iOS开发：能否用NSURLSession上传Azure Blob并实现后台续传？

Hey there! Let's tackle your two iOS Azure Blob Storage questions using NSURLSession:

Absolutely! NSURLSession is fully capable of handling blob uploads to Azure Storage containers. Here's the general approach to make this work:

First, you'll need an authorized URL for your target blob—this is typically a Shared Access Signature (SAS) URL, since embedding your Azure Storage account key directly in the app is a major security risk. You can generate the SAS either server-side or using Azure's iOS SDK (though you don't strictly need the SDK if you have the SAS URL ready to go).

Once you have the SAS URL, you can use an NSURLSessionUploadTask or NSURLSessionDataTask to send the blob data. For most blob uploads, using the PUT HTTP method is standard, as it aligns with Azure Blob Storage's REST API. You'll also need to set appropriate request headers, like Content-Type (matching your blob's content, e.g., image/jpeg for photos).

Here's a quick code snippet to illustrate the flow:

// Assume you have your SAS URL string and image data ready
guard let sasUrl = URL(string: "your-azure-blob-sas-url-here") else { return }
var request = URLRequest(url: sasUrl)
request.httpMethod = "PUT"
request.setValue("image/jpeg", forHTTPHeaderField: "Content-Type")

// Create a default session (or background session if you need background support)
let session = URLSession.shared

// Initiate the upload task
let uploadTask = session.uploadTask(with: request, from: imageData) { data, response, error in
    if let error = error {
        print("Upload failed: \(error.localizedDescription)")
        return
    }
    guard let httpResponse = response as? HTTPURLResponse, (200...299).contains(httpResponse.statusCode) else {
        print("Invalid response from server")
        return
    }
    print("Blob uploaded successfully!")
}
uploadTask.resume()

Yes, this is exactly what NSURLSession's background session configuration was built for. When you use a background session, the iOS system takes over the upload task even if your app is suspended, backgrounded, or even terminated. Once the upload completes (or fails), the system will wake your app (if needed) to let you handle the result.

Here's how to set this up properly:

1. Create a background session configuration: Use URLSessionConfiguration.background(withIdentifier:) to create a configuration with a unique identifier. This identifier helps the system reconnect to the session if your app is restarted.
2. Initialize the session with a delegate: Background sessions require a delegate to handle completion events—completion blocks aren't reliable for background tasks.
3. Handle background callbacks in AppDelegate: You'll need to capture the system-provided completion handler and call it once all background tasks are finished.

Example code breakdown: #

Step 1: Set up the background session manager

class UploadManager: NSObject, URLSessionDelegate, URLSessionTaskDelegate {
    static let shared = UploadManager()
    private var backgroundSession: URLSession!
    private var backgroundCompletionHandler: (() -> Void)?

    private override init() {
        super.init()
        let config = URLSessionConfiguration.background(withIdentifier: "com.your-app.azure-background-upload")
        config.isDiscretionary = false // Set to true if you want the system to optimize upload timing (optional)
        backgroundSession = URLSession(configuration: config, delegate: self, delegateQueue: nil)
    }

    func startBackgroundUpload(with sasUrl: URL, imageData: Data) {
        var request = URLRequest(url: sasUrl)
        request.httpMethod = "PUT"
        request.setValue("image/jpeg", forHTTPHeaderField: "Content-Type")
        
        let uploadTask = backgroundSession.uploadTask(with: request, from: imageData)
        uploadTask.resume()
    }

    // Delegate method to handle task completion
    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
        if let error = error {
            print("Background upload failed: \(error.localizedDescription)")
        } else {
            print("Background upload completed successfully!")
        }
        // Call the completion handler to let the system know we're done
        backgroundCompletionHandler?()
        backgroundCompletionHandler = nil
    }

    // Store the completion handler passed from AppDelegate
    func setBackgroundCompletionHandler(_ handler: (() -> Void)?) {
        backgroundCompletionHandler = handler
    }
}

Step 2: Update AppDelegate to handle background events

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        // Pass the completion handler to our upload manager
        UploadManager.shared.setBackgroundCompletionHandler(completionHandler)
    }
}

Step 3: Trigger the upload from your foreground code

// When the user selects a photo from the album
if let imageData = UIImageJPEGRepresentation(selectedImage, 0.8), let sasUrl = URL(string: "your-sas-url-here") {
    UploadManager.shared.startBackgroundUpload(with: sasUrl, imageData: imageData)
}

A few key notes to remember:

• Background sessions don't support completion blocks—always use delegate methods to handle results.
• If your app is terminated during the upload, the system will relaunch it in the background to let you process the outcome.
• Make sure to enable the "Background fetch" capability in your Xcode project's target settings (under Signing & Capabilities > Background Modes).

内容的提问来源于stack exchange，提问作者Rizwan