fixed some bugs

ios/OralTrainer/Services/StreamingVideoCache.swift

@@ -0,0 +1,188 @@
+import CryptoKit
+import Foundation
+
+/// iOS 端的持久化整文件视频缓存。
+///
+/// Android 端使用 ExoPlayer 的 CacheDataSource 边播边缓存；AVFoundation 没有公开
+/// 等价的持久字节缓存 API。这里在后台把远程 MP4 下载成完整本地文件，下次装载时
+/// 直接交给 AVPlayer，并在本次播放完成下载后热切换到本地文件，避免回跳句子时
+/// 再次依赖网络。
+@MainActor
+final class StreamingVideoCache {
+    static let shared = StreamingVideoCache()
+
+    static let maxCacheBytes: Int64 = 512 * 1024 * 1024
+
+    private static let supportedExtensions: Set<String> = ["mp4", "m4v", "mov"]
+    private static let fileExtension = "mp4"
+
+    private let directory: URL
+    private let fileManager = FileManager.default
+    private var downloadsByURL: [URL: Task<URL?, Never>] = [:]
+
+    private init() {
+        let caches = fileManager.urls(for: .cachesDirectory, in: .userDomainMask)[0]
+        directory = caches.appendingPathComponent("oral-trainer-streaming-videos", isDirectory: true)
+        try? fileManager.createDirectory(at: directory, withIntermediateDirectories: true)
+    }
+
+    func cachedFileIfAvailable(for remoteURL: URL) -> URL? {
+        guard isCacheable(remoteURL), let file = destinationFile(for: remoteURL) else {
+            return nil
+        }
+        guard fileManager.fileExists(atPath: file.path), isUsableMediaFile(file) else {
+            fileManager.removeItemIfPossible(at: file)
+            return nil
+        }
+
+        // AVPlayer 读取后刷新 LRU 时间，与 Android LeastRecentlyUsedCacheEvictor 的
+        // 语义保持接近。
+        try? fileManager.setAttributes(
+            [.modificationDate: Date()],
+            ofItemAtPath: file.path
+        )
+        return file
+    }
+
+    /// 重复调用同一 URL 时复用同一个后台下载；完成后返回本地文件。
+    func prefetch(_ remoteURL: URL) async -> URL? {
+        if let cached = cachedFileIfAvailable(for: remoteURL) {
+            return cached
+        }
+        guard isCacheable(remoteURL), let destination = destinationFile(for: remoteURL) else {
+            return nil
+        }
+
+        if let existing = downloadsByURL[remoteURL] {
+            return await existing.value
+        }
+
+        let task = Task {
+            await Self.download(from: remoteURL, to: destination)
+        }
+        downloadsByURL[remoteURL] = task
+
+        let result = await task.value
+        if downloadsByURL[remoteURL]?.isCancelled != true {
+            downloadsByURL[remoteURL] = nil
+        }
+        if result != nil {
+            evictIfNeeded(keeping: remoteURL)
+        }
+        return result
+    }
+
+    func clear() {
+        for task in downloadsByURL.values {
+            task.cancel()
+        }
+        downloadsByURL.removeAll()
+        try? fileManager.removeItem(at: directory)
+        try? fileManager.createDirectory(at: directory, withIntermediateDirectories: true)
+    }
+
+    private func isCacheable(_ url: URL) -> Bool {
+        guard let scheme = url.scheme?.lowercased(), scheme == "http" || scheme == "https" else {
+            return false
+        }
+        let ext = url.pathExtension.lowercased()
+        // HLS/DASH 播放列表不能通过下载单个清单完成离线缓存；当前云端课程均为 MP4。
+        return Self.supportedExtensions.contains(ext)
+    }
+
+    private func destinationFile(for remoteURL: URL) -> URL? {
+        let rawData = Data(remoteURL.absoluteString.utf8)
+        let digest = SHA256.hash(data: rawData)
+        let key = digest.map { String(format: "%02x", $0) }.joined()
+        return directory.appendingPathComponent(key).appendingPathExtension(Self.fileExtension)
+    }
+
+    private func isUsableMediaFile(_ url: URL) -> Bool {
+        let values = try? url.resourceValues(forKeys: [.isRegularFileKey, .fileSizeKey])
+        guard values?.isRegularFile == true, let fileSize = values?.fileSize else {
+            return false
+        }
+        let size = Int64(fileSize)
+        return size > 0
+    }
+
+    private func evictIfNeeded(keeping remoteURL: URL) {
+        guard let keeping = destinationFile(for: remoteURL) else {
+            return
+        }
+
+        let contents = (try? fileManager.contentsOfDirectory(
+            at: directory,
+            includingPropertiesForKeys: [.contentModificationDateKey, .fileSizeKey],
+            options: [.skipsHiddenFiles]
+        )) ?? []
+
+        var entries: [(url: URL, size: Int64, date: Date)] = contents.compactMap { url in
+            let values = try? url.resourceValues(forKeys: [.contentModificationDateKey, .fileSizeKey])
+            guard let fileSize = values?.fileSize, fileSize > 0 else {
+                fileManager.removeItemIfPossible(at: url)
+                return nil
+            }
+            let size = Int64(fileSize)
+            return (url, size, values?.contentModificationDate ?? .distantPast)
+        }
+
+        var totalBytes = entries.reduce(0) { $0 + $1.size }
+        entries.sort { $0.date < $1.date }
+        for entry in entries where totalBytes > Self.maxCacheBytes && entry.url != keeping {
+            guard fileManager.removeItemIfPossible(at: entry.url) else {
+                continue
+            }
+            totalBytes -= entry.size
+        }
+    }
+
+    private static func download(from remoteURL: URL, to destination: URL) async -> URL? {
+        var request = URLRequest(url: remoteURL)
+        request.timeoutInterval = AppConfig.connectTimeout
+        request.setValue(AppConfig.userAgent, forHTTPHeaderField: "User-Agent")
+        request.cachePolicy = .reloadIgnoringLocalCacheData
+
+        do {
+            let (tempFile, response) = try await URLSession.shared.download(for: request)
+            guard let http = response as? HTTPURLResponse, (200...299).contains(http.statusCode) else {
+                let status = (response as? HTTPURLResponse)?.statusCode ?? -1
+                FileManager.default.removeItemIfPossible(at: tempFile)
+                logFailure(remoteURL, message: "HTTP \(status)")
+                return nil
+            }
+
+            let fileManager = FileManager.default
+            try? fileManager.removeItem(at: destination)
+            try fileManager.createDirectory(
+                at: destination.deletingLastPathComponent(),
+                withIntermediateDirectories: true
+            )
+            try fileManager.moveItem(at: tempFile, to: destination)
+            return destination
+        } catch is CancellationError {
+            return nil
+        } catch {
+            logFailure(remoteURL, message: error.localizedDescription)
+            return nil
+        }
+    }
+
+    private static func logFailure(_ url: URL, message: String) {
+        #if DEBUG
+        print("[StreamingVideoCache] 预缓存失败: \(url), \(message)")
+        #endif
+    }
+}
+
+private extension FileManager {
+    @discardableResult
+    func removeItemIfPossible(at url: URL) -> Bool {
+        do {
+            try removeItem(at: url)
+            return true
+        } catch {
+            return false
+        }
+    }
+}