snapshotView of UIView render/draw in context with nothing

Question

I'm trying to render/draw the snapshotView of a UIView in contex to get a UIImage.then set it as a CAlayer.contents. I try this method to get snapshotView:

snapshotViewAfterScreenUpdates:

then convert UIView to UIImage:

UIGraphicsBeginImageContext(view.bounds.size);
[view.layer renderInContext:UIGraphicsGetCurrentContext()];
//I'm trying this method too
[view drawViewHierarchyInRect:view.bounds afterScreenUpdates:NO];
UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

the problem is I'm using the code above, but I get a empty image.

Answer 1

If you need a snapshot UIImage in the first place just use the method in your second code block. Create a UIView's category like

@implementation UIView (takeSnapshot)

- (UIImage *)takeASnapshot {
    UIGraphicsBeginImageContextWithOptions(self.bounds.size, NO, [UIScreen mainScreen].scale);

    [self drawViewHierarchyInRect:self.bounds afterScreenUpdates:YES];

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

@end

That's enough. You don't need snapshot a view and convert it to a image. In your case this process might cause the problem

Answer 2

Had a similar issue when building interactive animations with floating snapshots. Here what we did:

UIView extension:

extension UIView {

   public func snapshot(scale: CGFloat = 0, isOpaque: Bool = false, afterScreenUpdates: Bool = true) -> UIImage? {
      UIGraphicsBeginImageContextWithOptions(bounds.size, isOpaque, scale)
      drawHierarchy(in: bounds, afterScreenUpdates: afterScreenUpdates)
      let image = UIGraphicsGetImageFromCurrentImageContext()
      UIGraphicsEndImageContext()
      return image
   }


   public enum CASnapshotLayer: Int {
      case `default`, presentation, model
   }

   /// The method drawViewHierarchyInRect:afterScreenUpdates: performs its operations on the GPU as much as possible
   /// In comparison, the method renderInContext: performs its operations inside of your app’s address space and does
   /// not use the GPU based process for performing the work.
   /// https://stackoverflow.com/a/25704861/1418981
   public func caSnapshot(scale: CGFloat = 0, isOpaque: Bool = false,
                          layer layerToUse: CASnapshotLayer = .default) -> UIImage? {
      var isSuccess = false
      UIGraphicsBeginImageContextWithOptions(bounds.size, isOpaque, scale)
      if let context = UIGraphicsGetCurrentContext() {
         isSuccess = true
         switch layerToUse {
         case .default:
            layer.render(in: context)
         case .model:
            layer.model().render(in: context)
         case .presentation:
            layer.presentation()?.render(in: context)
         }
      }
      let image = UIGraphicsGetImageFromCurrentImageContext()
      UIGraphicsEndImageContext()
      return isSuccess ? image : nil
   }
}

Usage example (inside interactive animation):

private func makeSnapshot(view: UIView, snapshotLayer: UIView.CASnapshotLayer) -> UIView? {
   // There is 3 ways for taking snapshot:
   // 1. Replicate view: `view.snapshotView(afterScreenUpdates: true)`
   // 2. Draw Hierarchy: `view.drawHierarchy(in: bounds, afterScreenUpdates: true)`
   // 3. Render Layer: `layer.render(in: context)`
   //
   // Only #3 is working reliable during UINavigation controller animated transitions.
   // For modally presented UI also trick by combining #1 and #2 seems works.
   // I.e. `view.snapshotView(afterScreenUpdates: true).snapshot()`
   //
   // If this call causes error listed below, then this indicate that something wrong with one of the views layout.
   // [Snapshotting] View (_UIReplicantView) drawing with afterScreenUpdates:YES inside CoreAnimation commit is not supported.
   // See also: https://stackoverflow.com/a/29676207/1418981
   if let image = view.caSnapshot(layer: snapshotLayer) {
      let imageView = ImageView(image: image)
      imageView.clipsToBounds = true
      imageView.contentMode = .scaleAspectFit
      shapshots[view] = image
      return imageView
   }
   return nil
}