Windows 10 version 1709 (or “Fall Creators Update”) has
introduced the “Files On-Demand” feature, only for NTFS physical drives. It
allows users to see all their files stored on OneDrive from within the Windows
Explorer, without having to first download these files. On this screenshot we
can see an Image folder (note it’s a physical path) that contains 3 files:
The Status column is added automatically by the Windows
Shell because it knows this folder is special (in fact, it’s the OneDrive root
folder which is marked as a “sync root”). This column displays an icon that
correspond to the sync state of the Shell Item. This icon column exists for
items and folders. When the item is selected we can also see its status as a
text “Available when online” here. For folder items, the meaning is different,
it represents the combined state of child items.
The first two files are in sync, they exist locally. The
third “small.mp4” file exists on the Internet (represented by a “cloud” icon)
but is not present locally.
In fact, what happens is there is a “small.mp4” file that
exists physically in the folder. You can even see it using a Windows command
line “dir” command. But this file is just a “placeholder”. It has the correct
dates and size, but it’s content does not exist physically on the hard drive.
It will be downloaded as soon as any application will try to read it. In this
case, Windows will ask the sync provider (OneDrive) for the file’s content. If
OneDrive doesn’t run at that time, Windows will display an error message.
Although this technology is not strictly related to Shell
Namespace Extension development (in fact, it’s not even related to the Shell,
it operates at a lower level), ShellBoost provides some exclusive support for it,
in combination with Shell Namespace Extension development, through the OnDemandSynchronizer
This feature is demonstrated in the AmalgaDrive Sample.