutConstraint!
#IBOutlet weak var imageViewTrailingConstraint: NSLayoutConstraint!
var devicePhotosAsset : PHFetchResult!
var index = 0
var photo : UIImage!
var imgManager:PHImageManager!
#IBOutlet weak var imageView : UIImageView!
override func viewWillAppear(animated: Bool) {
super.viewWillAppear(animated)
}
override func awakeFromNib() {
self.imgManager = PHImageManager()
}
override func viewDidLoad() {
super.viewDidLoad()
self.displayPhoto()
}
override func viewWillLayoutSubviews() {
super.viewDidLayoutSubviews()
updateMinZoomScaleForSize()
updateConstraintsForSize()
}
func displayPhoto () {
_ = self.imgManager.requestImageForAsset(self.devicePhotosAsset[self.index] as! PHAsset, targetSize: PHImageManagerMaximumSize, contentMode: .AspectFit, options: nil, resultHandler: {(result, info) -> Void in
NSOperationQueue.mainQueue().addOperationWithBlock(){
self.imageView.image = result
}
})
}
private func targetSize() -> CGSize {
let scale = UIScreen.mainScreen().scale
let targetSize = CGSizeMake(CGRectGetWidth(self.imageView.bounds)*scale, CGRectGetHeight(self.imageView.bounds)*scale)
return targetSize
}
private func updateMinZoomScaleForSize() {
let size = scrollView.bounds.size
let widthScale = size.width / imageView.bounds.width
let heightScale = size.height / imageView.bounds.height
let minScale = min(widthScale, heightScale)
scrollView.minimumZoomScale = minScale
scrollView.zoomScale = minScale
}
func recenterImage(){
let scrollViewSize = scrollView.bounds.size
let imageSize = imageView.frame.size
let horizontalSpace = imageSize.width < scrollViewSize.width ? (scrollViewSize.width - imageSize.width)/2 : 0
let verticalSpace = imageSize.height < scrollViewSize.height ? (scrollViewSize.height - imageSize.height)/2 : 0
scrollView.contentInset = UIEdgeInsets(top: verticalSpace, left: horizontalSpace, bottom: verticalSpace, right: horizontalSpace)
}
private func updateConstraintsForSize() {
let size = scrollView.bounds.size
let yOffset = max(0, (size.height - imageView.frame.height) / 2)
imageViewTopConstraint.constant = yOffset
imageViewBottomConstraint.constant = yOffset
let xOffset = max(0, (size.width - imageView.frame.width) / 2)
imageViewLeadingConstraint.constant = xOffset
imageViewTrailingConstraint.constant = xOffset
view.layoutIfNeeded()
}
}
extension PhotoDetailViewController: UIScrollViewDelegate {
func viewForZoomingInScrollView(scrollView: UIScrollView) -> UIView? {
return imageView
}
func scrollViewDidZoom(scrollView: UIScrollView) {
updateConstraintsForSize()
}
}

Answer 1

Your existing constraints are enough to set the content size, it's just that it's based on the image view intrinsic content size and that doesn't really exist until the image view has an image.
You can add a width and height constraint to the image view with default values and deactivate those constraints when the image is set to the view. Or you could use a placeholder image and avoid those extra constraints because you'd always have an intrinsic content size for the image view.

Answer 2

You should set two more constraint to your imageView.
Horizontally in Container (or you can say it center X)
Fixed Height
Second thing you can put UIView on Scrollview with Constraints like,
Top,leading,trailing,bottom,Horizontally in container(center x),fixed height).
Then add your imageview to that view. And can change it's constraint after getting image to resize it's height and width.
You can connect outlet of any constraint and can change it's constant programmatically.

Answer 3

Xcode UI builder has special type of constraint for such cases (when you can setup constraint only in runtime). It's so called "placeholder constraint" which will be removed at build time but helps to remove constraints errors for developing.
So solution is
Add some sample constraints IB and mark them as placeholders
Add needed constraints in runtime

Answer 4

When you get the data, just add these lines
float sizeOfContent = 0;
UIView *lLast = [yourscrollview.subviews lastObject];
NSInteger wd = lLast.frame.origin.y;
NSInteger ht = lLast.frame.size.height;
sizeOfContent = wd+ht;
yourscrollview.contentSize = CGSizeMake(yourscrollview.frame.size.width, sizeOfContent);
Hope this helps