如何缩小视频文件大小(swift 5)

在Swift 5中，可以使用以下方法来缩小视频文件大小：

使用视频编码器：视频编码器可以将视频文件转换为较小的文件大小，同时保持较高的视频质量。在Swift中，可以使用AVAssetExportSession类来实现视频编码。首先，需要创建一个AVAsset对象，该对象表示要压缩的视频文件。然后，创建一个AVAssetExportSession对象，并设置输出文件的URL和文件格式。最后，调用exportAsynchronously(completionHandler:)方法来开始压缩过程。以下是一个示例代码：

import AVFoundation

func compressVideo(inputURL: URL, outputURL: URL, completion: @escaping (URL?, Error?) -> Void) {
    let asset = AVAsset(url: inputURL)
    guard let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetMediumQuality) else {
        completion(nil, NSError(domain: "com.example", code: 0, userInfo: [NSLocalizedDescriptionKey: "Failed to create AVAssetExportSession"]))
        return
    }
    
    exportSession.outputURL = outputURL
    exportSession.outputFileType = .mp4
    exportSession.shouldOptimizeForNetworkUse = true
    
    exportSession.exportAsynchronously {
        switch exportSession.status {
        case .completed:
            completion(outputURL, nil)
        case .failed:
            completion(nil, exportSession.error)
        case .cancelled:
            completion(nil, NSError(domain: "com.example", code: 0, userInfo: [NSLocalizedDescriptionKey: "Export cancelled"]))
        default:
            break
        }
    }
}

// 使用示例
let inputURL = URL(fileURLWithPath: "path/to/input/video.mov")
let outputURL = URL(fileURLWithPath: "path/to/output/compressed-video.mp4")

compressVideo(inputURL: inputURL, outputURL: outputURL) { (outputURL, error) in
    if let error = error {
        print("Failed to compress video: \(error.localizedDescription)")
    } else if let outputURL = outputURL {
        print("Video compressed successfully. Output URL: \(outputURL)")
    }
}

调整视频分辨率和帧率：降低视频的分辨率和帧率可以显著减小视频文件的大小。可以使用AVAssetWriter和AVAssetWriterInput来重新编码视频，并在编码过程中设置较低的分辨率和帧率。以下是一个示例代码：

import AVFoundation

func resizeVideo(inputURL: URL, outputURL: URL, targetSize: CGSize, targetFrameRate: Int32, completion: @escaping (URL?, Error?) -> Void) {
    let asset = AVAsset(url: inputURL)
    guard let videoTrack = asset.tracks(withMediaType: .video).first else {
        completion(nil, NSError(domain: "com.example", code: 0, userInfo: [NSLocalizedDescriptionKey: "Failed to get video track"]))
        return
    }
    
    let composition = AVMutableComposition()
    let compositionVideoTrack = composition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)
    
    do {
        try compositionVideoTrack?.insertTimeRange(CMTimeRange(start: .zero, duration: asset.duration), of: videoTrack, at: .zero)
    } catch {
        completion(nil, error)
        return
    }
    
    let videoSettings = [
        AVVideoCodecKey: AVVideoCodecType.h264,
        AVVideoWidthKey: targetSize.width,
        AVVideoHeightKey: targetSize.height,
        AVVideoCompressionPropertiesKey: [
            AVVideoAverageBitRateKey: targetSize.width * targetSize.height * targetFrameRate
        ]
    ] as [String : Any]
    
    let writer = try? AVAssetWriter(outputURL: outputURL, fileType: .mp4)
    let writerInput = AVAssetWriterInput(mediaType: .video, outputSettings: videoSettings)
    
    writer?.add(writerInput)
    
    writer?.startWriting()
    writer?.startSession(atSourceTime: .zero)
    
    let processingQueue = DispatchQueue(label: "com.example.processingqueue")
    
    writerInput.requestMediaDataWhenReady(on: processingQueue) {
        while writerInput.isReadyForMoreMediaData {
            let sampleBuffer = videoTrackOutput.copyNextSampleBuffer()
            
            if sampleBuffer != nil {
                writerInput.append(sampleBuffer!)
            } else {
                writerInput.markAsFinished()
                writer?.finishWriting(completionHandler: {
                    if writer?.status == .completed {
                        completion(outputURL, nil)
                    } else {
                        completion(nil, writer?.error)
                    }
                })
                break
            }
        }
    }
}

// 使用示例
let inputURL = URL(fileURLWithPath: "path/to/input/video.mov")
let outputURL = URL(fileURLWithPath: "path/to/output/resized-video.mp4")
let targetSize = CGSize(width: 640, height: 480)
let targetFrameRate: Int32 = 30

resizeVideo(inputURL: inputURL, outputURL: outputURL, targetSize: targetSize, targetFrameRate: targetFrameRate) { (outputURL, error) in
    if let error = error {
        print("Failed to resize video: \(error.localizedDescription)")
    } else if let outputURL = outputURL {
        print("Video resized successfully. Output URL: \(outputURL)")
    }
}

这些方法可以帮助你在Swift 5中缩小视频文件大小。请注意，这些示例代码仅涵盖了基本的视频压缩和调整分辨率/帧率的功能，实际应用中可能需要根据具体需求进行更多的优化和调整。

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