I really try to figure this stuff out. No kidding. I am trying this simple
experiment:
I define an NSView that has nothing more than a resize handle, defined as a
rectangular NSBezierPath in its lower right corner. There are no subviews and
no superviews involved here. I resize the NSView by implementing a mouse event
loop in the NSView code. I also drag the NSView within its window within the
same mouse event loop if the mouse down was not in the resize Bezier path.
In the view’s initializer I create two NSLayoutConstraints, to set a minimum
height and minimum width on the NSView, and add them to the view. I also set
the NSView’s translatesAutoresizingMaskIntoConstraints property to false, so
that these are the only two constraints defined on the the view.
This is all done programmatically. There is no NIB for this NSView. For
reference here is the code that creates and adds the constraints, found inside
the view’s initializer:
widthConstraint = NSLayoutConstraint(item: self, attribute:
NSLayoutAttribute.Width,
relatedBy: NSLayoutRelation.GreaterThanOrEqual, toItem: nil,
attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1.0, constant:
100.0)
heightConstraint = NSLayoutConstraint(item: self, attribute:
NSLayoutAttribute.Height,
relatedBy: NSLayoutRelation.GreaterThanOrEqual, toItem: nil,
attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1.0, constant:
100.0)
addConstraint(widthConstraint)
addConstraint(heightConstraint)
[Peeve, before I move on. I have to include the names of the two enum types
(NSLayoutAttribute and NSLayoutRelation) in this code in order for the compiler
to be happy — why? — I thought the compiler knows what the parameter types are
so only the enum constants should be needed. End peeve.]
When I instantiate one of these views and place it in an NSWindow, I can drag
it around and I can resize it, as expected. Notably, however, I can resize it
down to zero size (and even smaller!), even though the two constraints exist. I
kind of expected the program to crash when the constraints were violated, but
things keep on running. I can drag and resize the view at will with no
repercussions.
I would really like to be able to do resizing using a mouse event loop, but
have the layout constraints somehow involved. Reading through references for
NSView I don’t see how to do this. I guess I am looking for a way from within
the event loop to be able to check whether a proposed new frame rectangle for
the view obeys the view’s size constraints.
Anyone have advice of what I should be reading to figure out what to do? All
the examples I can find seem so simple. Whenever I get into one these, what
seems to me to be a simple, real world situations (how unusual is a resizable
view with constraints, after all?), I seem to get lost reading a plethora of
confusing documents. It’s either me or the documentation, or maybe a lack of
documentation. I can easily believe it’s me. But that doesn’t help me get this
experiment to work.
Clearly what I need is a better mental model of how the auto constraint system
operates at run time. Maybe someone can point me in the direction of a good
tutorial.
Thanks,
Tom Wetmore
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