下载进度来自URL的图像查看器

Question

我已经创建了一个imageViewer，它将使用一个String数组进行初始化，并将根据需要下载并显示图像。

在回顾的同时，你能给我一个关于在didReceiveMemoryWarning里做什么的建议吗？

PagedScrollViewController

class PagedScrollViewController:UIViewController,UIScrollViewDelegate {

    var imagesUrls:[String]?
    var imagesDownloads:[ImageDownload?]      = [ImageDownload]()
    var downloadsResponse:[DownloadResponse?] = [DownloadResponse?]()

    var pageImages:[UIImage?]          = [UIImage]()
    var pageViews:[ImageDownloadView?] = [ImageDownloadView]()

    var scrollView:UIScrollView   = UIScrollView()
    var pageControl:UIPageControl = UIPageControl()

    var viewingPage = -1

    init(images:[String])  {
        super.init(nibName: nil, bundle: nil)

        self.imagesUrls = images
        self.title = "Image viewer";

        self.view.backgroundColor = UIColor.blackColor()

        var pageCount = self.imagesUrls!.count

        self.scrollView.pagingEnabled = true
        self.scrollView.delegate = self
        self.scrollView.showsHorizontalScrollIndicator = false
        self.scrollView.showsVerticalScrollIndicator = false

        // Set up the page control
        self.pageControl.currentPage = 0;
        self.pageControl.numberOfPages = pageCount;

        //Add
        self.pageControl.setTranslatesAutoresizingMaskIntoConstraints(false)
        self.scrollView.setTranslatesAutoresizingMaskIntoConstraints(false)

        self.view.addSubview(self.pageControl)
        self.view.addSubview(self.scrollView)

        //Set layout
        var viewsDict = Dictionary <String, UIView>()
        viewsDict["control"] = self.pageControl;
        viewsDict["scrollView"] = self.scrollView;

        self.view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|-0-[scrollView]-0-|", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDict))
        self.view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|-0-[control]-0-|", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDict))
        self.view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-0-[scrollView(400)]-[control]-0-|", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDict))

        // Set up the array to hold the views for each page
        for (var i = 0; i < pageCount; ++i) {
            self.pageViews.append(nil)
            self.pageImages.append(nil)
            self.imagesDownloads.append(nil)
            self.downloadsResponse.append(nil)
        }

    }

    required init(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    func loadVisiblePages() {
        // First, determine which page is currently visible
        var pageWidth:CGFloat = self.scrollView.frame.size.width;
        var page = Int(floor((self.scrollView.contentOffset.x * 2.0 + pageWidth) / (pageWidth * 2.0)));

        /* 
            Check that page have changed, in case that user drag left in first page, or drag right in last page
            a 'scrollViewDidEndDecelerating' is fired
        */
        if self.viewingPage != page {

            self.viewingPage = page
            // Update the page control
            self.pageControl.currentPage = page;

            // Work out which pages we want to load
            var firstPage = page - 1;
            var lastPage = page + 1;

            // Purge anything before the first page
            for (var i=0; i<firstPage; i++) { self.purgePage(i) }
            for (var i=firstPage; i<=lastPage; i++) { self.loadPage(i) }
            for (var i = lastPage+1 ; i < self.pageImages.count ; i++) { self.purgePage(i) }
        }

    }

    func loadPage(page:Int) {
        if page < 0 || page >= self.pageImages.count {
            // If it's outside the range of what we have to display, then do nothing
            return;
        }

        // Load an individual page, first seeing if we've already loaded it
        var pageView:UIView? = self.pageViews[page];
        if pageView == nil {
            var frame:CGRect = self.scrollView.bounds;
            frame.origin.x = frame.size.width * CGFloat(page)
            frame.origin.y = 0.0

            var newPageView:ImageDownloadView = ImageDownloadView()
            newPageView.frame = frame;

