[Qt-interest] tools in a toolbar (like draw tool, select tool)

[Kishore]

In many applications i see a toolbar that set the current tool that acts on 
the central widget of the mainwindow. For example in an image manipulation 
application the move tool moves the image around, the crop  tool helps cut 
edges and say, a draw tool draws a line on the image. In other words the mouse 
interacts differently with the the central widget based on the currently 
selected tool.

What i am curious here is from a design point of view, is it the toolbar 
button that grabs the mouse (and maybe keyboard) and aids in performing the 
task or is it the central widget itself that has the capability while the 
toolbar just sets the mode on the central widget?

I am looking for a "standard procedure" as this seems to be a fairly popular 
use case. I am inclined to the former as it could easily allow one to add more 
"tools" as plugins
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Cheers!
Kishore