How to create a custom channel (and use yarn mapping)?

I saw discussion in https://github.com/MinecraftForge/ForgeGradle/pull/796 about creating custom channel.

So now I want to use yarn mapping instead of (not clearly licensed) official mapping, what should I do?

I tried this code:

channelProviders.addProvider(new ChannelProvider() {
    Set<String> getChannels() {
        println "channels"
        return ImmutableSet.of('yarn')
    }

    File getMappingsFile(MCPRepo mcpRepo, Project project, String channel, String version) {
        String desc = "net.fabricmc:yarn:" + version + ":v2@zip";
        println 'get' + desc
        return MavenArtifactDownloader.manual(project, desc, false);
    }
})

but it didn't work. It seems that `channelProviders` get reset after I changed it. error says:

Error getting artifact: net.minecraft:mappings_yarn:1.0.1:null@zip from  MCPRepo
java.lang.IllegalArgumentException: Unknown mapping provider: yarn_1, currently loaded: [stable_nodoc, snapshot_nodoc, stable, official, snapshot]

I would expect this yarn project has different syntax from mcp, but in that case I can write another program to fix that.