Swift版PhotoStackView——照片叠放视图

前言

之前流行过一种图片展示视图——photo stack，即照片叠放视图。大致上是这个样子的：

（图片出自code4app）
现在我们已经能够使用UICollectionViewLayout来实现这种视图了。Apple给的示例代码中就有这样一个layout，并且示例代码中不仅仅是展示这样的视图，还有非常棒的layout过度动画（结合手势）。在这之前，也有非常多的开源代码能实现这样的效果。本文正是借鉴了开源的源代码“PhotoStackView”,使用Objective-C实现，并且带手势移动图片的功能。由于这是上一学期课设的时候拿来用的库，结果现在找不到了，无法给出链接，还望见谅。

最后的效果图如下

• 移动、添加与删除
  

• 下一张
  

这样重复造轮子的目的是什么呢？一方面，不使用UICollectionViewLayout而是纯粹的用UIView来实现，提高灵活性，方便“私人订制”。另一方面，学习大神的源代码，从中学习一下自定义库的书写方式等。最后，swift。。天杀的swift，是谁说swift对新手友好来着
当然，这里也不是简单的对源代码的搬运、抄袭与翻译，我还根据自己的需要给他改了一个bug，添加了一些功能，比如全屏展示与返回等。示例图如下：

• 全屏展示
  

（图片有些卡，测试运行时还是非常流畅的）

思路

• 使用若干和自己一样大的view来装载图片，每次添加新图片到视图上时为其随机旋转一定角度，把最前面的这张旋转角度设为0。
• “view装载图片”的意思是，view里面放一个imageView，为什么不直接用imageVIew呢？因为我们要放边框，边框可以是图片，另外在highlighted状态下也可以用图片或自定义颜色铺在图片上，因此使用一个view统一管理
• 添加pan手势，当手指移动时让view跟着做相应平移，手指松开时根据velocity决定返回还是移到最后
• 综上，我们还需要不少辅助方法，例如获取顶部图片和其下标、使用动画移动view、让view旋转到某一度数等

代码

computed property & stored property

oc中可以声明属性然后覆写setter或getter，从而实现赋值或取值时进行一些操作的功能，如下代码

@property(strong, nonatomic) NSString *someString

- (void)setSomeString() {...}
- (NSString *)someString() {...}

swift中相对应的写法，目前我知道的有两种，一种是使用computed property 的set和get，缺点是必须同时声明一个stored property（？可以不用吗，求科普），很像oc2.0之前的属性。另一种是使用监听器（didSet和willSet），缺点是只能对setter操作，不能对getter操作。
但是，要想实现重写父类的setSomeThing这样的功能，只能通过监听器的方法。否则会报错

由上，该类用到的属性如下（部分）：

    //MARK: computed property

    var s_rotationOffset: CGFloat = 0.0
    /// the scope of offset of rotation on every photo except the first one. default is 4.0.
    /// ie, 4.0 means rotate iamge with degree between (-4.0, 4.0)
    var rotationOffset: CGFloat {
        set {
            if s_rotationOffset == newValue {
                return
            }

            s_rotationOffset = newValue
            reloadData()
        }
        get {
            return s_rotationOffset
        }
    }

    var s_photoImages: [UIView]?
    var photoImages: [UIView]? {
        set {
            //remove all subview and prepare to re-add all images from data source
            for view in subviews {
                view.removeFromSuperview()
            }

            if let images = newValue {
                for view in images {
                    //keep the original transfrom for the existing images
                    if let index = find(images, view), count = s_photoImages?.count where index < count {
                        let existingView = s_photoImages![index]
                        view.transform = existingView.transform
                    } else {
                        makeCrooked(view, animated: false)
                    }

                    insertSubview(view, atIndex: 0)
                }
            }

            s_photoImages = newValue
        }
        get {
            return s_photoImages
        }
    }

    override var highlighted: Bool {
        didSet {
            let photo = self.topPhoto()?.subviews.last as! UIImageView
            if highlighted {
                let view = UIView(frame: self.bounds)
                view.backgroundColor = self.highlightColor
                photo.addSubview(view)
                photo.bringSubviewToFront(view)
            } else {
                photo.subviews.last?.removeFromSuperview()
            }
        }
    }

    override var frame: CGRect {
        didSet {
            if CGRectEqualToRect(oldValue, self.frame) {
                return
            }

            reloadData()
        }
    }

上面大部分还有其他省略的大都是一样的思路：设置新值时调用reloadData刷新，主要是上面那个photoImage数组的setter：首先移除当前所有的子视图，接下来遍历新数组，那句判断if let index = find(images, view), count = s_photoImages?.count where index < count的作用是判断此次循环中view是否是之前已经添加到界面上的，如果是，则保留其transform不变，否则为其重新生产一个旋转角度（产生照片堆效果），这样做保证了添加照片时原先的照片形状不会变。

