stream.lua

-- WirePlumber script to handle inputs based on processing program state. This allows me to use
-- my virtual microphone in all programs and have it still work with or without REAPER open.
-- There is almost definitely a much more sane way of doing this, but I'm neither an audio
-- expert nor a Lua expert.
-- 
-- The three nodes we care about are Scarlett (hardware microphone), REAPER (processor), and Voice (virtual microphone)
--
-- When REAPER is open:
--   Scarlett->REAPER->Voice
--
-- When REAPER is not open:
--   Scarlett->Voice
--   
-- Object Managers for the relevant nodes
-- REAPER
reaper_om = ObjectManager {
    Interest {
        type = 'node',
        Constraint { 'node.name', 'equals', 'REAPER' },
    },
}

-- Focusrite Scarlett Solo
scarlett_om = ObjectManager {
    Interest {
        type = 'node',
        Constraint { 'node.name', 'equals', 'alsa_input.usb-Focusrite_Scarlett_Solo_USB_Y7YRMX017CBC1D-00.analog-stereo' },
    },
}

scarlett_fr_om = ObjectManager {
    Interest {
        type = 'port',
        Constraint { 'port.alias', 'equals', 'Scarlett Solo USB:capture_FR', },
    },
}

-- Virtual microphone
voice_om = ObjectManager {
    Interest {
        type = 'node',
        Constraint { 'node.name', 'equals', 'stream.voice' },
    },
}

-- Desktop virtual output
desktop_om = ObjectManager {
    Interest {
        type = 'node',
        Constraint { 'node.name', 'equals', 'stream.desktop.in' },
    },
}

-- Link object manager
link_om = ObjectManager {
    Interest {
        type = 'link',
    },
}

-- Links table so they don't get garbage collected
links = {}

-- Port name : Port map
ports = {}

-- Port (OUT->IN) mappings for REAPER inactive/active
default_port_map = {
    ['Scarlett.capture_FL']={ 'Voice.playback_FL', 'Voice.playback_FR' },
}
active_port_map = {
    ['Scarlett.capture_FL']={ 'REAPER.in1', 'REAPER.in2' },
    ['REAPER.out1']={ 'Voice.playback_FL' },
    ['REAPER.out2']={ 'Voice.playback_FR' },
}

-- Connect signals
reaper_om:connect('object-added', function(om, node)
    Log.message('REAPER connected')
    establish_port('REAPER', node, true)
    reset_links()
end)

scarlett_om:connect('object-added', function(om, node)
    Log.message('Scarlett connected')
    establish_port('Scarlett', node)
end)

voice_om:connect('object-added', function(om, node)
    Log.message('Voice connected')
    establish_port('Voice', node)
end)

-- Reset links when REAPER disconnects
reaper_om:connect('object-removed', function(om, node)
    reset_links()
end)

-- Reset links to default state
function reset_links()
    for pout, pins in pairs(default_port_map) do
        for _, pin in ipairs(pins) do
            if ports[pout] and ports[pin] then
                if reaper_om:lookup() then
                    try_destroy_link(ports[pout], ports[pin])
                else
                    try_create_link(ports[pout], ports[pin])
                end
            end
        end
    end
end

-- Establish port connection rules for a given node
function establish_port(prefix, node)
    port_om = ObjectManager {
        Interest {
            type = 'port',
            Constraint { 'node.id', 'equals', node.properties['object.id'] },
        },
    }
    -- Create port ObjectManager for associated ports in order to make connections
    port_om:connect('object-added', function(om, port)
        local key = add_port_to_map(prefix, port)
        local pin, pouts = get_ports(key)
        if not pin or not pouts then return end
        for _, pout in ipairs(pouts) do
            try_create_link(pin, pout)
        end
    end)
    port_om:activate()
end

-- Adds port to port map keyed by given prefix
function add_port_to_map(prefix, port)
    ports[prefix .. '.' .. port.properties['port.name']] = port
    return prefix .. '.' .. port.properties['port.name']
end

-- Get ports associated with a key, return them in (in, out) order.
-- This could probably just be handled with port.direction instead.
function get_ports(key)
    local port_map
    -- Determine which port map to use based on REAPER active/inactive
    if reaper_om:lookup() then
        port_map = active_port_map
    else
        port_map = default_port_map
    end
    -- Key is on output side
    if port_map[key] then
        local ps = {}
        for _, p in ipairs(port_map[key]) do
            table.insert(ps, ports[p])
        end
        return ports[key], ps
    -- Key is on input side
    else
        for pin, pouts in pairs(port_map) do
            for _, p in ipairs(pouts) do
                if p == key then
                    return ports[pin], { ports[p] }
                end
            end
        end
    end
end

-- Create link from port_out->port_in
function try_create_link(port_out, port_in)
    link = Link("link-factory", {
        ["link.output.port"] = port_out['bound-id'],
        ["link.input.port"] = port_in['bound-id'],
    })
    link:activate(Features.ALL)
    local key = tostring(port_out['bound-id']) .. '.' .. tostring(port_in['bound-id'])
    links[key] = link
end

-- Destroy link from port_out->port_in
function try_destroy_link(port_out, port_in)
    local key = tostring(port_out['bound-id']) .. '.' .. tostring(port_in['bound-id'])
    local link = links[key]
    if link then
        link:request_destroy()
    end
end

-- Automatically remove REAPER->Desktop virtual output and Scarlett Right-> connections
-- There is probably a better way to do this.
link_om:connect('object-added', function(om, link)
    local reaper = reaper_om:lookup()
    local desktop = desktop_om:lookup()
    local scarlett = scarlett_fr_om:lookup()
    if desktop and reaper then
        if link.properties['link.output.node'] == tostring(reaper['bound-id']) and link.properties['link.input.node'] == tostring(desktop['bound-id']) then
            link:request_destroy()
        end
    end
    if scarlett then
        if link.properties['link.output.port'] == tostring(scarlett['bound-id']) then
            link:request_destroy()
        end
    end
end)

reaper_om:activate()
scarlett_om:activate()
scarlett_fr_om:activate()
voice_om:activate()
desktop_om:activate()
link_om:activate()

I also wanted to thank you for posting this, I don't know why replicating what you can do with a few commands of "pw-link" is so difficult to translate to Wireplumber. I was able to modify it to my needs but you weren't joking about it being a "cursed" script, I really have no idea how this is working and where the documentation to do something like this was.

I'm glad folks found this helpful. This is still basically what I'm using on Linux just with some different devices now. It's possible there is a more straightforward way to accomplish this sort of thing now but I actually have no idea. If anyone does find something simpler I would appreciate hearing about it, though!