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README: jot down some notes regarding XTest vs libei

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Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
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Peter Hutterer 2020-08-28 13:11:39 +10:00
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@ -151,6 +151,39 @@ to this implementation. For example, a portal can set the client name
using `libreis` based on the app-id. A client cannot override this name
later.
Differences between XTest vs libei
----------------------------------
**libei** functionality is a superset of XTest's input emulation which
consists of a single request, `XTestFakeInput`. This request allows
emulation of button, key and motion events, including X Input 1.x events
(but not XI2). So **libei** can be a drop-in replacement since it supports
the same functionality and more.
However, XTest is an X protocol extension and users of XTest usually obtain
more information out-of-band ("out-of-band" here means "not through XTest
but instead other X protocol requests").
One example is `xdotool` which does window focus and modifier mangling (see
below). Window focus notification is not available to a pure **libei**
client and would have to be obtained or handled on a separate channel, e.g.
X or Wayland. Having said that, a Wayland client does not usually have acess
to query or modifiy the window focus.
Modifiers in `xdotool` are handled by obtaining the modifier mask from the X
server, identifying any difference to the intended mask and emulating key
events to change the modifier state to the intended one. For example, if
capslock is on, xdotool would send a capslock key event first (thus
disabling capslock) and then the actual key sequence. This is followed by
another capslock key event to restore the modifier mask.
This is not possible for a pure **libei** client as the modifier state is
maintained by the windowing system (if any). A client can obtain the
modifier state on Wayland on `wl_keyboard.enter` but when the client is
in-focus, there is rarely a need to emulate events.
Overall, it is best to think of **libei** devices as virtual equivalents to
a hardware device.
Open questions
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