Backoff计时器是一种在网络请求失败时采取递增时间间隔重试的策略，通过URLRequest可实现在iOS应用中。以下是实现Backoff计时器的示例代码：

class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Make initial request
        makeRequest(successCompletion: {
            print("Request succeeded")
        }, failedCompletion: {
            print("Request failed")
        })
    }

    func makeRequest(successCompletion: @escaping () -> (), failedCompletion: @escaping () -> ()) {
        let url = URL(string: "https://sampleurl.com")!
        var request = URLRequest(url: url)
        
        // Set timeout interval to 10 seconds
        request.timeoutInterval = 10
        
        // Set number of retries to 3
        let numberOfRetries = 3
        
        // Set initial time interval to 1 second
        var timeInterval = 1.0
        
        let queue = DispatchQueue(label: "com.example.requestQueue")
        
        // Create a function that handles the data task and performs retries
        func performRequest() {
            let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
                if let error = error {
                    // Check if the request can be retried
                    if numberOfRetries > 0 {
                        print("Request failed, will retry in \(timeInterval) seconds")
                        
                        // Dispatch retry after a backoff interval
                        queue.asyncAfter(deadline: .now() + timeInterval, execute: {
                            // Decrease the number of retries and double the backoff interval
                            timeInterval *= 2
                            let newNumberOfRetries = numberOfRetries - 1
                            
                            // Perform retry
                            print("Retrying \(newNumberOfRetries) more times...")
                            makeRequest(successCompletion: successCompletion, failedCompletion: failedCompletion)
                        })
                    } else {
                        // Request failed and can no longer be retried
                        DispatchQueue.main.async {
                            failedCompletion()
                        }
                    }
                } else {
                    // Request succeeded
                    DispatchQueue.main.async {
                        successCompletion()
                    }
                }
            }
            
            task.resume()
        }
        
        // Perform initial request
        performRequest()