[bp:FileUpload] UploadStarted event

[exyi (Standa Lukeš)]

There isn’t an event for a started upload, as there isn’t much you can do with it server-side. On client, you can monitor the file collection and maybe its properties (IsBusy, Progress) and react on changes. You can do this either by subscribing the specific knockout observable, or subscribing the entire DotVVM state with dotvvm.events.newState and checking if your property of interest has changed:

// option 1: knockout observable subscribe
dotvvm.viewModels.root.viewModel.Files.subscribe(() => {
    // change in files
})

// option 2: new
let knownFiles = new Set()
dotvvm.events.newState.subscribe(state => {
   for (const f of state.Files)
      if (f.IsBusy) // is uploading
          if (!knownFiles.has(f.FileId)) {
              knownFiles.add(f.FileId)
              // whatever you wanted to do
          }
})