add a .then() method to the module-creating function in MODULARIZE

[kripken]

This lets it be used as a promise, and works in both sync and async mode (we need async for wasm).

In modularize mode, the entire output is a function Module() (or another name, set by EXPORT_NAME). This is useful for libraries, so they don't influence the global scope. Til now most libraries, like box2d.js and ammo.js, assumed synchronous startup, which was made possible by not having a mem init file (which was convenient anyhow as for libraries having extra files is less good). But for wasm it looks like we can't depend on sync compilation.

This PR adds a then() method, usable as

Module().then(function(Module) { .. });

Basically like a promise. The callback is called when the module is all compiled and ready to be used. It has the module as a parameter.

This is also perhaps a nicer API than the other methods for finding out when it is possible to call compiled code (e.g., onRuntimeInitialized), so in the long term, maybe we'd want to recommend people use modularize more, and this with it.

The wasm branch of ammo.js uses this. Looks like it works ok.

[bvibber]

Looks good to me!

I ran into same problem when adding wasm mode to ogv.js; this looks like a handier solution than the manual use of onRuntimeInitialized I initially used, though I'll have to retool some of my initialization code away from the 'init function looks like a constructor' pattern to 'init function looks like a Promise' pattern.

[bvibber]

(Would it make sense to also have a .catch() that calls if we never initialize due to an error?)

[kripken]

catch is a little trickier. We'd need to add a try-catch in a bunch of places - basically anywhere that can throw during startup, and their async callbacks. Or we could use window.error perhaps, but that could have weird interactions if the rest of the page uses it?

If all the rest of our startup were Promises that had catches, this might be simple, and maybe that's worth working towards eventually.

[kripken]

Tested also on box2d.js, in the wasm branch there. Looks good there too.

[AlexPerrot]

Shouldn't old be called before func ?
I mean, in Module().then(f1).then(f2), I want f1 to always be called before f2.

[kripken]

Makes sense, I reordered it, thanks.

[drola]

Working towards a .catch() is preferable because:

1. then() hints at usage of promises and error handling using promise rejection is consistent with that pattern.
2. Encapsulating all possible error conditions and exposing them as a promise rejection will simplify usage of modules
3. It's a general principle to avoid using globals as much as possible. We should avoid using window.error. These modules will eventually be packed together with other frameworks and packaging tools such as Webpack or dependency injection frameworks.

[kripken]

Agreed, yeah, we should work towards a catch as a followup. (Perhaps we can refactor all the code that runs after it into a scope and use a normal Promise, or something like that? I don't know offhand if Promises are that easy to build on.)

Merging this for now as it's an improvement in the right direction, and already used by ammo.js and box2d.js successfully.