UIDragInteractionDelegate：如何在dragInteraction(_:previewForLifting:session:)返回的拖动预览中显示透明部分

Question

我正在为 iOS 应用构建 drag and drop 交互。我想让用户能够拖放包含透明部分的图像。

但是，拖动内容的默认预览是一个覆盖我的应用程序背景的不透明白色背景的矩形。

当我通过实现 UIDragInteractionDelegate 方法创建自定义预览时 dragInteraction(_:previewForLifting:session:), as in Apple's code sample Adopting Drag and Drop in a Custom View，我的源图像的透明度仍然没有被考虑在内，这意味着我的预览图像仍然显示在一个不透明的白色背景的矩形中：

func dragInteraction(_ interaction: UIDragInteraction, previewForLifting item: UIDragItem, session: UIDragSession) -> UITargetedDragPreview? {
    guard let image = item.localObject as? UIImage else { return nil }

    // Scale the preview image view frame to the image's size.
    let frame: CGRect
    if image.size.width > image.size.height {
        let multiplier = imageView.frame.width / image.size.width
        frame = CGRect(x: 0, y: 0, width: imageView.frame.width, height: image.size.height * multiplier)
    } else {
        let multiplier = imageView.frame.height / image.size.height
        frame = CGRect(x: 0, y: 0, width: image.size.width * multiplier, height: imageView.frame.height)
    }

    // Create a new view to display the image as a drag preview.
    let previewImageView = UIImageView(image: image)
    previewImageView.contentMode = .scaleAspectFit
    previewImageView.frame = frame

    /*
         Provide a custom targeted drag preview that lifts from the center
         of imageView. The center is calculated because it needs to be in
         the coordinate system of imageView. Using imageView.center returns
         a point that is in the coordinate system of imageView's superview,
         which is not what is needed here.
     */
    let center = CGPoint(x: imageView.bounds.midX, y: imageView.bounds.midY)
    let target = UIDragPreviewTarget(container: imageView, center: center)
    return UITargetedDragPreview(view: previewImageView, parameters: UIDragPreviewParameters(), target: target)
}

我试图强制预览不透明，但没有帮助：

previewImageView.isOpaque = false

如何在电梯预览中获得透明部分？

Answer 1

覆盖 backgroundColor in the UIDragPreviewParameters，因为它定义了拖动项目预览的背景颜色。

设置为UIColor.clear，即灰度和alpha值均为0.0的彩色对象。

let previewParameters = UIDragPreviewParameters()
previewParameters.backgroundColor = UIColor.clear // transparent background
return UITargetedDragPreview(view: previewImageView,
                             parameters: previewParameters,
                             target: target)

Answer 2

您可以根据自己的形象定义一个UIBezierPath，设置为previewParameters.visiblePath

示例 (Swift 4.2):

let previewParameters = UIDragPreviewParameters()
previewParameters.visiblePath = UIBezierPath(roundedRect: CGRect(x: yourX, y: yourY, width: yourWidth, height: yourHeight), cornerRadius: yourRadius)
//... Use the created previewParameters

UIDragInteractionDelegate：如何在dragInteraction(_:previewForLifting:session:)返回的拖动预览中显示透明部分

UIDragInteractionDelegate: How to display transparent parts in the drag preview returned by dragInteraction(_:previewForLifting:session:)

transparency

drag-and-drop

uikit

ios