先上效果图吧，框框被限制在了image内了。

这个feature我把它用在了我的app里了，博客写得不容易，来star下啦? https://github.com/Imputes/Nike-Collection/

就好像Ray Wenderlich的iOS Apprentice一书说的一样，要实现任何feature都要列个任务清单。所以我们先列个任务清单:

1. 实现把一个可拖动的View限制在一个特定的View里。

2. 实现把框到的image放大。

首先讲第一点:

既然要实现拖动的feature，Google之后就知道Cocoa Touch框架里的UIKit module里有一个UIPanGestureRecognizer类，有了这个类之后就可以实现拖拽feature了。

那么Google之后又知道了有两种方式:

一种是纯代码(在要放大的View所在的controller里或者view里, 在willAppear时刻或者didLoad时刻或者layoutSubView时刻写上, xxxView是拖拽的View):

let panGesture = UIPanGestureRecognizer.init(target: self, action: #selector(handlePan(sender: )))
xxxView.addGestureRecognizer(panGesture)

之后再在放大的View所在的类中写selector, 就是观察者panGestureRecognizer观察到被观察者(在这里是要拖拽的View)的变化之后执行的function。

@objc func handlePan(sender: UIPanGestureRecognizer) {
...
}

另一种是直接在storyboard里面拖panGesture到要拖拽的view上即可，再用IBAction连接到代码里，这个Action里写的东西和前面的handlePan一样。

那么如何限制在所在view里呢？

Easy playing! 只要在action里添加(assistView是铺在上面的UIView，):

let transition = sender.translation(in: self.assistView)let changeX = (sender.view?.center.x)! + transition.xlet changeY = (sender.view?.center.y)! + transition.yvar limitCenter = CGPoint.init(x: changeX, y: changeY)limitCenter.y = max((sender.view?.frame.size.height)! / 2, limitCenter.y)limitCenter.y = min(self.assistView.frame.size.height - (sender.view?.frame.size.height)! / 2, limitCenter.y)limitCenter.x = max((sender.view?.frame.size.width)! / 2, limitCenter.x)limitCenter.x = min(self.assistView.frame.size.width - (sender.view?.frame.size.width)! / 2, limitCenter.x)sender.view?.center = limitCentersender.setTranslation(CGPoint.zero, in: self.assistView)

到了这一步之后就可以把这个view限制在imageView里了。

至此第一步实现。

那么第二步就简单了(因为可以直接Google到),

在所在类里写上这两个functions:

private func snapshotTargetView(view: UIView!, inRect rect: CGRect!) -> UIImage! {let scale = UIScreen.main.scale//Snapshot of viewUIGraphicsBeginImageContextWithOptions(rect.size, false, scale)UIGraphicsGetCurrentContext()?.translateBy(x: -rect.origin.x, y: -rect.origin.y)view.layer.render(in: UIGraphicsGetCurrentContext()!)//Need this to stop screen flashing, but it's slowerlet snapshotImage = UIGraphicsGetImageFromCurrentImageContext()UIGraphicsEndImageContext()return snapshotImage}private func resizeImage(image: UIImage, toNewSize newSize:CGSize) -> UIImage {let scale = UIScreen.main.scaleUIGraphicsBeginImageContextWithOptions(newSize, false, scale)image.draw(in: CGRect(x:0, y:0, width:newSize.width, height:newSize.height))let newImage = UIGraphicsGetImageFromCurrentImageContext()UIGraphicsEndImageContext()return newImage!
}

再在Action里加上这两句(resultImageView是放大后要put的地方):

 let transition = sender.translation(in: self.assistView)let changeX = (sender.view?.center.x)! + transition.xlet changeY = (sender.view?.center.y)! + transition.yvar limitCenter = CGPoint.init(x: changeX, y: changeY)limitCenter.y = max((sender.view?.frame.size.height)! / 2, limitCenter.y)limitCenter.y = min(self.assistView.frame.size.height - (sender.view?.frame.size.height)! / 2, limitCenter.y)limitCenter.x = max((sender.view?.frame.size.width)! / 2, limitCenter.x)limitCenter.x = min(self.assistView.frame.size.width - (sender.view?.frame.size.width)! / 2, limitCenter.x)sender.view?.center = limitCentersender.setTranslation(CGPoint.zero, in: self.assistView)// add this two lineslet imges = snapshotTargetView(view: self.productImageView, inRect: sender.view?.frame)resultImageView.image = resizeImage(image: images!, toNewSize: resultImageView.frame.size)

OK了?。

效果: