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Pods/Kingfisher/Sources/Networking/RetryStrategy.swift

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+//
+//  RetryStrategy.swift
+//  Kingfisher
+//
+//  Created by onevcat on 2020/05/04.
+//
+//  Copyright (c) 2020 Wei Wang <onevcat@gmail.com>
+//
+//  Permission is hereby granted, free of charge, to any person obtaining a copy
+//  of this software and associated documentation files (the "Software"), to deal
+//  in the Software without restriction, including without limitation the rights
+//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+//  copies of the Software, and to permit persons to whom the Software is
+//  furnished to do so, subject to the following conditions:
+//
+//  The above copyright notice and this permission notice shall be included in
+//  all copies or substantial portions of the Software.
+//
+//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+//  THE SOFTWARE.
+
+import Foundation
+
+/// Represents a retry context which could be used to determine the current retry status.
+public class RetryContext {
+
+    /// The source from which the target image should be retrieved.
+    public let source: Source
+
+    /// The last error which caused current retry behavior.
+    public let error: KingfisherError
+
+    /// The retried count before current retry happens. This value is `0` if the current retry is for the first time.
+    public var retriedCount: Int
+
+    /// A user set value for passing any other information during the retry. If you choose to use `RetryDecision.retry`
+    /// as the retry decision for `RetryStrategy.retry(context:retryHandler:)`, the associated value of
+    /// `RetryDecision.retry` will be delivered to you in the next retry.
+    public internal(set) var userInfo: Any? = nil
+
+    init(source: Source, error: KingfisherError) {
+        self.source = source
+        self.error = error
+        self.retriedCount = 0
+    }
+
+    @discardableResult
+    func increaseRetryCount() -> RetryContext {
+        retriedCount += 1
+        return self
+    }
+}
+
+/// Represents decision of behavior on the current retry.
+public enum RetryDecision {
+    /// A retry should happen. The associated `userInfo` will be pass to the next retry in the `RetryContext` parameter.
+    case retry(userInfo: Any?)
+    /// There should be no more retry attempt. The image retrieving process will fail with an error.
+    case stop
+}
+
+/// Defines a retry strategy can be applied to a `.retryStrategy` option.
+public protocol RetryStrategy {
+
+    /// Kingfisher calls this method if an error happens during the image retrieving process from a `KingfisherManager`.
+    /// You implement this method to provide necessary logic based on the `context` parameter. Then you need to call
+    /// `retryHandler` to pass the retry decision back to Kingfisher.
+    ///
+    /// - Parameters:
+    ///   - context: The retry context containing information of current retry attempt.
+    ///   - retryHandler: A block you need to call with a decision of whether the retry should happen or not.
+    func retry(context: RetryContext, retryHandler: @escaping (RetryDecision) -> Void)
+}
+
+/// A retry strategy that guides Kingfisher to retry when a `.responseError` happens, with a specified max retry count
+/// and a certain interval mechanism.
+public struct DelayRetryStrategy: RetryStrategy {
+
+    /// Represents the interval mechanism which used in a `DelayRetryStrategy`.
+    public enum Interval {
+        /// The next retry attempt should happen in fixed seconds. For example, if the associated value is 3, the
+        /// attempts happens after 3 seconds after the previous decision is made.
+        case seconds(TimeInterval)
+        /// The next retry attempt should happen in an accumulated duration. For example, if the associated value is 3,
+        /// the attempts happens with interval of 3, 6, 9, 12, ... seconds.
+        case accumulated(TimeInterval)
+        /// Uses a block to determine the next interval. The current retry count is given as a parameter.
+        case custom(block: (_ retriedCount: Int) -> TimeInterval)
+
+        func timeInterval(for retriedCount: Int) -> TimeInterval {
+            let retryAfter: TimeInterval
+            switch self {
+            case .seconds(let interval):
+                retryAfter = interval
+            case .accumulated(let interval):
+                retryAfter = Double(retriedCount + 1) * interval
+            case .custom(let block):
+                retryAfter = block(retriedCount)
+            }
+            return retryAfter
+        }
+    }
+
+    /// The max retry count defined for the retry strategy
+    public let maxRetryCount: Int
+
+    /// The retry interval mechanism defined for the retry strategy.
+    public let retryInterval: Interval
+
+    /// Creates a delay retry strategy.
+    /// - Parameters:
+    ///   - maxRetryCount: The max retry count.
+    ///   - retryInterval: The retry interval mechanism. By default, `.seconds(3)` is used to provide a constant retry
+    ///   interval.
+    public init(maxRetryCount: Int, retryInterval: Interval = .seconds(3)) {
+        self.maxRetryCount = maxRetryCount
+        self.retryInterval = retryInterval
+    }
+
+    public func retry(context: RetryContext, retryHandler: @escaping (RetryDecision) -> Void) {
+        // Retry count exceeded.
+        guard context.retriedCount < maxRetryCount else {
+            retryHandler(.stop)
+            return
+        }
+
+        // User cancel the task. No retry.
+        guard !context.error.isTaskCancelled else {
+            retryHandler(.stop)
+            return
+        }
+
+        // Only retry for a response error.
+        guard case KingfisherError.responseError = context.error else {
+            retryHandler(.stop)
+            return
+        }
+
+        let interval = retryInterval.timeInterval(for: context.retriedCount)
+        if interval == 0 {
+            retryHandler(.retry(userInfo: nil))
+        } else {
+            DispatchQueue.main.asyncAfter(deadline: .now() + interval) {
+                retryHandler(.retry(userInfo: nil))
+            }
+        }
+    }
+}