ability to specify a type for derived properties

[flipside]

By specifying a dataType for derived properties, a derived property with type:"state" can dynamically have listeners updated when it returns a different state.

Derived types also allow for custom behavior by adding new dataTypes.

[latentflip]

Hey @flipside, thanks for submitting this! I think I'm heading towards +1 on this idea. I've a couple of thoughts though - forgive me if I'm being slow.

First up: what does init: true do here? Do we need it?

[aaronmccall]

@latentflip, by my read it looks like init: true is intended to ensure that the derived prop is populated to the cache at instantiation rather than being lazy.

[flipside]

@aaronmccall @latentflip i wasn't sure if it should always automatically set a derived property on load if a type is set so I made them separate params. i'm open for init to default to true when a type is specified but i can imagine situations where it would be important to override.

[flipside]

the other thing worth discussing is that since this approaches uses dataType, should it go all the way with using get/set/default and not just compare?

[flipside]

adding support for default and the rest of the dataType functions turned out to be easy and didn't break tests so i added them.

[wraithgar]

Any reason we're getting rid of flushCache?

[latentflip]

I'm guessing cos it's not used. Doesn't appear to be being used by any other modules as per a github search. What do you think about removing it @HenrikJoreteg? Can you remember if it's useful/needed?

[HenrikJoreteg]

I know at one point or another i wanted to be able to do this forcibly. But can't remember why. If it's not being used and everything passes, then we're probably good.

[flipside]

one debatable part is supporting default. removing it from this update would simplify things a bit.