Set up & Touches

初始化的工作非常简单，一方面为属性设置默认值，另一方面添加手势监听。

    //MARK: Set up

    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }

    required init(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setup()
    }

    func setup() {
        //default value
        borderWidth = 5.0
        showBorder = true
        rotationOffset = 4.0

        let panGR = UIPanGestureRecognizer(target: self, action: Selector("handlePan:"))
        addGestureRecognizer(panGR)

        let tapGR = UITapGestureRecognizer(target: self, action: Selector("handleTap:"))
        addGestureRecognizer(tapGR)

        reloadData()
    }

    override func sendActionsForControlEvents(controlEvents: UIControlEvents) {
        super.sendActionsForControlEvents(controlEvents)
        highlighted = (controlEvents == .TouchDown)
    }

sendActionsForControlEvents是自定义UIControl时可能会使用或重写的方法，作用是发送事件。为了说明这一点，这里顺带附上view的触摸方法：

    //MARK: Touch Methods

    override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
        super.touchesBegan(touches, withEvent: event)

        sendActionsForControlEvents(.TouchDown)
    }

    override func touchesMoved(touches: Set<NSObject>, withEvent event: UIEvent) {
        super.touchesMoved(touches, withEvent: event)

        sendActionsForControlEvents(.TouchDragInside)
    }

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

        sendActionsForControlEvents(.TouchCancel)
    }

例如，当用户点击该控件时，发送UIControlEventTouchDown事件，这样使用该控件的人就可以通过addTarget:selector:forControlEvent:方法对此事件添加监听了。我们平时最经常使用的button不就是对TouchUpInside事件进行监听的吗。

reloadData

刷新视图时，要做的事情有：重新获取size，计算frame，添加border，设置images

    /**
    use this method to reload photo stack view when data has changed
    */
    func reloadData() {
        if dataSource == nil {
            photoImages = nil
            return
        }

        if let number = dataSource?.numberOfPhotosInStackView(self) {
            var images = [UIView]()
            let border = borderImage?.resizableImageWithCapInsets(UIEdgeInsets(top: borderWidth, left: borderWidth, bottom: borderWidth, right: borderWidth))
            let topIndex = indexOfTopPhoto()

            for i in 0..<number {
                if let image = dataSource?.stackView(self, imageAtIndex: i) {

                    //add image view for every image
                    let imageView = UIImageView(image: image)
                    var viewFrame = CGRectMake(0, 0, image.size.width, image.size.height)

                    if let ds = dataSource where ds.respondsToSelector(Selector("stackView:sizeOfPhotoAtIndex:")) {
                        let size = ds.stackView!(self, sizeOfPhotoAtIndex: i)
                        viewFrame.size = size
                    }

                    imageView.frame = viewFrame

                    let view = UIView(frame: viewFrame)

                    //add border for view
                    if showBorder {
                        if let b = border {
                            viewFrame.origin = CGPoint(x: borderWidth, y: borderWidth)
                            imageView.frame = viewFrame
                            view.frame = CGRect(x: 0, y: 0, width: imageView.frame.width + 2 * borderWidth, height: imageView.frame.height + 2 * borderWidth)

                            let backgroundImage = UIImageView(image: b)
                            backgroundImage.frame = view.frame
                            view.addSubview(backgroundImage)
                        } else {
                            view.layer.borderWidth = borderWidth
                            view.layer.borderColor = UIColor.whiteColor().CGColor
                        }

                    }
                    view.addSubview(imageView)

                    //add view to array
                    images.append(view)
                    view.tag = i
                    view.center = CGPoint(x: CGRectGetMidX(bounds), y: CGRectGetMidY(bounds))
                }
            }

            photoImages = images
            goToImageAtIndex(topIndex)
        }

    }

这里要干的仅有前三件事，添加photos的任务交给photoImages的setter去做，这个之前已经说过了。

逻辑相关

在谈这个之前，让我们先来了解一下view的组织方式。如果一个view内有若干个subview，你知道subviews.lastObject和subviews.firstObject分别指哪个吗？
事实上，越是靠近我们的，下标识越小。换句话说，subviews[0]指的是子视图中位于最底层的，被其他子视图遮住了的那一个，而subviews.lastObject指的则是最顶层的，能被我们看到（一般来说）的。
这样，很容易能得出下面几个函数

