Parameterize ProgressEvent

[saschanaz]

@sandersn, how about this?

[sandersn]

Is this useful anywhere else? I expected that lots of other event types would want to have custom targets?

[saschanaz]

Might be better to start rather than doing all in once. IMO this will at least prevent a future breaking change "Where is my FileReaderProgressEvent" as we already saw one.

[sandersn]

If this is a start that will have some follow-up, I want to understand what the follow-up will be. Are there other subtypes of ProgressEvent that will provide different arguments to T?

If the only subtype of ProgressEvent is FileReader, then I think it's too expensive and complex to create a generic type for just one case.

[saschanaz]

Are there other subtypes of ProgressEvent

Current users of ProgressEvent in lib.d.ts include ApplicationCache, Document, GlobalEventHandlers, and XMLHttpRequest,

[sandersn]

How would ApplicationCache customise ProgressEvent.target? Would T=ApplicationCache ?

[saschanaz]

Yep, and T=XMLHttpRequest.

This can be generalized for nearly all Event sub-interfaces, so we even could do some automation in the future rather than manual generic field. (...while generics are known to be heavy.)

[sandersn]

OK, that makes sense. Can you make the change for ApplicationCache, Document, GlobalEventHandlers, and XMLHttpRequest in this PR?

[sandersn]

One naming request and one question.