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

Question

我正在为iOS应用程序构建拖放交互.我想让用户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带有不透明白色背景的矩形,覆盖了我的应用程序的背景.

当我通过实现UIDragInteractionDelegate方法dragInteraction(_:previewForLifting:session :)来创建自定义预览时 ,就像Apple的代码示例采用自定义视图中的拖放一样,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显示在具有不透明白色背景的矩形中:

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)
}

我试图强制预览不是不透明的,但它没有帮助:

previewImageView.isOpaque = false

如何在电梯预览中获得透明部件？

Answer 1

覆盖的backgroundColor在UIDragPreviewParameters,因为它定义为一拖项目预览的背景颜色.

将其设置为UIColor.clear,它是一个颜色对象,其灰度和alpha值均为0.0.

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

Answer 2

您可以UIBezierPath根据图像定义一个，并将其设置为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