[Scripting] Try to ensure that the WillPrint/DidPrint respectively DidSave events are always dispatched

[Snuffleupagus]

• [Scripting] Try to ensure that the WillPrint/DidPrint respectively DidSave events are always dispatched

  Note that currently the DidSave event is not guaranteed to actually be dispatched if there's any errors during saving, which is easily fixed by simply moving it to occur in the finally-handler in PDFViewerApplication.save method.

  For the WillPrint/DidPrint events, things are unfortunately more complicated. Currently these events will only be dispatched iff the printing request comes from within the viewer itself (e.g. by the user clicking on the "Print" toolbar button), however printing can be triggered in a few additional ways:

  • In the GENERIC viewer:
    • By the Ctrl+P keyboard shortcut.
  • In the MOZCENTRAL viewer, i.e. the Firefox built-in viewer:
    • By the Ctrl+P keyboard shortcut.
    • By the "Print" item, as found in either the Firefox "Hamburger menu" or in the browser-window menu.

  In either of the cases described above, no WillPrint/DidPrint events will be dispatched. In order to guarantee that things work in the general case, we thus have to move the dispatchEventInSandbox calls to the "beforeprint"/"afterprint" event handlers instead.

• [Scripting] Await manually triggered dispatchEventInSandbox calls in the viewer

  Given that the dispatchEventInSandbox method (on the scripting-classes) is asynchronous, there's a very real risk that the events won't be dispatched/handled until after their associated functionality has actually run (with the "Will..." events being particularily susceptible to this issue).
  To reduce the likelihood of that happening, we can simply await the dispatchEventInSandbox calls as necessary. A couple of methods are now marked as async to support these changes, however that shouldn't be a problem as far as I can tell.

  Please note: Given that the browser "beforeprint"/"afterprint" events are synchronous, we unfortunately cannot await the WillPrint/DidPrint event dispatching. To fix this properly the web-platform would need support for asynchronous printing, and we'll thus have to hope that things work correctly anyway.

/cc @calixteman

[Snuffleupagus]

/botio integrationtest

[pdfjsbot]

From: Bot.io (Windows)

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Received

Command cmd_integrationtest from @Snuffleupagus received. Current queue size: 0

Live output at: http://3.101.106.178:8877/08ff9f0af4470b6/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

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Received

Command cmd_integrationtest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.67.70.0:8877/7b1b5445fe690da/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

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Success

Full output at http://54.67.70.0:8877/7b1b5445fe690da/output.txt

Total script time: 2.63 mins

• Integration Tests: Passed

[pdfjsbot]

From: Bot.io (Windows)

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Success

Full output at http://3.101.106.178:8877/08ff9f0af4470b6/output.txt

Total script time: 3.61 mins

• Integration Tests: Passed

[calixteman]

I added some integration tests here: #12774
to test print/save actions.
Anyway this patch lgtm, thanks a lot for doing this: your help is very useful and valuable.