API to get network and WiFi interfaces w/o connecting

[JanneKiiskila]

These might be useful, if you want to for example scan the WiFi
for access points BEFORE connecting.

Any device with UI/keyboard or some external input method would
likely use that, rather than have some hard-coded value.

[JanneKiiskila]

This would enable then mbed-os-example-wifi to use the easy-connect as well.... :-)
(Changes done already in my work area).

[kjbracey]

Yes, makes sense. If mbed OS had something equivalent, it would be a "get default interface" call, not "connect default interface". Having "get specific type" is a good idea - makes sense.

It would be useful for the default interface returned from the generic call to be preconfigured with set_credentials if possible such that connect() with no parameters did something sensible. (As connect with no parameters is the only thing you can do on a generic NetworkInterface).

I wonder now, should we have a "get_type" call on NetworkInterface? RTTI is disabled in the compilers, so there's no other way to check if the default interface is actually wifi, as opposed to asking specifically for a wifi interface. Maybe a bit late...

[SeppoTakalo]

By printing error messages, instead of just returning NULL, you are forcing inclusion of whole C stdio libraries.
Most of the applications use them anyway, but there might be cases where we wan't to really minimise the flash usage.

[janjongboom]

Yes, please use debug() from mbed_debug.h. Then stdio will not be included in release build.

[JanneKiiskila]

Well, it's behind the boolean flag - if those are false the compiler should optimize it all out. Should, I know they fail sometimes. I'll try it out.

[kjbracey]

I said:

I wonder now, should we have a "get_type" call on NetworkInterface?

I am actually going to add this - turns out I'm going to need it. It will allow code like this:

NetworkInterface *default_if = get_default_interface();
WiFiInterface *wifi = default_if->wifiInterface(); 
if (wifi) {
     wifi->set_credentials(...)
}

[JanneKiiskila]

@kjbracey-arm - I was too fast? Anyways, feel free to make another PR and for sure we'll merge it in.
How would you know though if you get_default_interface, if it's a wifi or not? How can you know you can validly put it as wifi * ptr? Should there be some kind of "ask type" method and it would return from one enum?

[kjbracey]

No, your stuff is fine. What I'm working on was for the future EMAC work, over at ARMmbed/mbed-os#5558 - commit ARMmbed/mbed-os@d32c56d . But we could maybe accelerate that specific "check type" change if it's useful to you - the API could be added now on its own for 5.8.

The call works like dynamic_cast - you only get a WiFiInterface pointer if it actually is one. If it isn't, you get NULL. So it also functions as a "check type" call.

I think that's probably sufficient and you could live without a "type-id" enum. Only significant draw-back is that you wouldn't be able to neatly switch on the type-id, you'd have to have a chain of "if (wifi), if (cellular)".