Liveview: is_connected macro?

Phoenix Forum Proposals/Suggestions
1

I came across a blog post that implemented a function guard for a liveview socket connection’s presence. It seems like a really ergonomic way to separate out mount functions for connected sockets and disconnected sockets, and I was surprised I wasn’t able to find any existing proposals or suggestions to implement connected? as a macro (very possibly my search fu failing me).

Is there any appetite in implementing this function guard in core liveview? Are there reasons it’s not implemented as a guard yet.

Code referenced in the blog post:

defguard is_connected?(socket) when socket.transport_pid != nil

Enabling:

def mount(params, session, socket) when is_connected?(socket) do
  # Connected mount
end

def mount(params, session, socket) do
  # Disconnected mount
end
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2

One reason this may not exist already - the map.field notation wasn’t supported in guards until this PR, released in 1.11:

Phoenix 1.6 only requires Elixir 1.9+, so Phoenix 1.7 is the earliest version this guard could have been included in.

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3

definetly some new feature I didn’t see yet

4

Glad you liked the article! I have been thinking about making a PR to add it to LiveView. I wanted to get some feedback on it first. I suppose this thread might serve that purpose

I haven’t tested it, but I imagine the map.field syntax could be worked around by matching on the fields in the struct:

defguard connected?(%Socket{transport: transport}) when transport != nil

But that’s just a guess. I wrote that in the forum post editor

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5

Gave it a whirl and it doesn’t look like you can pattern match when defining guards:

** (ArgumentError) invalid syntax in defguard is_connected?(%Socket{transport_pid: transport_pid})

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6

:frowning:

Thanks for giving it a shot though!

I guess that makes sense because guards have to be compile time constants and pattern matching is not

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