AVFoundation框架解析看这里（6）- AVAssetExportSession

public enum Status : Int {
        case unknown
        case waiting
        case exporting
        case completed
        case failed
        case cancelled
    }

/*
asset: 用于导出的AVAsset对象。

presetName: NSString，指定导出的预设模板的名称。
*/
public init?(asset: AVAsset, presetName: String)

//下面这些export选项可用于生成视频大小适合设备的视频文件。export不会从较小的尺寸缩放较大尺寸的视频。视频将使用H.264压缩和音频将使用AAC进行压缩。
public let AVAssetExportPresetLowQuality: String
public let AVAssetExportPresetMediumQuality: String
public let AVAssetExportPresetHighestQuality: String

//下面这个export(导出)方式视频方面使用HEVC压缩和音频方面将使用AAC进行压缩。有些设备可能无法支持某些尺寸。
public let AVAssetExportPresetHEVCHighestQuality: String
public let AVAssetExportPresetHEVCHighestQualityWithAlpha: String

//下面的导出方式视频采用HEVC压缩，音频采用AAC压缩。有些设备可能无法支持某些尺寸。
public let AVAssetExportPreset640x480: String
public let AVAssetExportPreset960x540: String
public let AVAssetExportPreset1280x720: String
public let AVAssetExportPreset1920x1080: String
public let AVAssetExportPreset3840x2160: String

下面的导出方式视频采用HEVC压缩，音频采用AAC压缩。有些设备可能无法支持某些尺寸。
public let AVAssetExportPresetHEVC1920x1080: String
public let AVAssetExportPresetHEVC1920x1080WithAlpha: String
public let AVAssetExportPresetHEVC3840x2160: String
public let AVAssetExportPresetHEVC3840x2160WithAlpha: String

下面的导出选项将生成仅具有音频的.m4a文件，其中包含适当的iTunes无间隙播放数据
public let AVAssetExportPresetAppleM4A: String

//下面的导出选项将使所有轨道的媒体完全按照存储在源资源中的方式传递到output，除了通道无法通过的轨道，通常是由于指定的outputFileType指示的容器格式的约束。
此选项不包含在-allExportPresets和-exportPresetsCompatibleWithAsset返回的数组中。
public let AVAssetExportPresetPassthrough: String

typealias TranscodingVideoBlock = (URL?) -> ()        //转码视频回调

/*
    转码视频
     
     asset:           需要转码的视频
     presetName:      视频转码质量，默认为高清
     outputName:      转码后视频的名称
     completedBlock:  转码完成后的回调
    */
    class func transcodingVideo(asset: AVURLAsset, presetName: String = AVAssetExportPresetMediumQuality, outputName: String, completedBlock: @escaping TranscodingVideoBlock) {
        let compatiblePresets = AVAssetExportSession.exportPresets(compatibleWith: asset)
        if !compatiblePresets.contains(presetName) {
            print("不支持的转码 presetName")
            
            return
        }
        
        guard let documentsDir = try? FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true) else {
            return
        }
        
        guard let fileURL = URL(string: outputName, relativeTo: documentsDir) else {
            return
        }
        do {
            try FileManager.default.removeItem(at:fileURL)
        } catch {
        }
        
        guard let exportSession = AVAssetExportSession(asset: asset, presetName: presetName) else {
            return
        }
        exportSession.outputURL = fileURL
        exportSession.outputFileType = .mp4
        exportSession.shouldOptimizeForNetworkUse = true
        
        exportSession.exportAsynchronously {
            switch exportSession.status {
            case .completed:
                completedBlock(fileURL)
            default:
                completedBlock(nil)
            }
        }
    }

asset:           需要裁剪的视频
outputName:      裁剪后视频的名称
startTime:       起始时间
durationTime:    裁剪时长
completedBlock:  裁剪完成后的回调
*/
class func tailoringVideo(asset: AVURLAsset, outputName: String, startTime: CMTime, durationTime: CMTime, completedBlock: @escaping TailoringVideoBlock) {
    guard let documentsDir = try? FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true) else {
        return
    }
    
    guard let fileURL = URL(string: outputName, relativeTo: documentsDir) else {
        return
    }
    do {
        try FileManager.default.removeItem(at:fileURL)
    } catch {
    }
    
    guard let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetPassthrough) else {
        return
    }
    exportSession.outputURL = fileURL
    exportSession.outputFileType = .mp4
    let timeRange = CMTimeRange(start: startTime, duration: durationTime)
    exportSession.timeRange = timeRange
    
    exportSession.exportAsynchronously {
        switch exportSession.status {
        case .completed:
           completedBlock(fileURL)
        default:
            completedBlock(nil)
        }
    }
}