TShiftState in MouseDown and MouseUp event handlers
lazarus / 31-LCL

----------------------------------------------------------------------
stefanv · 2019-04-03 14:44

I just ran into an interesting problem, which I've already resolved,
but I thought I'd share it here in case it bites anyone else.

I'm working on an app, one feature of which is that you can introduce
labels at runtime, and drag them around over your document (kind of
like attaching annotations in a PDF viewer). In my MouseDown event
handler, I wanted to check that it was the left button that was
pressed, and that no modifier keys were being used, so I wrote:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i if (Button
= mbLeft) and (Shift = []) then begin ... end
At the time I wrote this, I was developing on a Raspberry Pi,
connected remotely using TightVNC. The code above didn't work because
Shift contained ssLeft. So I changed it to:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i if (Button
= mbLeft) and (Shift = [ssLeft]) then begin ... end
After that, it worked as expected. However, when I went to go test the
app on the Raspberry Pi's own local X server, it no longer worked. I
discovered that there, Shift was actually an empty set, as I had
originally expected it to be.

My fix was to rewrite the code like this:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i if (Button
= mbLeft) and (Shift * [ssShift,ssCtrl,ssAlt] = []) then begin ...
endThis worked properly in both environments.

----------------------------------------------------------------------
Blaazen · 2019-04-03 15:12

Yes, because part of ShiftState is also NumLock (ssNum) and many
others, therefore you must always test:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i if ssAlt in
Shift then ...or what you did
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i ... (Shift
* [ssShift,ssCtrl,ssAlt] = []) ...
Note: there exists handy constant ssModifier defined in unit Controls:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i ssModifier
= {$if defined(darwin) or defined(macos) or defined(iphonesim)} ssMeta
{$else} ssCtrl {$endif};which is ssCtrl everywhere except MacOS.

----------------------------------------------------------------------
stefanv · 2019-04-03 15:17

Thanks Blaazen.

I'm curious though, why there's a difference in the presence or
absence of ssLeft depending on which X server I am using.