Saving files in a Chrome App

Summary

Normally I could download a bunch of files, but Chrome Apps won't show the download shelf when a download occurs. What would be the best way of getting around this limitation of Chrome Apps?

Ideas

• I could go about this by creating a zip file, but this would require the user to perform an extra step of unzipping the file.
• I'm able to silently download the files, and so I could display a prompt to the user when the file is downloaded, but this would require the user to manually search for the file in their downloads folder.

What I've Learned

• Everywhere on the internet tells me to use Chrome's download API, but this only works for Chrome extensions and not Chrome apps.
• I can't bring up a save as window because 50 save as windows for 50 files is unacceptable
• I can, however, bring up a prompt using chrome.fileSystem.chooseEntry({'type': "openDirectory"} to ask the user to choose a directory, but I can't find a way of saving to that directory.
• My question is basically the same as How can a Chrome extension save many files to a user-specified directory? but for a Chrome app instead of an extension.

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Project and Example Code

The app I'm building will be the same as this webpage I've built, but with a few modifications to make it work as a web-app.

This is how my website solves the problem

let example_pic = "data:image/png;base64,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"
let a = document.createElement("a");
a.href = example_pic;

document.body.appendChild(a)
a.click();

window.URL.revokeObjectURL(a.href);
a.remove()

I can, however, bring up a prompt using chrome.fileSystem.chooseEntry({'type': "openDirectory"}) to ask the user to choose a directory, but I can't find a way of saving to that directory.

That's what you need to work on.

Suppose you declare all the sub-permissions for the fileSystem API:

"permissions": [
   {"fileSystem": ["write", "retainEntries", "directory"]}
]

Then you can:

1. Get an entry from the user:

   chrome.fileSystem.chooseEntry({'type': "openDirectory"}, function(dirEntry) {
     // Check for chrome.runtime.lastError, then use dirEntry
   });
   

2. Retain it, so you can reuse it later without asking the user again:

   dirEntryId = chrome.fileSystem.retainEntry(dirEntry);
   
   // Use chrome.storage to save/retrieve it
   
   chrome.fileSystem.restoreEntry(dirEntryId, function(entry) { /* ... */ });
   

3. Using the HTML FileSystem API, create files in the directory:

   dirEntry.getFile(
     "test.txt",
     {create: true}, // add "exclusive: true" to prevent overwrite
     function(fileEntry) { /* write here */ },
     function(e) { console.error(e) }
   );