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I am fumbling around with the free Chrome Dev Editor on my Chromebook. I am trying to use the fileSystem to read and write .txt files. It is all very wrapped up, not at all like in C. I can no more tell if I am even allowed to do something, let alone where the proper place is to find out how.

I think the files I can see using the Files thingy are in the sandbox that I am allowed to play in (meaning, folders that are accessible by the app?) The root is called Downloads. Sure enough, if I use all the dot calls and callback arguments for the read, as in the examples at developer.chrome.com/apps/filesystem, it works. But I have to have a prompt every time for both reads and writes.

A little more Googling came up with this trick: (I think it was here in stackoverflow, in fact) a chrome.runtime call, getPackagedDirectoryEntry, that seems to give me a handle to the folder of my app. Great! That's all I need to not have to go through the prompting. For the readfile, anyway.

But then trying to apply the same trick to the writefile did not work. In fact, it did nothing discernible. No errors, no complaints. Nothing. Even though the write file with prompting works fine (so presumably I have the permissions and Blob construction right.) What to do?

Here is my code:

function test(){
// Samsung 303C Chromebook - Chrome Dev Editor - /Downloads/Daily/main.js

// prompted write

  chrome.fileSystem.chooseEntry({type:'saveFile'},function(a){
    a.createWriter(function(b){
      b.write(new Blob(["Programming fun"],{type:'text/plain'}));
    },function(e){trace.innerText = 'error is ' + e;});
  });

// unprompted read

  chrome.runtime.getPackageDirectoryEntry(function(a){
    a.getFile('text.txt',{},function(b){
      b.file(function(c){
        var d = new FileReader();
        d.onloadend = function(){trace.innerText = this.result;};
        d.readAsText(c);
      });
    });
  });

// unprompted write - why not?

  chrome.runtime.getPackageDirectoryEntry(function(a){
    a.getFile('new.txt',{create:true},function(b){
      b.createWriter(function(c){
        c.write(new Blob(["Miss Manners fan"],{type:'text/plain'}));
      },function(e){trace.innerText = 'error is ' + e;});
    });
  });
}
Xan
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Ian G Coville
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  • Unclear what you mean by "example code" and "does not work". – Xan Aug 26 '15 at 20:47
  • Yes, with sample code (though please format it properly - all lines must start with 4 spaces) it's clear what your problem is. I'll be answering shortly. – Xan Aug 27 '15 at 12:37
  • By the way, your question no longer contains the question, only code. Please edit it properly - I have an answer ready when you do. – Xan Aug 27 '15 at 12:44
  • Thanks a lot for your edit! I cut a bit of extra fluff from the question, upvoted for the effort and answered. – Xan Aug 27 '15 at 20:25

2 Answers2

2

To be fair, Filesystem API is a big mess of callbacks and it's not unreasonable to get drowned in it.

It's not currently documented, but chrome.runtime.getPackageDirectoryEntry returns a read-only DirectoryEntry, and there is no way to make it writable (it's specifically blacklisted).

You probably don't see an error, because it fails at the getFile stage, for which you don't have an error handler.

Unfortunately, for a Chrome App the only option to write out to a real filesystem is to prompt the user. However, you can retain the entry and ask only once.

If you don't need to write out to the real filesystem but need only internal storage, HTML Filesystem API can help you (yes, it's marked as abandoned, but Chrome maintains it since chrome.fileSystem is built on it).

Extensions additionally have access to chrome.downloads API that enables writing to (but not reading) the Downloads folder.

P.S. What you see in Files app is your "real" local filesystem in ChromeOS + mounted cloud filesystems (e.g. Google Drive)

Xan
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  • Thank you so much! You saved me from a lot of testing vague suggestions and reading irrelevant tutorials and incomplete documentation. – Ian G Coville Aug 27 '15 at 21:37
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You can use the basic web Filesystem API. First, add the "unlimitedStorage" permission. Then, copy the packaged files to the sandboxed filesystem, like this:

chrome.runtime.getPackageDirectoryEntry(function(package) {
  package.getMetadata(function(metadata) {
    webkitRequestFileSystem(PERSISTENT, metadata.size, function(filesystem) {
      package.copyTo(filesystem.root)
    })
  })
})
Daniel Herr
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