getCustomTransformers are not being used when running with dev server

[Igorbek]

I'm using options.getCustomTransformers to supply custom transformations to the TypeScript emitter.
It looks like they only work when webpack creates static bundles.
The method doesn't get called at all, when dev-server is running with HMR.
Is anything can be done in order to support customer transformers in that mode?

[s-panferov]

In "watch" mode our type-checking happens in another process and we need to find some way to pass transformers to that process.

[Igorbek]

Do you use emit api in that mode? Is that where the transpiling in the watch mode happening: https://github.com/s-panferov/awesome-typescript-loader/blob/master/src/checker/runtime.ts#L385 ?

[Igorbek]

If so, looks like ts.transpileModule is pretty simple (https://github.com/Microsoft/TypeScript/blob/master/src/services/transpile.ts#L26-L112) and creates a compiler host internally. So that it would be a good optimization to do this work manually.

Other option, which obviously is easier, is to pass transformers?: CustomTransformers as part of transpileOptions.

[guncha]

I really needed this to work so I made the necessary changes. Basically what you suggested, @Igorbek. @s-panferov if you're interested, I'm happy to write specs, document and open a proper PR.

master...guncha:fix-custom-transformers

[Igorbek]

That's great @guncha. That would be great if you open a PR. I have a suggestion though, that I raised here #423 (comment), but it can be discussed later in your PR.