screen_cast/ios/Receiver/Decode/FormatDescription.swift

import CoreMedia
import Foundation

/// Builds a `CMVideoFormatDescription` for H.264 from in-band SPS + PPS.
///
/// VideoToolbox needs the parameter sets up front; the sender repeats them
/// before every keyframe, so any keyframe carries a complete set. This is the
/// Core Foundation recipe for an H.264 "config" format description.
enum H264FormatDescription {
    static func create(sps: [UInt8], pps: [UInt8]) -> CMVideoFormatDescription? {
        let pointersArray = CFArrayCreateMutable(kCFAllocatorDefault, 0, &kCFTypeArrayCallBacks)
        for ps in [sps, pps] {
            guard let descriptor = makeParameterSetDescriptor(ps) else {
                CFRelease(pointersArray)
                return nil
            }
            CFArrayAppendValue(pointersArray, descriptor.takeUnretainedValue())
            CFRelease(descriptor) // the array now owns it
        }

        let key = kCMFormatDescriptionExtension_SampleDescriptionPointers as CFString
        let attrs = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, &kCFDictionaryKeyCallBacks, &kCFDictionaryValueCallBacks)
        CFDictionarySetValue(attrs, key, pointersArray)

        var config: Unmanaged<CMVideoFormatDescription>?
        let status = CMVideoFormatDescriptionCreate(
            kCFAllocatorDefault,
            kCMVideoCodecType_H264,
            0, 0, 0,
            attrs,
            &config)
        CFRelease(attrs)
        CFRelease(pointersArray)
        guard status == noErr, let c = config else { return nil }
        return c.takeRetainedValue()
    }

    private static func makeParameterSetDescriptor(_ ps: [UInt8]) -> Unmanaged<CMVideoFormatDescription>? {
        let cfData = Data(ps) as CFData
        let oneElement = CFArrayCreateMutable(kCFAllocatorDefault, 0, &kCFTypeArrayCallBacks)
        CFArrayAppendValue(oneElement, cfData)
        let key = kCMFormatDescriptionExtension_SampleDescriptionPointers as CFString
        let attrs = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, &kCFDictionaryKeyCallBacks, &kCFDictionaryValueCallBacks)
        CFDictionarySetValue(attrs, key, oneElement)

        var desc: Unmanaged<CMVideoFormatDescription>?
        let status = CMVideoFormatDescriptionCreateForCodecType(
            kCFAllocatorDefault,
            kCMVideoCodecType_H264,
            attrs,
            &desc)
        CFRelease(oneElement)
        CFRelease(attrs)
        guard status == noErr, let d = desc else { return nil }
        return d
    }
}