The Open Screen Library implements the Open Screen Protocol, Multicast DNS and DNS-SD, and the Cast protocols (discovery, application control, and media streaming).
The library consists of feature modules that share a common platform API that must be implemented and linked by the embedding application.
The major feature modules in the library can be used independently and have their own documentation:
Library dependencies are managed using gclient, from the depot_tools repo.
To get gclient, run the following command in your terminal:
git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
Then add the depot_tools folder to your PATH environment variable.
Note that openscreen does not use other features of depot_tools like repo or drover. However, some git-cl functions do work, like git cl try, git cl format, git cl lint, and git cl upload.
From the parent directory of where you want the openscreen checkout (e.g., ~/my_project_dir), check out openscreen with the following command:
fetch openscreen
This command will create an openscreen/ subdirectory, downloads the source code, all third-party dependencies, and the toolchain needed to build things; and at their appropriate revisions.
To update your local checkout from the openscreen reference repository, just run
cd ~/my_project_dir/openscreen git pull gclient sync
This will rebase any local commits on the remote top-of-tree, and update any dependencies that have changed.
The main tools required for development are listed below. The gclient tool, which you installed as part of “Getting the code,” will automatically download and install a pre-built toolchain that includes gn, clang-format, ninja, and clang.
gn (in buildtools/)clang-format (in buildtools/)ninjaclangBuilding the library and its executables requires certain system-specific dependencies (such as libstdc++ on Linux or XCode on macOS). For certain targets, like the standalone cast_sender and cast_receiver, additional external media libraries (e.g., FFmpeg, SDL2) are also required.
For a comprehensive guide on setting up these system-specific and external library dependencies for Linux and macOS, please refer to:
→ cast/docs/external_libraries.md
Setting the gn argument is_debug=true enables debug build.
gn gen out/debug --args="is_debug=true"
Running gn args opens an editor that allows to create a list of arguments passed to every invocation of gn gen. gn args --list will list all of the possible arguments you can set.
gn args out/debug
In order for Language Server Protocol (LSP) tools like clangd to properly interpret the codebase, they need access to a compile_commands.json file.
This file can be generated by GN by using the following command:
gn gen out/<Default> --add-export-compile-commands="*"
We use the Open Screen Protocol demo application as an example, however, the instructions are essentially the same for all executable targets.
mkdir out/debug gn gen out/debug # Creates the build directory and necessary ninja files ninja -C out/debug osp_demo # Builds the executable with ninja ./out/debug/osp_demo # Runs the executable
The -C argument to ninja works just like it does for GNU Make: it specifies the working directory for the build. So the same could be done as follows:
./gn gen out/debug cd out/debug ninja osp_demo ./osp_demo
After editing a file, only ninja needs to be rerun, not gn. If you have edited a BUILD.gn file, ninja will re-run gn for you.
We recommend using autoninja instead of ninja, which takes the same command-line arguments but automatically parallelizes the build for your system, depending on number of processor cores, amount of RAM, etc.
Also, while specifying build targets is possible while using ninja, typically for development it is sufficient to just build everything, especially since the Open Screen repository is still quite small. That makes the invocation to the build system simplify to:
autoninja -C out/debug
For details on running osp_demo, see its README.md.
Running ninja -C out/debug gn_all will build all non-test targets in the repository.
gn ls --type=executable out/debug will list all of the executable targets that can be built.
If you want to customize the build further, you can run gn args out/debug to pull up an editor for build flags. gn args --list out/debug prints all of the build flags available.
ninja -C out/debug openscreen_unittests ./out/debug/openscreen_unittests
Open Screen library code should follow the Open Screen Library Style Guide.
This library uses Chromium Gerrit for patch management and code review (for better or worse). You will need to register for an account at chromium-review.googlesource.com to upload patches for review.
The following sections contain some tips about dealing with Gerrit for code reviews, specifically when pushing patches for review, getting patches reviewed, and committing patches.
The git cl tool handles details of interacting with Gerrit (the Chromium code review tool) and is recommended for pushing patches for review. Once you have committed changes locally, simply run:
git cl format
git cl upload
The first command will will auto-format the code changes using clang-format. Then, the second command runs the PRESUBMIT.py script to check style and, if it passes, a newcode review will be posted on chromium-review.googlesource.com.
If you make additional commits to your local branch, then running git cl upload again in the same branch will merge those commits into the ongoing review as a new patchset.
It's simplest to create a local git branch for each patch you want reviewed separately. git cl keeps track of review status separately for each local branch.
If conflicting commits have been landed in the repository for a patch in review, Gerrit will flag the patch as having a merge conflict. In that case, use the instructions above to rebase your commits on top-of-tree and upload a new patchset with the merge conflicts resolved.
Clicking the Cq Dry Run button (also, confusingly, labeled Commit Queue +1) will run the current patchset through all LUCI builders and report the results. It is always a good idea get a green tryjob on a patch before sending it for review to avoid extra back-and-forth.
You can also run git cl try from the commandline to submit a tryjob.
Send your patch to one or more owners from the OWNERS file for code review. All patches must receive at least one LGTM by a committer and endorsement by two Google employees to pass the Review-Enforcement check before it can be submitted.
After your patch has received one or more LGTMs, commit it by clicking the Submit button (or, confusingly, Commit Queue +2) in Gerrit. This will run your patch through the builders again before committing to the main openscreen repository.