[whatwg] Drag-and-drop folders/files support with directory structure using DirectoryEntry

[Glenn Maynard]

On Wed, Nov 16, 2011 at 5:33 PM, Daniel Cheng <dcheng at chromium.org> wrote:

> I'm trying to better understand the use case for DataTransfer.entries.
> Using the example you listed in your first post, if I dragged those folders
> into a browser, I'd expect to see File objects with the following names in
> DataTransfer.files:
>     trip/1.jpg
>     trip/2.jpg
>     trip/3.jpg
>     halloween/a.jpg
>     halloween/b.jpg
>     tokyo/1.jpg
>     tokyo/2.jpg
> It seems like with that, a web app could implement a progress meter and
> handle subdirectories easily while using workers. What does the FileSystem
> API provide on top of that?
>

The issue isn't when you have seven files; it's when you have seven
thousand.  File trees can be very large.  In order to implement the above
API, you need to traverse the entire tree in advance to discover what files
exist.  The DirectoryEntry API lets you traverse the directory explicitly,
without having to read the entire tree into memory first, so you don't
waste time reading file metadata that you don't care about.

For example, you might drag a SVN working copy into a page, which allows
viewing logs and other data about the repository.  It might easily contain
tens of thousands of files, but you rarely need to enumerate all of them in
advance to do useful things with it.

(If the trees are on a slow medium, like a DVD drive or a high-latency
network drive, even a much smaller number of files can take a long time.)

Even when you do want to traverse it all, there are many other advantages:
the traversal can be done asynchronously without blocking the page; the
page can have a cancel button to abort the operation; the page can show
other information about what it's doing (eg. number of new files, number of
unrecognized filenames); the page can allow dragging more directories to be
queued up for processing without having to wait for the first set to
complete; and so on.

Also, if a page caches a DirectoryEntry from entries, does that mean it can
> continuously poll the DirectoryEntry to see if the contents have changed to
> contain something interesting? That seems undesirable.
>

Nothing needs to be cached.  The DirectoryEntry just represents the
directory that was dragged; you don't have to look inside the directory at
all until the page uses it.

-- 
Glenn Maynard