UIView render/draw 的 snapshotView 在没有上下文的情况下

Question

我正在尝试 render/draw contex 中 UIView 的快照视图以获取 UIImage.then 并将其设置为 CAlayer.contents。我尝试使用此方法获取快照视图：

snapshotViewAfterScreenUpdates:

然后将 UIView 转换为 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();

问题是我正在使用上面的代码，但我得到的是一张空图像。

Answer 1

如果您首先需要快照 UIImage，只需使用第二个代码块中的方法即可。创建一个 UIView 的类别

@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

够了。您不需要快照视图并将其转换为图像。在您的情况下，此过程可能会导致问题

Answer 2

在使用 floating snapshots 构建交互式动画时遇到了类似的问题。这是我们所做的：

UIView 扩展：

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.
   /// 
   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
   }
}

使用示例（交互式动画内）：

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: 
   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
}

UIView render/draw 的 snapshotView 在没有上下文的情况下

snapshotView of UIView render/draw in context with nothing

iphone

objective-c

uiview

uiimage

ios