iOS Camera Development：4K Resolution And High FrameRate Settings

there are two ways to set 4k resolution.

• w1: through the sessionPreset of AVCaptureSession
• w2: through the activeFormat of AVCaptureDevice 

the most important different of them is you can set high frameRate through the activeFormat

1、sessionPreset

A preset value that indicates the quality level or bit rate of the output.

Specify a preset value to configure a capture session’s format and settings. The default preset is high, which produces high-quality video and audio output, but you can specify any preset value that returns true for a call to canSetSessionPreset(_:).

i don't want to talk sessionPreset because i'll need set high frameRate for my app.

2、activeFormat

The currently active media data format of the capture device.

我花了一段时间才找到4k和60fps录制视频的答案。我的设置是iphone，Swift 4和iOS 12.3

• 确保捕获设备设置为AVCaptureDevice.default(for：.video)！不要使用AVCaptureDevice.DiscoverySession，因为它不会返回AVCaptureDevice.Format中的4k和60fps设置。
• 循环通过格式数组查找和设置所需的格式。4k和60fps所需的格式应该如下所示: AVCaptureDeviceFormat: 0x2820d8700‘vide’/‘420 v’。采购产品3840x2160，{ 3-60 fps}

最后一件事是使用AVOutputSettingsAssistant类在AVAssetWriter中配置输出设置字典。输出设置应该是AVOutputSettingsPresetHEVC3840x2160。如果视频设置为AVOutputSettingsPreset3840x2160，则视频输出将不稳定。