为绘图应用程序添加撤消功能

我关注了一个raywenderlich 教程 http://www.raywenderlich.com/87899/make-simple-drawing-app-uikit-swift关于使用 UIKit 制作绘图应用程序。我现在正在尝试添加撤消最后一笔的功能。理想情况下，我想撤消最多 10 次笔画。我正在尝试找出执行此操作的最佳方法是什么。我正在考虑创建另一个只有最后一笔的 ImageView 并制作ImageView.image = nil当用户按下返回键时。在教程的代码中有与此类似的内容。当触摸结束时，最新的笔划将合并到图像视图上，所有旧的笔划都具有正确的不透明度。我不太确定如何将第三个（可能还有更多）imageivews 添加到此代码中以使其工作。有什么想法/更好的方法来解决这个问题吗？ TouchsEnded 的代码如下。

Code

override func touchesEnded(touches: Set<NSObject>, withEvent event: UIEvent) {

        if !swiped {
            // draw a single point
            drawLineFrom(lastPoint, toPoint: lastPoint)
        }

        // Merge tempImageView into mainImageView
        UIGraphicsBeginImageContext(mainImageView.frame.size)
        mainImageView.image?.drawInRect(CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height), blendMode: kCGBlendModeNormal, alpha: 1.0)
        tempImageView.image?.drawInRect(CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height), blendMode: kCGBlendModeNormal, alpha: opacity)
        mainImageView.image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        tempImageView.image = nil
    }

@SpaceShroomies：我发现/想出了一个很好的解决方案..

我使用的零件来自希普瑟 http://nshipster.com/nsundomanager，并且从雷文德利希教程 http://www.raywenderlich.com/87899/make-simple-drawing-app-uikit-swift.

这是我的解决方案：

override func touchesEnded(touches: Set<NSObject>, withEvent event: UIEvent) {
    //undoManager
    let undo: UIImageView = undoManager?.prepareWithInvocationTarget(Temp2Image) as! UIImageView

    UIGraphicsBeginImageContext(Temp2Image.frame.size)
    Temp2Image.image?.drawInRect(CGRect(x: 0, y: 0, width: Temp2Image.frame.size.width, height: Temp2Image.frame.size.height), blendMode: kCGBlendModeNormal, alpha: 1.0)
    TempImage.image?.drawInRect(CGRect(x: 0, y: 0, width: TempImage.frame.size.width, height: TempImage.frame.size.height), blendMode: kCGBlendModeNormal, alpha: opacity)
    undo.image = Temp2Image.image
    Temp2Image.image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    TempImage.image = nil
}

这里重要的是设置什么UNDO应该回到即undo.image = Temp2Image.image否则它不会把“它”改回来