Check-in [608b4bc12b]

# TIP 532: Re-implementation of event loop processing.
	Author:         Gregor Cramer <[email protected]sourceforge.net>
	State:          Final
	Type:           Project
	Vote:           Pending
	Created:        09-Jan-2019
	Post-History:  
	Keywords:       Tk, bind, event, event loop
	Tcl-Version:    8.7
	Tk-Branch:      bug6e8afe516d
----

# Abstract

Current implementation of event loop processing suffers from a limited event ring.
This limitation is causing unexpected behavior. Moreover some issues in event loop
handling are known.

Increasing the event size of the event ring can solve the problems caused by event
queue overflow locally, but not in general. Therefore the author decided to re-implement
the event processing, the revised implementation is working as if the event ring is
unlimited. Moreover new implementation is more efficient (in time), and all known
issues in event handling have been eliminated.

This project (re-implementing the event loop handling) has been started with error report
[Severe bugs in binding (tkBind.c)](https://core.tcl-lang.org/tk/tktview/6e8afe516df85f6213f436ef7c2fab2ec2d11c76).

# Rationale

The following problems, caused by event ring overflow, have been solved:

1. Sometimes double-clicks with mouse will not be detected, nothing happens although this
event is bound (see test case
[bind-32.2](https://core.tcl-lang.org/tk/artifact/6377cb0d762b7261?ln=6123-6143)).
This problem occurs often in applications[Scidb](http://scidb.sourceforge.net),
because this application is using tooltips heavily, causing a lot of intervening
expose events.

2. Immediately after startup of application [Scidb](http://scidb.sourceforge.net)
(same with applications [Scid](http://scid.sourceforge.net), and
[Scid vs PC](http://scidvspc.sourceforge.net))
it's not possible to open the menu via a shortcut like \<Alt-m\>. This event will be
gobbled, because of many intervening events, causing an event ring overflow.

3. After switching a tab pane in a notebook window the tab is losing the focus sometimes.
This has been observed in applications [Scid](http://scid.sourceforge.net), and
[Scid vs PC](http://scidvspc.sourceforge.net).

Moreover the following issues have been solved:

4. It's possible to bind an event like `\<Quadruple-1\>`, but it's nearly impossible to
trigger this event (with mousepad). Even a triple-click is not so easy. This behavior is
user-unfriendly, and it seems that it is caused by an erroneous implementation.

5. If a statement like `event generate . \<1\>` is executed, and after some time
(\> 500 ms) this statement is executed again, then it's likeky that a double-click event
will be triggered, even if a single-click event is expected, because the triggering
of double-click events has to fit time requirements (due to manual; see test case
[bind-32.4](https://core.tcl-lang.org/tk/artifact/6377cb0d762b7261?ln=6158-6171)).

6. `event generate . \<FocusIn\> -sendevent 1` is not working, the argument of
`sendevent` get lost (test case
[bind-32.6](https://core.tcl-lang.org/tk/artifact/6377cb0d762b7261?ln=6192-6204)).

7. See following code:

		bind . \<Double-1\> { lappend x "Double" }  
		bind . \<1\>\<1\> { lappend x "11" }  
		event generate . \<1\> -x 0 -y 0 -time 0  
		event generate . \<1\> -x 0 -y 0 -time 0
		set x

	This gives the result `11`, but `Double` is expected, because the time (and space)
	constraints for a double click are fulfilled. With other words, the legacy implementation
	is not preferring the most specialized event. But it should, because the manual says
	(`man bind`):

	> If more than one binding matches a particular event and they have the
	> same tag, then the most specific binding is chosen and its script is
	> evaluated.

	And the sequence `\<Double-1\>` is more specific than `\<1\>\<1\>` because of time and
	space requirements (in `\<Double-1\>`). Note that constant `PREFER_MOST_SPECIALIZED_EVENT=1`
	has to set when compiling to enable this new feature.

