0 15 asyncwebserver 2.1.0 features

[willmmiles]

This PR leverages the new features in the proposed AsyncWebServerWLED 2.1.0 build. In particular:

• AsyncWebServerWLED 2.1.0 exposes its PROGMEM ContentType constants; use them where appropriate
• AsyncWebServerWLED 2.1.0 accepts Strings for server.on() requests, so we don't have to cast PROGMEM to const char * and rely on the alignment fault handler to correctly construct;
• AsyncWebServerWLED 2.1.0 introduces a SharedBuffer type that can be explicitly constructed and moved without the extra heap allocation and indirection of AsyncWebSocketMessageBuffer. This eliminates the deprecation warnings for that type.

[willmmiles]

@blazoncek You were asking about the warning fixes - these are the relevant patches.

[blazoncek]

Thanks.

So basically it is

AsyncWebSocketSharedBuffer newBuff(size);
...
ws->text(std::move(newBuff));

If I understand this (new to me) C++ syntax it says "use move constructor" when calling text() method? Is this correct?

[willmmiles]

Correct. The move operation is intended to make it clear that the buffer is being "given" to the web server code to manage. Moves like this are meant to communicate the ownership semantics to a code reader, without requiring a reader to know a priori that some random function is expecting to take responsibility for future deallocation. (And all this with no - or at least minimal - runtime cost.)

...though as I write this, I realize there's probably still an API design weakness there; the accepting function should probably insist on an unshared buffer (and internally promote it to shared if necessary) to be clear that it's not OK to write to the buffer after passing it. Next iteration!

[willmmiles]

Superseded by #3828