Data Module: Add isFulfilled API for advanced resolver use cases

[youknowriad]

In some advanced use cases, the memoization of the resolvers is not enough to avoid similar requests to be triggered multiple times:

• When using complex args (new instance of objects)
• Multiple generators dealing with the same data.

To solve this, I'm introducing an isFulfilled API to the resolvers.

We can now write resolvers like this:

const myResolver: { 
  fulfill: ( state, ...args ) => // do something.
  isFulfilled: ( state, ...args ) => // returns a boolean. 
},

This resolver will be triggered each time we call the selector unless isFulfilled returns true.

Testing instructions

Nothing is changing for now, I'm planning to use this in #5875

[aduth]

Curious whether you have any thoughts on just making this a property of the resolver function. Seems like it's a bit more intuitive to migrate a function to having more advanced isFulfilled behavior, as it's just adding a property instead of reshaping as an object:

function myResolver( state, ...args ) {
	// do something.
}

myResolver.isFulfilled = ( state, ...args ) => {
	// returns a boolean.
};

[aduth]

Typo: "Fullfill" -> "Fulfill"

[aduth]

Personal preference: Since we're only re-shaping this if not already a plain object, seems we'd only need a condition for that, not if/else, avoiding repetition of property access:

let resolver = newResolvers[ key ];
if ( ! isPlainObject( resolver ) ) {
	resolver = { fulfill: resolver };
}

[youknowriad]

Curious whether you have any thoughts on just making this a property of the resolver function.

That was an option I considered and honestly I don't feel strongly either way. I opted for the object approach in anticipation for future potential properties (caching invalidation etc...), I feel the object scales better instead of assigning several keys.

[atimmer]

Looks good.

I think I agree with @youknowriad that the object approach is the way to go. It looks more straightforward.

[aduth]

I wonder if we really need to be opinionated on it, or if we just check for the property whether it's an object or other:

let resolver = newResolvers[ key ];
if ( ! resolver ) {
	return selector;
}

if ( ! resolver.fulfill ) {
	resolver = { fulfill: resolver };
}

I'm fine with as-is too.

[youknowriad]

Update the PR to be less opinionated about it :)

[ocean90]

Edit: Please ignore, I had a typo in fullfill, it's fulfill.

Previous comment

Found this while looking through the code to search exactly for something like this. I guess we should document this somewhere?

Unfortunately it doesn't seem to work with generator functions:

// This works.
export function* getProfile() {
	yield something();
}

// This does not.
export const getProfile = {
	*fullfill() {
		yield something();
	},
	isFulfilled( state ) {
		return ! isEmpty( state.profile.data );
	},
};

This is the error I'm seeing:

Uncaught (in promise) TypeError: resolver.fulfill.apply is not a function
    at _callee2$ (namespace-store.js:348)

Can someone confirm that I'm not doing something wrong?