關於 URLSession - part2

接下來，我們嘗試加入progress bar，讓這個專案更完整一點。
如果要progress bar有作用，我們需要用到URLSession delegation的一些methods。

ps. 這篇原文來自Apple的Fetching data into memory

好，我們開始吧！
我們一樣用Part 1的程式碼，只是稍做修改。在Viewcontroller上面加入UIProgressView並且設定IBOutlet。

然後加入兩個property:

    //作為接收data的容器

    var receivedData:Data?

    //檔案大小

    var expectedContentLength = 0

UI部份都設定好後，開始後面邏輯的處理。
在原來的startLoad方法，把URLSession用closure的方式給予初始化設定，並設定delegate給自己。

	//Session with configuration
	private lazy var session: URLSession = {
	let config = URLSessionConfiguration.default
	config.waitsForConnectivity = true
	return URLSession(configuration: config, delegate: self, delegateQueue: nil)
	}()

view raw urlsessionclousure.swift hosted with ❤ by GitHub

在這邊我們會用到3個屬於URLSessionDataDelegate的方法:

• urlSession(_:dataTask:didReceive:completionHandler:) 在這個方法裡面我們會驗證status code是否正確來判斷有沒有錯誤產生，並且取得檔案的大小.
• urlSession(_:dataTask:didReceive:) 將收到的data丟到receivedData並且更新progress bar進度.
• urlSession(_:task:didCompleteWithError:) 將收到的完整data轉換成UIImage並呈現在ImageView

	//MARK: - URLSessionDataDelegate
	extension ViewController: URLSessionDataDelegate {
	func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition) -> Void) {
	guard let httpResponse = response as? HTTPURLResponse, (200...299).contains(httpResponse.statusCode) else {
	completionHandler(.cancel)
	return
	}
	//get full lenth of your content
	expectedContentLength = Int(response.expectedContentLength)
	completionHandler(.allow)
	}
	func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
	self.receivedData?.append(data)
	//count percent and update progress
	if let receivedCount = self.receivedData{
	let percentageDownload = Float(receivedCount.count / self.expectedContentLength)
	DispatchQueue.main.async {
	self.progressView1.progress = percentageDownload
	}
	}
	}
	func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?){
	DispatchQueue.main.async {
	self.button1.isEnabled = true
	self.progressView1.progress = 1.0
	if let error = error {
	self.handleClientError(error)
	}
	//when completed receive data, put data to imageview
	if let receivedData = self.receivedData {
	self.imageview1.image = UIImage(data: receivedData)
	}
	}
	}
	}

view raw urlsessiondatadelegate.swift hosted with ❤ by GitHub

個人習慣用extension把delegate和原本的class區隔開來，覺得這樣比較簡潔不會都混在一塊。

接下來我們就試著在simulator上build並看看結果！儘量找一些檔案大一點的圖片，比較能看出效果: