Tuesday 2 April 2019

Mod player in Rust - part 6. Creating and publishing the crate

The module music player has progressed quite well. It now handles most of the effects and unusual storage types.
I think it is now good enough to create and publish a crate so that anyone can use the mod player functionality. In order to publish a crate there are a couple of areas I need to address;
  • documentation
  • examples
  • creating and publishing the crate

Creating documentation

I like crates that come with good documentation that make it easy to understand what the crate does and explains how to use it. I should try to do the same for my crate.
Rust comes with all the functionality for building good documentation. Typing the following command in the project folder ( the same folder the .toml) builds the html documentation from the source files and opens it in a browser.
cargo doc --open
The doc compiler looks for special comments that start with /// or //! to use for building the documentation. Any lines starting with /// are assumed to document the item ( function, enum, structure etc.) that follows the comment.
So for example the following code adds documentation for the Note structure.
/// Describes what sound sample to play and an effect (if any) that should be applied. 
pub struct Note{
    sample_number: u8,
    period: u32,            // how many clock ticks each sample is held for
    effect: Effect,
In the above example the second comment does not get included in the documentation for two reasons; it uses normal // comment and it is not public. Only public items can be documented. It would make very little sense to have documentation for items that are not visible to the library users.
The other type of comment, //! is documentation for the surrounding module. When I add the following lines to the too of my lib.rs file they get added to documentation's index page.
//! # mod_player
//! mod_player is a crate that reads and plays back mod audio files. The mod_player decodes the audio one sample pair (left,right) at a time
//! that can be streamed to an audio device or a file. 

Documentation examples

Good libraries should make it easy to find working example code. Rust lets you insert tested example code right into the documentation.
Inserting ``` after the /// in the comment section tells Rust's doc compiler to treats it like a code block. So inserting the code following comment
//! To use the library to decode the a mod file and save it to disk
//! ```rust
//! fn main() {
//!     //...
//! } 
produces the following documentation:
To use the library to decode the a mod file and save it to disk
fn main() {
The really cool thing is that the included example code can be fully tested code. So if you ever change the way the library should be used, the tests will catch any outdated documentation examples.
Running these tests could not be simpler. Typing
cargo test
on the command line will instruct cargo to run all the tests in the folder, including the code examples in the documentation.
You can also use rustdoc directly but I found it much harder to get this to work correctly when the examples have external dependencies. For now, I have decided to let cargo handle this for me.
The example compiler does its best to make it easy to create and maintain examples. It will automatically wrap the example code inside a fn main() {} block if one does not exist.


In addition to the examples in the documentation I want to include a couple of more fully fleshed out examples. The examples are placed into the examples folder.
My examples use several dependencies to illustrate how to use the library; I use hound for writing WAV files and cpal for playing to an audio device. The library itself does not care about how the audio is used so I do not want my library to have a dependency on either of these crates. This is where the [dev-dependencies] section in the toml becomes useful.
I have the following section in my toml file
cpal = "0.8.2"
hound = "3.4.0"
This tells Rust that the examples and tests have a dependency on these crates. The crate itself does not have these dependencies and these will not be picked up by any project using the crate.
To run an example I use the cargo command run
cargo run --example streaming_player
The above commmand tells cargo to go into the examples folder and compiled and run the file streaming_player.rs.
The examples replace the various versions of main.rs I used for testing the library. By turning that code into examples I have converted previous throw-away code into a useful part of the library documentation.

Publishing the crate

This too turned out to be fairly easy. First I had to create an account at crates.io. I needed to use my github account to create a crates.io account and grant it access to my github account. It was not terribly clear exactly why the access was needed and what it would be used for. I wish crates.io would have a clearer explanation of this.
I also needed to create an a access token for crates.io that my installed cargo can use to access my crates. This just involved going to my crates.io Account settings and creating a new token buy giving it a name, clicking on create and copying the resulting command line into my local session
cargo login 
my cargo can access crates.io
Now I need to package up my crate. Unsurprisingly, the command line for this is
cargo package
This builds the package, runs all the tests and checks that the .toml file has all the relevant sections filled in correctly.
One of the required fields is license, so I needed to pick the license for my crate. The spdx.org has a long list of standard open source licenses and their identifiers. I picked the MIT license because it is widely used and permits the use of the code in open source and proprietary projects.
Once the package builds it can be published. The command for this is;
cargo publish
This uploads the crate to crates.io and makes the crate visible to all.

Crate documentation

This gets the crate published but the its page looks a bit barren. The only text is the description tag from the .toml file.
Turns out that crates.io does not extract the documentation from the source code comments but expects to find a separate readme.md file. This is a bit of a pain because the top level module documentation would be ideally suited for the crate description.
The crate is available at crates.io and the source code is at github

Next Steps

There are parts of the code that have become quite messy and could do with refactoring. Before I do any such thing I need to makes sure my changes don't break the functionality so I need to add tests.
As I am nor publishing the crate I think I should add options for different playing modes and what to do when encountering unhandled effects

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