ios: Use H264 to send screen sharing frames from extension to main app #1745
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Screen share on iOS gets captured at Broadcast Extension (that is implemented by the library consumer) and then is sent to the main app via unix socket. However, previously it was not very efficient mainly for 2 reasons: 1. The extension was sending the frames as JPEG images. Encoding to JPEG is relatively expensive and cannot output 60fps (which as of now is the maximum provided by the ReplayKit). 2. Those frames were wrapped in HTTP message. Receiving HTTP message chunk by chunk requires reallocating memory each time, making it inefficient. To adress these bottlenecks, H264 video codec is now used to send the frames from the extension to the main app. H264 is hardware-accelerated on all the iOS devices, and is designed for the video, so is very efficient. Decoding is done with the help of [Transcoding](https://github.com/finnvoor/Transcoding/) library. Since that library is only available as SPM package, its source code was vendored into the project. The library is also written in Swift, which means the part of the project interacting with it (ScreenCapturer class) had to be written in Swift as well. Messages are now sent in binary format, with prefixed length, meaning there is no need to reallocate memory for each message chunk as the length is already known.
Since Transcoding library requires iOS 15, we need to bump the target iOS version to 15, otherwise it won't compile.
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Newer version of Xcode (16) is not compatible with CocoaPods. It broke when a broadcast extension target was added to the project with the following error: > RuntimeError - PBXGroup attempted to initialize an object with unknown ISA > PBXFileSystemSynchronizedRootGroup from attributes: Fix was found here: CocoaPods/CocoaPods#12583 (comment)
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Screen share on iOS gets captured at Broadcast Extension (that is implemented by the library consumer) and then is sent to the main app via unix socket. However, previously it was not very efficient mainly for 2 reasons:
To adress these bottlenecks, H264 video codec is now used to send the frames from the extension to the main app. H264 is hardware-accelerated on all the iOS devices, and is designed for the video, so is very efficient. Decoding is done with the help of Transcoding library. Since that library is only available as SPM package, its source code was vendored into the project. The library is also written in Swift, which means the part of the project interacting with it (ScreenCapturer class) had to be written in Swift as well.
Messages are now sent in binary format, with prefixed length, meaning there is no need to reallocate memory for each message chunk as the length is already known.