class DeviceWatcher

Creates a device watcher associated with the current message loop.

Asynchronously invokes |exists_callback| for all existing devices within

the specified directory as well as any subsequently added devices until

the device watcher is destroyed. Ignores the "." directory.

Equivalent to:

CreateWithIdleCallback(directory_path, exists_callback, []{});

|directory_path| is the directory to watch (without a trailing slash).

|exists_callback| gets called with each existing or new filename, or with

an empty string after existing files if empty_after_existing is true.

Creates a device watcher associated with the current message loop.

Asynchronously invokes |exists_callback| for all existing devices within

the specified directory as well as any subsequently added devices until

the device watcher is destroyed. Ignores the "." directory.

The |idle_callback| is invoked once immediately after all pre-existing

devices have been reported via |exists_callback| shortly after creation.

After |idle_callback| returns, any newly-arriving devices are reported via

|exists_callback|.

|directory_path| is the directory to watch (without a trailing slash).

|exists_callback| gets called with each existing or new filename, or with

an empty string after existing files if empty_after_existing is true.

|idle_callback| gets called after |exists_callback| has returned for all

pre-existing devices, and returns before |exists_callback| is called for

any subsequently-added devices.

|idle_callback| will be deleted after it is called, so captured context

is guaranteed to not be retained.