Stay variables, what's the point?

[Alex Birkett]

Hi,

I'm new to cassowary and I'm trying to get my head around the basics. I think I now understand what a stay variable is after reading this constructor:

    public ClEditOrStayConstraint(ClVariable var, ClStrength strength, double weight) {

        super(strength, weight);

        _variable = var;

        _expression = new ClLinearExpression(_variable, -1.0, _variable.getValue());

    }

If I understand correctly it's an expression that sets a variable to its own initial value.

An expression that sets a variable to a value is obviously useful. I don't understand the point of setting a variable to it's own initial value though. It just confuses newbies like me. Why not just have an expression that sets a variable to a value? Then nobody would ever need to explain the concept of stays.

In the java implementation at least, if I want to change the value of the stay variable. I need to re-add it to the solver.

Any comments. Have I misunderstood?

Thanks,

Alex

[Ken Ferry]

Think about how when you resize a window, the window always stays at whatever size it is when you let go of the mouse.

Stay constraints are like that. A stay says, "don't change this unless you have to. If you _do_ have to change it, then try to stick at the new value, forgetting the original entirely."

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[Alex Birkett]

Thanks, Ken! Just to clarify, does the solver treat the linear expression created by the stay constraint as a special case?

Or to put it a different way, how is this code different from adding a stay variable (assuming the strength is the same a stay variable)?

            ClVariable myvar = new ClVariable();

            double INITAL_VALUE = 100;

            solver.addConstraint(new ClLinearEquation(new ClLinearExpression(myvar, -1.0, INITAL_VALUE)));

[kenf...@gmail.com]

On Wednesday, Oct 8, 2014 at 5:26 AM, Alex Birkett <al...@birkett.co.uk>, wrote:

Thanks, Ken! Just to clarify, does the solver treat the linear expression created by the stay constraint as a special case?

Or to put it a different way, how is this code different from adding a stay variable (assuming the strength is the same a stay variable)?

            ClVariable myvar = new ClVariable();

            double INITAL_VALUE = 100;

            solver.addConstraint(new ClLinearEquation(new ClLinearExpression(myvar, -1.0, INITAL_VALUE)));

Yes, this is different. In the window resize example from before, if this was used instead of a stay constraint then the window would snap back to its original frame every time you let go of the mouse. 

[Cacaodev cacaodev]

Le mercredi 8 octobre 2014 09:34:07 UTC+2, Alex Birkett a écrit :

In the java implementation at least, if I want to change the value of the stay variable. I need to re-add it to the solver.

Hi, I am using the javascript version of cassowary and I also had issues with externally modified stay variables. All I can say is that there were "unexpected behavior" (very difficult to reduce) and the only way to fix them was to remove and re-add stay variables with their new values and constant.

I submitted a test to this repository showing that the constant (constraint.expression.constant) in the stayConstraint is not updated when the value is successfully edited. Is it a bug or i'm missing something ?

I would be very interested if you have an example of a stay variable who refuses to change its value or the constant it is supposed to stick to.

Martin.

N.B: I am currently trying to implement autolayout into cappuccino, a javascript framework: https://github.com/cacaodev/cappuccino/tree/Autolayout