iOS AVAssetWriter 实现高分辨率录制视频，生成低体积的视频文件

在 iOS 开发中，录制视频是一项常见的任务。使用 AVAssetWriter 可以实现高分辨率录制视频并生成低体积的视频文件。本文将详细介绍如何使用 AVAssetWriter 来完成这个任务。

一、使用 AVAssetWriter 录制高分辨率视频

AVAssetWriter 是 iOS 提供的用于处理媒体数据的类。它可以用来录制和编辑音频和视频数据。要使用 AVAssetWriter 录制高分辨率视频，首先需要创建一个 AVAssetWriter 对象。

let outputURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!.appendingPathComponent("output.mp4")
let writer = try AVAssetWriter(outputURL: outputURL, fileType: .mp4)

然后，需要设置视频的输出属性。在这里，我们将视频的尺寸设置为 1920x1080，帧率为 30fps。

let videoSettings = [AVVideoCodecKey : AVVideoCodecH264, AVVideoWidthKey : 1920, AVVideoHeightKey : 1080, AVVideoFrameRateKey : 30]
writer.videoSettings = videoSettings

接下来，创建一个 AVAssetWriterInput 对象，它将用于接收视频数据。

let input = AVAssetWriterInput(mediaType: .video, outputSettings: videoSettings)
input.expectsMediaDataInRealTime = true
writer.add(input)

最后，开始录制视频。在这个例子中，我们将使用 AVCaptureSession 来捕获视频数据并传递给 AVAssetWriter。

let session = AVCaptureSession()
session.sessionPreset = AVCaptureSession.Preset.high
let captureDevice = AVCaptureDevice.default(for: .video)
let input = AVCaptureDeviceInput(device: captureDevice!)
session.addInput(input)
session.startRunning()

将捕获的视频数据传递给 AVAssetWriter。
```swift
let queue = DispatchQueue(label: “com.example.video-writer”)
writer.beginWriting()
queue.async { [weak writer] in
let inputPixelBufferAdaptor = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: input, sourcePixelBufferAttributes: nil)
var nextPixelBuffer: CVPixelBuffer? = nil
while let pixelBuffer = inputPixelBufferAdaptor.dequeuePixelBuffer() {
if let pixelBuffer = pixelBuffer {
nextPixelBuffer = CVPixelBufferGetImageArray(pixelBuffer)[0] // assumes a single-plane image buffer for video frames in this example.
} else { // pixelBuffer == nil, which means the source has no more data to provide. This is a signal to stop processing.
break // Exit the loop when there’s no more data to process.
}
// Process the pixel buffer here. This could involve operations such as applying filters, resizing, etc. Note that this code assumes a single-plane image buffer for video frames in this example. Adjust as needed for your specific use case.
} // end while loop. This loop processes data until the source has no more data to provide (pixelBuffer == nil). Then it breaks out of the loop and returns to the caller. This could involve operations such as applying filters, resizing, etc. Note that this code assumes a single-plane image buffer for video frames in this example. Adjust as needed for your specific use case. 112 end while loop. This loop processes data until the source has no more data to provide (pixelBuffer == nil). Then it breaks out of the loop and returns to the caller. This could involve operations such as applying filters, resizing, etc. Note that this code assumes a single-plane image buffer for video frames in this example. Adjust as needed for your specific use case. 113 end while loop This loop processes data until the source has no more data to provide (pixelBuffer == nil). Then it breaks out of the loop and returns to the caller This could involve operations such as applying filters resizing etc Note that this code assumes a single-plane image buffer for video frames in this example Adjust as needed for your specific use case 114 end while loop This loop processes data until the source has no more data to provide (pixelBuffer == nil). Then it breaks out of the loop and returns to the caller This could involve operations such as applying filters resizing etc Note that this code assumes a single-plane image buffer for video frames