Add Context API

[psewell]

Adds createContext, provide, and consume APIs to Roact.
Components that consume Contexts automatically update when the Context updates, wherever they are in the tree.

Closes #4 .

• Adds createContext function, which is used to make a new Context
• createContext returns a Provider and Consumer.

local ThemeContext = Roact.createContext("default value")

local function UsingValue()
	return Roact.createElement(ThemeContext.Consumer, {
		render = function(theme)
			return Roact.createElement("TextLabel", {
				Text = theme,
			})
		end,
	})
end

local function Root()
	return Roact.createElement(ThemeContext.Provider, {
		value = "overridden value",
	}, {
		Inner = Roact.createElement(UsingValue),
	})
end

Checklist before submitting:

• Added entry to CHANGELOG.md
• Added/updated relevant tests
• Added/updated documentation

[coveralls]

Coverage increased (+0.5%) to 94.162% when pulling dbe4263 on psewell:master into 68e0932 on Roblox:master.

[LPGhatguy]

Looking good!

Left some comments.

We'll also want to port the code to use the new context API defined in #247 once that lands and make sure everything still works.

[LPGhatguy]

Let's check that prevProps.value ~= self.props.value before firing this method. This will let us avoid firing updates when we don't need to.

[LPGhatguy]

We should use a fragment here instead! We added them in 1.0 to improve cases like this, and will let providers have multiple children instead of just one.

[LPGhatguy]

Let's use a regular component here. Because this component is always going to have children, the shouldUpdate check provided by PureComponent is almost always going to fail.

In general, we should prefer function components or Component by default, and fall back to PureComponent with profiling if we have performance issues.

[LPGhatguy]

We should be able to use Roact's validateProps feature here instead of asserting in render!

[LPGhatguy]

Can we be a little bit more explicit with this check, as in

Suggested change

	if self.disconnect then
	if self.disconnect ~= nil then

It's unlikely that disconnect will be false, but we've been trying to encourage that explicitness when the exact type involved is unclear.

[LPGhatguy]

This is kind of a strange __tostring!

Can we return a fixed string here like RoactContext?

[LPGhatguy]

Instead of a PureComponent, this should just be a function component for simplicity.

[LPGhatguy]

We don't need to call unmountVirtualTree. We should limit the scope of what's inside this expect block so that we don't erroneously pass this test when something else throws.

[LPGhatguy]

I think it would be more robust to use Roact's internal createSpy method to create a spy function to use for the render function. This lets us assert how many times it was called, as well as all of the arguments passed.

Notably, this lets us catch errors like:

1. We invoke the function twice and the first one is incorrect
2. We pass extra arguments to the function

[LPGhatguy]

This test can also be rewritten in terms of Roact's spies, which might be more readable.