    /**
    find the index of top photo

    :returns: index of top photo
    */
    func indexOfTopPhoto() -> Int {
        if let images = photoImages, let photo = topPhoto() {
            if let index = find(images, photo) {
                return index
            }
        }
        return 0
    }

    /**
    get the top photo on photo stack

    :returns: current first photo
    */
    func topPhoto() -> UIView? {
        if subviews.count == 0 {
            return nil
        }
        return subviews[subviews.count - 1] as? UIView
    }

    /**
    jump to photo at index
    */
    func goToImageAtIndex(index: Int) {
        if let photos = photoImages {
            for view in photos {
                if let idx = find(photos, view) where idx < index {
                    sendSubviewToBack(view)
                }
            }
        }
        makeStraight(topPhoto()!, animated: false)
    }

swift知识补充：swift中没有indexForObject:这样的方法，众所周知，swift更多的是“函数式编程”，因此Int、Float甚至Array、Dictionary都是结构体（虽然和类挺相似），我们也是使用一些函数来对这些结构体操作。例如这里取代上述方法的是find()函数

有了这些方法，我们就能使用动画后处理view的层级关系了。

动画相关

这里用到的动画相关的都非常简单，所以直接上代码：

    //MARK: Animations

    func returnToCenter(view: UIView) {
        UIView.animateWithDuration(0.2, animations: { () -> Void in
            view.center = CGPoint(x: CGRectGetMidX(self.bounds), y: CGRectGetMidY(self.bounds))
        })
    }

    func flickAway(view: UIView, withVelocity velocity: CGPoint) {
        if let del = delegate where del.respondsToSelector(Selector("stackView:willFlickAwayPhotoFromIndex:toIndex:")) {
            let from = indexOfTopPhoto()
            var to = from + 1
            if let number = dataSource?.numberOfPhotosInStackView(self) where to >= number {
                to = 0
            }

            del.stackView!(self, willFlickAwayPhotoFromIndex: from, toIndex: to)
        }

        let width = CGRectGetWidth(bounds)
        let height = CGRectGetHeight(bounds)

        var xPosition: CGFloat = CGRectGetMidX(bounds)
        var yPosition: CGFloat = CGRectGetMidY(bounds)

        if velocity.x > 0 {
            xPosition = CGRectGetMidX(bounds) + width
        } else if velocity.x < 0 {
            xPosition = CGRectGetMidX(bounds) - width
        }
        if velocity.y > 0 {
            yPosition = CGRectGetMidY(bounds) + height
        } else if velocity.y < 0 {
            yPosition = CGRectGetMidY(bounds) - height
        }

        UIView.animateWithDuration(0.1, animations: { () -> Void in
            view.center = CGPoint(x: xPosition, y: yPosition)
        }) { (finished) -> Void in
            self.makeCrooked(view, animated: true)
            self.sendSubviewToBack(view)
            self.makeStraight(self.topPhoto()!, animated: true)
            self .returnToCenter(view)

            if let del = self.delegate where del.respondsToSelector("stackView:didRevealPhotoAtIndex:") {
                del.stackView!(self, didRevealPhotoAtIndex:self.indexOfTopPhoto())
            }
        }
    }

    func rotate(degree: Int, onView view: UIView, animated: Bool) {
        let radian = CGFloat(degree) * CGFloat(M_PI) / 180

        if animated {
            UIView.animateWithDuration(0.2, animations: { () -> Void in
                view.transform = CGAffineTransformMakeRotation(radian)
            })
        } else {
            view.transform = CGAffineTransformMakeRotation(radian)
        }
    }

    func makeCrooked(view: UIView, animated: Bool) {
        let min = Int(-rotationOffset)
        let max = Int(rotationOffset)

        let scope = UInt32(max - min - 1)
        let randomDegree = Int(arc4random_uniform(scope))
        let degree: Int = min + randomDegree

        rotate(degree, onView: view, animated: animated)
    }

    func makeStraight(view: UIView, animated: Bool) {
        rotate(0, onView: view, animated: animated)
    }

swift相关：一直不是太明白swift中可选值的意义是什么。到是给我们带来了不少麻烦，因为不怎么想全都用强解(!)，所以用了大量if let解包的方式。其中oc中很简单就能完成的操作：

if (self.delegate responseToSelector:@selector(@"someMethod")) {
    [self.delegate someMethod];
}

