从AVAssetExportSession准确估计输出视频大小的方法

import AVFoundation

func estimateOutputVideoSize(asset: AVAsset, outputURL: URL, outputFileType: AVFileType) -> String? {
    let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetPassthrough)
    exportSession?.outputURL = outputURL
    exportSession?.outputFileType = outputFileType
    
    // 开始导出视频
    let semaphore = DispatchSemaphore(value: 0)
    exportSession?.exportAsynchronously {
        semaphore.signal()
    }
    semaphore.wait()
    
    // 导出完成后，获取输出文件的大小
    if exportSession?.status == .completed {
        let fileManager = FileManager.default
        do {
            let attributes = try fileManager.attributesOfItem(atPath: outputURL.path)
            if let fileSize = attributes[.size] as? Int64 {
                let sizeInMB = Double(fileSize) / (1024 * 1024)
                return String(format: "%.2f MB", sizeInMB)
            }
        } catch {
            print("Failed to get file attributes: \(error)")
        }
    }
    
    return nil
}

// 使用示例
let asset = AVAsset(url: URL(fileURLWithPath: "path/to/input/video.mov"))
let outputURL = URL(fileURLWithPath: "path/to/output/video.mp4")
let outputFileType = AVFileType.mp4

if let estimatedSize = estimateOutputVideoSize(asset: asset, outputURL: outputURL, outputFileType: outputFileType) {
    print("Estimated output video size: \(estimatedSize)")
} else {
    print("Failed to estimate output video size")
}