Add events to the viewer API.

Based on https://github.com/hypothesis/h/blob/document-pdfjs/docs/internals/pdfjs.rst:

• Need an event when document loaded – documentloaded
• Need an event when metadata is loaded – metadataloaded
• Need an event when page is cleared (removed from the cache) – pageremoved

Also add state properties to indicate that those events were already triggered.

/cc @csillag

[moll]

While there’s now an event bus, do I read the viewer code correctly when I say that there’s no way to know from the outside (e.g. an <iframe> referring to the viewer.html) when PDFViewerApplication itself has finished initializing? Before it finishes there’s no eventBus property to at all…

[Snuffleupagus]

Based on https://github.com/hypothesis/h/blob/document-pdfjs/docs/internals/pdfjs.rst:

That document is no longer available, and hence the use-cases for having the default viewer dispatch new events which are used nowhere in the code-base are difficult to justify at this point (in my opinion).

Need an event when document loaded – documentloaded

There’s already both documentinit and documentloaded events dispatched on the eventBus, representing when the document is ready for user-interaction respectively when the entire document has been loaded.

Need an event when metadata is loaded – metadataloaded

This seems like a fairly specialized use-case, and it could probably be achieved by simply waiting for the documentinit event and then manually access the metadata through e.g. the PDFViewerApplication.pdfDocument.getMetadata() method instead.

Need an event when page is cleared (removed from the cache) – pageremoved

There’s no real substitute for such an event (which based on the “removed from the cache” part probably ought to be placed in PDFPageView.destroy, if it were ever implemented).

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Given the age of this issue, and that the use-cases might be “special”, should this be closed as WONTFIX at this point?