UIView 的快照视图在上下文中渲染/绘制，没有任何内容

Posted 2023-03-12

【中文标题】UIView 的快照视图在上下文中渲染/绘制，没有任何内容

【英文标题】：snapshotView of UIView render/draw in context with nothing

【发布时间】：2015-03-15 12:49:15

【问题描述】：

我正在尝试在上下文中渲染/绘制 UIView 的快照视图以获取 UIImage。然后将其设置为 CAlayer.contents。 我尝试这种方法来获取快照视图：

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，但我得到一个空图像。

【问题讨论】：

在 Apple 的文档中，snapshotViewAfterScreenUpdates 无法在上下文中呈现。如果需要快照图像，请尝试[window.layer.presentationLayer drawViewHierarchyInRect. window.bounds afterScreenUpdates:NO]

presentationLayer 将复制图层对象，该对象代表图层当前显示在屏幕上的状态。

【参考方案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。创建一个 UIView 的类别，如

@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

【讨论】：

谢谢，这对我有用。我通过更改一些代码来修复我的项目，但我真的很想知道我的情况会发生什么。

Apple 'doc: 如果您想对快照应用图形效果（例如模糊），请改用 drawViewHierarchyInRect:afterScreenUpdates: 方法。

【参考方案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.
   /// https://***.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
   

使用示例（互动动画内）：

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: https://***.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

【讨论】：

哇，多么棒的解决方案。在 Swift 5 上测试并运行

当控制器模态呈现时我无法工作