            if(self.imagesDownloads[page] == nil && self.pageImages[page] == nil) {
                self.downloadsResponse[page] = { (pageNumber:Int!, actionType:ImageDownload.ActionType, progress:Float, result:UIImage!) -> Void in
                    switch actionType {
                    case ImageDownload.ActionType.STARTING :
                        if self.pageViews[pageNumber] != nil {
                            //DO NOTHING FOR NOW
                        }
                    case ImageDownload.ActionType.DOWNLOADING :
                        if self.pageViews[pageNumber] != nil {
                            self.pageViews[pageNumber]?.setDownloadProgress(progress)
                        }
                    case ImageDownload.ActionType.COMPLETED :
                        self.pageImages[pageNumber] = result;
                        if self.pageViews[pageNumber] != nil {
                            self.pageViews[pageNumber]?.showImage(result)
                        }
                    }
                }
                self.imagesDownloads[page] = ImageDownload()
                self.imagesDownloads[page]?.connect(self.imagesUrls![page], pageNumber:page, downloadReponse:self.downloadsResponse[page]!)
            } else if self.pageImages[page] != nil {
                newPageView.showImage(self.pageImages[page]!)
            }

            self.scrollView.addSubview(newPageView)
            self.pageViews[page] = newPageView
        }
    }

    func purgePage(page:Int) {
        if page < 0 || page >= self.pageImages.count {
            // If it's outside the range of what we have to display, then do nothing
            return;
        }

        // Remove a page from the scroll view and reset the container array
        var pageView:UIView? = self.pageViews[page];
        if pageView != nil {
            pageView?.removeFromSuperview()
            self.pageViews[page] = nil
        }
    }

    override func viewDidAppear(animated:Bool) {
        super.viewDidAppear(animated)

        // Set up the content size of the scroll view
        var pagesScrollViewSize:CGSize = self.scrollView.frame.size;
        self.scrollView.contentSize = CGSizeMake(pagesScrollViewSize.width * CGFloat(self.pageImages.count), pagesScrollViewSize.height);

        // Load the initial set of pages that are on screen
        self.loadVisiblePages()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        //Clean not visible images ?
    }

    func scrollViewDidEndDecelerating(scrollView: UIScrollView!) {
        self.loadVisiblePages()
    }
}

ImageDownloadView

class ImageDownloadView: UIView {
    var imageViewer:UIImageView = UIImageView()
    var roundProgress:RoundProgress = RoundProgress(frame:CGRectMake(0, 0, 60, 60))

    override init() {
        super.init()

        self.imageViewer.contentMode = UIViewContentMode.ScaleAspectFit;

        self.imageViewer.setTranslatesAutoresizingMaskIntoConstraints(false)
        self.roundProgress.setTranslatesAutoresizingMaskIntoConstraints(false)
        self.addSubview(self.imageViewer)
        self.addSubview(self.roundProgress)

        //Set layout
        var viewsDict = Dictionary <String, UIView>()
        viewsDict["image"] = self.imageViewer
        viewsDict["progress"] = self.roundProgress

        self.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|-0-[image]-0-|", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDict))
        self.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-0-[image]-0-|", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDict))

        self.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:[progress(60)]", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDict))
        self.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:[progress(60)]", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDict))


        self.addConstraint(NSLayoutConstraint(item:self.roundProgress,
            attribute:NSLayoutAttribute.CenterX,
            relatedBy:NSLayoutRelation.Equal,
            toItem:self,
            attribute:NSLayoutAttribute.CenterX,
            multiplier:1,
            constant:0))

        self.addConstraint(NSLayoutConstraint(item:self.roundProgress,
            attribute:NSLayoutAttribute.CenterY,
            relatedBy:NSLayoutRelation.Equal,
            toItem:self,
            attribute:NSLayoutAttribute.CenterY,
            multiplier:1,
            constant:0))
    }

    func setDownloadProgress(progress:Float) {
        self.roundProgress.setProgress(progress / 100)
    }

    func showImage(image:UIImage) {
        self.imageViewer.image = image
        self.roundProgress.removeFromSuperview()
    }

    required init(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

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

}

ImageDownload

typealias DownloadResponse = (pageNumber:Int!, actionType:ImageDownload.ActionType, progress:Float, result:UIImage!) -> Void

class ImageDownload:NSObject, NSURLConnectionDataDelegate {

    var data = NSMutableData()
    var pageNumber:Int?
    var downloadReponse:DownloadResponse?
    var contentLength:Int64?

