yelouafi/redux-saga

An alternative side effect model for Redux apps

Christian Maniewski

@chmanie

Currently we’re using an eventChannel to emit certain external events. What I’d like to do is to just take a certain pattern from it. I guess I’d have to do it like this:

const chan = eventChannel(/* do stuff */);
while (true) {
  const event = yield take(channel);
  if (event.type === ’TYPE_THAT_I_WANT’) {
   // Do a certain thing with an event here
  }
}

I thought I might be able to yield take(channel, ‘TYPE_I_WANT’) but that’s just semantics I guess

Tormen Raßmann

@trassmann

apparently you can in 1.0 with the new multicastchannel

Christian Maniewski

@chmanie

Yeah that’s what I thought!

The API is a little different though and I wonder whether the multicast channel is meant for internalizing these external sources (like the eventChannel does)

I guess I’m just confused of when to use which channel

Tormen Raßmann

@trassmann

i've never worked with eventChannels, but if you're connecting outside sources that seems to be the way to go

i guess it depends on your use case

you could also write a wrapper that accepts a pattern from that particular channel

or funnel the events into another multicast channel

Christian Maniewski

@chmanie

Mhmm that sounds like a good idea. I’ll think about that, thanks!

Christian Maniewski

@chmanie

I wonder why it’s not possible to have a buffer and support patterns in channels

Tormen Raßmann

@trassmann

you could check the different take implementations

it might make the restrictions of the different channel types more apparent

or just wait till @Andarist chimes in :P

Mateusz Burzyński

@Andarist

if you want standard take(channel) behaviour (unicast) but you want only certain items to be received from it then just do an if statement after take

or dont put unwanted actions on the channel in the first place (not sure if feasible though)

A Flores

@unflores

@chmanie eventChannels use buffers no?

Note: messages on an eventChannel are not buffered by default. You have to provide a buffer to the eventChannel factory in order to specify buffering strategy for the channel (e.g. eventChannel(subscriber, buffer)). See the API docs for more info

https://github.com/redux-saga/redux-saga/blob/master/docs/advanced/Channels.md

Christian Maniewski

@chmanie

@Andarist if I do an if-statement, I basically have to re-implement the matching against strings (types) and patterns. It’s not much but it would be great if I could re-use that

Tormen Raßmann

@trassmann

@chmanie i get the point though, usually you have strict control over what goes into your custom channel no? assuming that, generic matching logic would be overkill

Christian Maniewski

@chmanie

Yeah, I guess you’re right, I probably won’t be needing anything besides string comparisons

Christian Maniewski

@chmanie

I have another question: I’m not entirely sure around the workings of the try-finally construct when using a while(true). How do we get in the finally clause there?

(third example in the eventChannel: https://github.com/redux-saga/redux-saga/blob/master/docs/advanced/Channels.md#using-the-eventchannel-factory-to-connect-to-external-events)

Tormen Raßmann

@trassmann

from w3c: The finally statement lets you execute code, after try and catch, regardless of the result.

i dont understand the question, do you mean why it triggers?

Christian Maniewski

@chmanie

Yeah, what kicks us out of the while(true)

Oh, ok probably an error is thrown

Tormen Raßmann

@trassmann

when any code wrapped in a try block throws an exception the catch/finally will trigger

Christian Maniewski

@chmanie

So take(END) actually throws an error?

Tormen Raßmann

@trassmann

END just kills all takers on the standard channel

so take(END) is kinda redundant

Christian Maniewski

@chmanie

But then it’s caught internally by redux-saga somehow, I guess

Tormen Raßmann

@trassmann

its contradictary actually

A Flores

@unflores

right but to get to finally there has to be an exception thrown right?

Tormen Raßmann

@trassmann

right but to get to finally there has to be an exception thrown right?

yea

Christian Maniewski

@chmanie

Note also we're executing the whole while (true) {...} loop inside a try/finally block. When the interval terminates, the countdown function closes the event channel by invoking emitter(END).

Tormen Raßmann

@trassmann

But then it’s caught internally by redux-saga somehow, I guess

probably you are getting an exception because take(END) is not meant to be done, thats what take.maybe is for

Christian Maniewski

@chmanie

Yeah, I think I was just confused that there’s no error but redux-saga seems to catch that specific one for me

A Flores

@unflores

er nm what am I saying.

This message was deleted

try{ console.log('derp') } catch(err){console.log(err)}finally{console.log('yerp')}

gives 'derp' 'yerp'

so no need for an exception to be thrown

Christian Maniewski

@chmanie

wait, where’s your while(true) @unflores

A Flores

@unflores

I was confused about the usage of finally, no idea how they drop out of while(true) maybe a break somehow?

Christian Maniewski

@chmanie

An exception is needed to break out of this

Tormen Raßmann

@trassmann

so no need for an exception to be thrown

yea it also executes when the try block finishes executing, but it wont in this case since its an endless loop

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