screen_cast/ios/Receiver/Decode/RenderSink.swift

import AVFoundation
import CoreMedia

/// A render target for decoded frames. The decoder enqueues a `CMSampleBuffer`
/// (wrapping a CVPixelBuffer) per frame; the sink renders it. Mirrors the role
/// of the Android `Surface` + MediaCodec surface-mode rendering.
protocol RenderSink: AnyObject {
    var isAttached: Bool { get }
    func attach()
    func detach()
    /// Updates the layer's video format description (called when the in-band
    /// SPS/PPS establish a (new) stream size).
    func setFormatDescription(_ formatDescription: CMVideoFormatDescription)
    /// Enqueues one decoded frame for display.
    func enqueue(_ sampleBuffer: CMSampleBuffer)
}

/// Renders decoded CVPixelBuffers via `AVSampleBufferDisplayLayer`, which does
/// the video-scaling for us. `.resizeAspect` gives letterbox directly, so the
/// host view can stay full-screen without a manual transform (the same lesson
/// as the Android TextureView sizing: size the surface to the aspect, not a
/// transform matrix).
final class AVSampleBufferRenderSink: RenderSink {
    private let layer: AVSampleBufferDisplayLayer
    private var session: AVSampleBufferDisplayLayerSession?
    private let lock = NSLock()
    private(set) var isAttached = false

    init(layer: AVSampleBufferDisplayLayer) {
        self.layer = layer
    }

    func attach() {
        lock.lock()
        defer { lock.unlock() }
        guard session == nil else { return }
        let s = AVSampleBufferDisplayLayerSession(layer)
        s.start()
        session = s
        isAttached = true
    }

    func detach() {
        lock.lock()
        defer { lock.unlock() }
        session?.stop()
        session = nil
        isAttached = false
    }

    func setFormatDescription(_ formatDescription: CMVideoFormatDescription) {
        layer.formatDescription = formatDescription
    }

    func enqueue(_ sampleBuffer: CMSampleBuffer) {
        lock.lock()
        let s = session
        lock.unlock()
        s?.enqueue(sampleBuffer)
    }
}