8. Legacy implementation cannot handle homogeneous equal sequences properly, see this script:

		bind . <1><Control-1> { lappend x "first" }
		bind . <Control-1><1> { lappend x "last" }
		event generate . <Control-1>
		event generate . <Control-1>
		set x
	
	Manual (`man bind`) says:

	> If these tests fail to determine a winner, then the most recently registered
	> sequence is the winner.

	In this script there is no winner, so the latter defined one has to be chosen, and
	revised implementation is doing this.

Legacy code also suffers from causing memory holes, revised implementation is tested
to be memory friendly.

The revised implementation supports an additional syntax for binding motion events
(if constant `SUPPORT_ADDITIONAL_MOTION_SYNTAX=1` is set when compiling). E.g.,
the following bindings

	bind . \<B2-Motion\> { ... }  
	bind . \<B1-B2-Motion\> { ... }

can be expressed in a different way:

  	bind . \<Motion-2\> { ... }  
  	bind . \<Motion-1-2\> { ... }

The additional syntax is easier to remember, because button press/release events will also
be expressed in the latter form, for example `bind . \<ButtonPress-1\>`. The former
syntax (`\<B2-Motion\>`) form will still be supported.

# Specification

The whole handling in file `general/tkBind.c` has been re-implemented. This implementation
is passing all test cases in `tests/bind.test`. Note that legacy implementation is failing
in some (of the new) test cases.

Case (4): Legacy implementation is computing the time difference of nth click with fst click,
and tests whether it is less than 500 ms. But this seems to be an implementation bug. Revised
implementation computes the difference of nth and (n+1)th click. This behavior also conforms
better to the behavior of other toolkits. With new implementation the use of quadruple clicks
(and triple clicks) is unproblematic. See also test case
[bind-32.6](https://core.tcl-lang.org/tk/artifact/6377cb0d762b7261?ln=6172-6191).

Case (5) is only a minor bug, and there exists a work-around. But the author decided to
eliminate this design bug, with revised implementation option **-time** is recognizing new
special value **current**, and is using the current event time in this case. This extension
is fully backward compatible. See also test case
[bind-32.4](https://core.tcl-lang.org/tk/artifact/6377cb0d762b7261?ln=6158-6171).

For the fix of case (6) the author decided that non-zero values (given with option
**-send_event**) will be converted to **1**. This is conform to the manual, see
`man bind` (search for **Sendevent**), see also lines 3287ff in legacy file
[generic/tkBind.c](http://core.tcl.tk/tk/artifact/e41f45f7f6ac3447?ln=4178-4203).

The fix of (7) is not fully backwards compatible. But in the author's opinion this is not
a real problem, nobody is using sequences like `\<1\>\<1\>`, it is not expected that
applications have to be adjusted.

Fix of (8) is correcting a major bug, see test case
[bind-33.13](https://core.tcl-lang.org/tk/artifact/6377cb0d762b7261?ln=6550-6566).

# Implementation

Please refer to the
[bug6e8afe516d](https://core.tcl-lang.org/tk/timeline?r=bug6e8afe516d)
branch of the core Tcl repository.

The event ring has been removed. Revised implementation is working with promoted
event bindings, and remembers the latest event per event type. This technique works
as if we have an inifinite event ring, so no overflow is possible.

Based on tests the performance in time is better than with legacy implementation. This
result is expected, because a triple-nested loop, executed for each incoming event, has
been changed to a quasi-double-nested loop (only in very seldom cases it is still
triple-nested). Furthermore the traversed lists are shorter than with legacy implementation,
because the event ring, always containing 30 items, has been eliminated. Only unbinding
a tag is a bit slower than before. Memory consumption did not change significantly.

# Backwards Compatibility

Fix of issue (7) is not fully backwards compatible (more details in section **Rationale**).

Moreover fix of issue (8) is not backwards compatible, but here erroneous behavior has been
corrected.

Beside these two exceptions the revised implementation is fully backwards compatible, even
if the additional syntax style for motion bindings is enabled.

# Copyright

This document has been placed in the public domain.