到了swift中

if let del = delegate where del.responseToSelector(Selector("someMethod") {
    del.someMethod()
}

且不说swift的Selector机制，每次都这样写可真是要累死人了:[

手势

和之前说的一样，pan手势中要做的就是通知代理，让view随手指移动，释放手指后根据velocity将view归位或切换到最后一张。

    //MARK: Gesture Recognizer

    func handlePan(recognizer: UIPanGestureRecognizer) {
        if let topPhoto = self.topPhoto() {
            let velocity = recognizer.velocityInView(recognizer.view)
            let translation = recognizer.translationInView(recognizer.view!)

            if recognizer.state == .Began {
                sendActionsForControlEvents(.TouchCancel)

                if let del = delegate where del.respondsToSelector(Selector("stackView:willBeginDraggingPhotoAtIndex")) {
                    del.stackView!(self, willBeginDraggingPhotoAtIndex: self.indexOfTopPhoto())
                }
            } else if recognizer.state == .Changed {
                topPhoto.center = CGPoint(x: topPhoto.center.x + translation.x, y: topPhoto.center.y + translation.y)
                recognizer.setTranslation(CGPoint.zeroPoint, inView: recognizer.view)
            } else if recognizer.state == .Ended || recognizer.state == .Cancelled {
                if abs(velocity.x) > 200 {
                    flickAway(topPhoto, withVelocity: velocity)
                } else {
                    returnToCenter(topPhoto)
                }
            }

        }

    }

    func handleTap(recognizer: UIGestureRecognizer) {
        sendActionsForControlEvents(.TouchUpInside)

        if let del = delegate where del.respondsToSelector(Selector("stackView:didSelectPhotoAtIndex:")) {
            del.stackView!(self, didSelectPhotoAtIndex: self.indexOfTopPhoto())
        }
    }

Show All Images

创建一层黑色的遮罩（view），然后将每个view从自己原来的位置移到计算好的新位置，为了产生顺序关系，为每个view 的动画设置各自的delay。点击黑色遮罩后所有view回位，view消失。

    func showAllPhotos() {
        let screenBounds = UIScreen.mainScreen().bounds
        let maskView = UIView(frame: screenBounds)
        maskView.backgroundColor = UIColor.blackColor()
        maskView.alpha = 0
        UIApplication.sharedApplication().keyWindow?.addSubview(maskView)

        UIView.animateWithDuration(0.1, delay: 0.0, options: nil, animations: { () -> Void in
            maskView.alpha = 1.0
        }) { (_) -> Void in

        }


        let column = 3
        let imageWidth = 80
        let padding = (Int(screenBounds.width) - column * imageWidth) / (column + 1)

        if let photos = photoImages {
            for view in photos {

                //set the initial location
                view.removeFromSuperview()
                maskView.addSubview(view)
                view.frame = frame

                if let index = find(photos, view) {
                    UIView.animateWithDuration(0.1, delay: NSTimeInterval(Double(index) * 0.1), options: nil, animations: { () -> Void in
                        view.frame = CGRect(x: padding + (index % column) * (imageWidth + padding), y: padding + (index / column) * (padding + imageWidth), width: imageWidth, height: imageWidth)
                    }, completion: { (finished) -> Void in

                    })
                }
            }
        }

        let tapGR = UITapGestureRecognizer(target: self, action: Selector("removeMaskView:"))
        maskView.addGestureRecognizer(tapGR)

    }

    func removeMaskView(recognizer: UITapGestureRecognizer) {
        let maskView = recognizer.view!
        for i in stride(from: maskView.subviews.count - 1, through: 0, by: -1) {
            let photo = maskView.subviews[i] as? UIView
            UIView.animateWithDuration(0.25, animations: { () -> Void in
                photo?.frame = self.frame
            }, completion: { (_) -> Void in

            })
        }

        UIView.animateWithDuration(0.25, animations: { () -> Void in
            maskView.alpha = 0.0
        }) { (_) -> Void in
            maskView.removeFromSuperview()
            self.reloadData()
        }
    }

源代码

本文的源代码可以从这里下载

总结

刚接触swift不久就直接写东西，确实不好写，swift为了安全做了很多努力，但却增加了一些注意事项。一个Int乘以一个Double都报错的“强类型“检查实在不习惯。但是不得不说这门语言确实有意思，省略小括号，不用分号，switch，where…以后多接触，说不定就能喜欢上这门语言了:]