    enum ActionType:CInt {
        case STARTING = 1, DOWNLOADING, COMPLETED
    }

    func connect(query:String, pageNumber:Int, downloadReponse:DownloadResponse) {
        self.pageNumber = pageNumber
        self.downloadReponse = downloadReponse

        var url =  NSURL.URLWithString(query)
        var request = NSURLRequest(URL: url)
        var conn = NSURLConnection(request: request, delegate: self, startImmediately: false)
        conn.start()
    }

    func connection(didReceiveResponse: NSURLConnection!, didReceiveResponse response: NSURLResponse!) {
        contentLength = response.expectedContentLength
        downloadReponse!(pageNumber: self.pageNumber, actionType: ActionType.STARTING, progress:0.0, result: nil)
    }

    func connection(connection: NSURLConnection!, didReceiveData conData: NSData!) {
        self.data.appendData(conData)
        var progress:Float = Float(self.data.length * 100) / Float(self.contentLength!)
        downloadReponse!(pageNumber: self.pageNumber, actionType: ActionType.DOWNLOADING, progress:progress, result: nil)
    }

    func connectionDidFinishLoading(connection: NSURLConnection!) {
        downloadReponse!(pageNumber: self.pageNumber, actionType: ActionType.COMPLETED, progress:100.0, result: UIImage(data: self.data))
    }

}

RoundProgress

class RoundProgress:UIView {
    var progressCircle = CAShapeLayer();
    var progress:Float = 0.1;

    override init(frame: CGRect) {
        super.init(frame: frame)
        self.bounds = frame
    }


    override func didMoveToSuperview() {
        super.didMoveToSuperview()

        let centerPoint = CGPoint (x: self.bounds.width / 2, y: self.bounds.width / 2);
        let circleRadius : CGFloat = self.bounds.width / 2 * 0.83;
        var circlePath = UIBezierPath(arcCenter: centerPoint, radius: circleRadius, startAngle: CGFloat(-0.5 * M_PI), endAngle: CGFloat(1.5 * M_PI), clockwise: true    );

        progressCircle.path = circlePath.CGPath;
        progressCircle.strokeColor = UIColor.whiteColor().CGColor;
        progressCircle.fillColor = UIColor.clearColor().CGColor;

        progressCircle.lineWidth = 2.5;
        progressCircle.strokeStart = 0;
        progressCircle.strokeEnd = 0.0;

        self.layer.addSublayer(progressCircle);

    }

    func setProgress(progress:Float) {
        self.progress = progress
        progressCircle.strokeEnd = CGFloat(self.progress)
        self.setNeedsDisplay()
    }


    required init(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }


}

Answer 1

您的ImageDownload类有一些问题。

首先，我要指出我不喜欢这个名字。这应该更像ImageDownloader，但这是一个相当小的抱怨。

更大的问题是，我们采取一个完成块和完成块预期将响应3个不同的步骤，在下载图像的过程中。也许更大的问题是，我们根本不在乎下载可能会失败。我们还没有包括连接失败条件的委托方法。

我们也不公开允许对象的用户取消下载请求的任何方法(例如，当视图滚动到屏幕时)。因此，如果终端用户不在wi上，我们就有可能浪费他们的移动数据。

因此，我们需要添加一个方法来取消下载。我们还需要一种方法来处理下载失败或被取消的异步情况。

抑制将两个状态添加到您的enum中的冲动，并大幅增加完成块的复杂性。相反，我们需要实现一个协议-委托模式。

protocol ImageDownloaderDelegate {
    required func imageDownloaderDidConnect()
    required func imageDownloaderDidComplete(image: UIImage)
    optional func imageDownloaderDidReceiveData(progress: Float)
    optional func imageDownloaderDidFail(error: NSError)
    optional func imageDownloaderWasCanceled()
}

显然，如果您认为合适，可以向这些方法添加任何附加参数。

现在，我们向我们的ImageDownloader类添加一个值：

var delegate: ImageDownloaderDelegate?

我们在适当的时候在委托上调用这些方法。我们应该确保在调用这些方法时使用可选链接，特别是在可选方法上：

self.delegate?.imageDownloaderDidReceiveData?(0.3)

当然，这意味着无论是谁使用ImageDownloader，现在都需要实现这些方法，并准备好在调用它们时响应它们。

这使我们避免了巨大的、丑陋的开关，它被关闭而变得更加丑陋，并且只有在添加我指出的其他两种场景的情况时才会变得更丑。相反，我们只实现了这些2-5个方法。总的来说，它的代码行数并不少。差不多是一样的。但是每一种方法都是独立的。它们更小更分散。这是5个非常不同的事件，它们需要作为这样的事件来处理。

NSURLConnection使用协议委托模式有一个很好的理由，我们不应该试图将其压缩到一个完成块中。