在Objective-C 截屏视频中编码？

在Objective-C中截屏视频，可以通过以下步骤实现：

首先，需要导入AVFoundation框架，以便使用视频编码和处理功能。
创建一个AVCaptureSession实例，并设置视频输入源。
创建一个AVAssetWriter实例，并设置视频输出格式。
使用AVCaptureVideoDataOutput将捕获到的视频数据写入AVAssetWriter中。
当需要停止截屏时，调用AVAssetWriter的finishWriting方法，将视频数据写入文件。

以下是一个简单的示例代码：

#import <AVFoundation/AVFoundation.h>

@interface ViewController () <AVCaptureVideoDataOutputSampleBufferDelegate>

@property (nonatomic, strong) AVCaptureSession *captureSession;
@property (nonatomic, strong) AVCaptureVideoDataOutput *videoOutput;
@property (nonatomic, strong) AVCaptureConnection *videoConnection;
@property (nonatomic, strong) AVAssetWriter *assetWriter;
@property (nonatomic, strong) AVAssetWriterInput *videoWriterInput;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // 创建捕获会话
    self.captureSession = [[AVCaptureSession alloc] init];
    self.captureSession.sessionPreset = AVCaptureSessionPresetHigh;
    
    // 创建视频输入源
    AVCaptureDevice *videoDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    NSError *error = nil;
    AVCaptureDeviceInput *videoInput = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:&error];
    if (error) {
        NSLog(@"Error: %@", error.localizedDescription);
        return;
    }
    [self.captureSession addInput:videoInput];
    
    // 创建视频输出
    self.videoOutput = [[AVCaptureVideoDataOutput alloc] init];
    self.videoOutput.videoSettings = @{(id)kCVPixelBufferPixelFormatTypeKey : @(kCVPixelFormatType_32BGRA)};
    self.videoOutput.alwaysDiscardsLateVideoFrames = YES;
    [self.captureSession addOutput:self.videoOutput];
    
    // 设置视频输出代理
    dispatch_queue_t queue;
    queue = dispatch_queue_create("videoOutputQueue", DISPATCH_QUEUE_SERIAL);
    [self.videoOutput setSampleBufferDelegate:self queue:queue];
    
    // 创建视频写入器
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *filePath = [documentsDirectory stringByAppendingPathComponent:@"output.mp4"];
    NSURL *outputURL = [NSURL fileURLWithPath:filePath];
    self.assetWriter = [[AVAssetWriter alloc] initWithURL:outputURL fileType:AVFileTypeMPEG4 error:&error];
    if (error) {
        NSLog(@"Error: %@", error.localizedDescription);
        return;
    }
    
    // 设置视频编码器
    NSDictionary *videoCompressionProperties = @{(id)kVTCompressionPropertyKey_ProfileLevel: (id)kVTProfileLevel_H264_High_4_0,
                                                    (id)kVTCompressionPropertyKey_AverageBitRate: @(1000000)};
    NSDictionary *videoOutputSettings = @{(id)kCVPixelBufferPixelFormatTypeKey : @(kCVPixelFormatType_32BGRA),
                                            (id)kCVPixelBufferWidthKey : @(self.view.bounds.size.width),
                                            (id)kCVPixelBufferHeightKey : @(self.view.bounds.size.height),
                                            (id)kCVPixelBufferBytesPerRowAlignmentKey : @(4),
                                            (id)kCVPixelBufferCGBitmapContextCompatibilityKey : @(YES),
                                            (id)kCVPixelBufferCGImageCompatibilityKey : @(YES),
                                            (id)kCVPixelBufferIOSurfacePropertiesKey : @{}};
    self.videoWriterInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:videoOutputSettings];
    self.videoWriterInput.expectsMediaDataInRealTime = YES;
    self.videoWriterInput.transform = CGAffineTransformMakeScale(-1, 1);
    [self.assetWriter addInput:self.videoWriterInput];
    
    // 开始捕获视频
    [self.captureSession startRunning];
}

- (void)captureOutput:(AVCaptureOutput *)output didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
    if (self.videoWriterInput.readyForMoreMediaData) {
        if (self.videoConnection == nil) {
            self.videoConnection = connection;
        }
        if (connection == self.videoConnection) {
            [self.videoWriterInput appendSampleBuffer:sampleBuffer];
        }
    }
}

- (void)dealloc {
    [self.captureSession stopRunning];
    [self.assetWriter finishWritingWithCompletionHandler:^{
        NSLog(@"视频已保存");
    }];
}

@end

这段代码将捕获屏幕上的视频，并将其写入一个MP4文件中。在视频捕获过程中，可以通过AVCaptureVideoDataOutputSampleBufferDelegate协议的回调方法captureOutput:didOutputSampleBuffer:fromConnection:来处理每一帧视频数据。在视频捕获结束时，可以通过AVAssetWriter的finishWriting方法将视频数据写入文件。