[custom-handler] Allow specifying a custom file reader handler

[kensnyder]

No description provided.

[stargazing-dino]

File reading is not my thing but would this line

const blob = file.getAsFile ? file.getAsFile() : file;

be better served before the fileReader callback is checked for and called? That way we can be semi sure a File is returned in the callback instead of an Item?

[kensnyder]

@Nolence Let me update the PR.

[kensnyder]

@Nolence have a look now.

[stargazing-dino]

Hi Ken, first off, thanks for working on this. I used to go to school at the U and was always running about Salt Lake, but it's rare that I meet another Utahn online.

It seems like this line is disregarding a lot of valid cases:

if (!file.getAsFile) {
  // dragged object is not a file
  return;
}

As files themselves don't have that method, only vague DataTransferItems. Here's what I had in mind:

const blob = file.getAsFile ? file.getAsFile() : file;

if (!(blob instanceof Blob)) {
  // dragged object is not a file
  return;
}

// defer to custom file reader handler if specified
if (typeof this.options.fileReader === "function") {
  this.options.fileReader(blob, callback);
  return;
}
// otherwise set up file reader
const reader = new FileReader();
reader.onload = evt => {
  callback(evt.target.result);
};
// read the clipboard item or file
reader.readAsDataURL(blob);

I tested this on FireFox, Chrome and Edge. Edge didn't seem to support drag and drop but that seems like a wholly different PR.

[zachzoup]

Is this going to be merged? Doesn't seem to be any issues but it's been almost three months.