5 ms·

I'm a huge fan of constraint programming, but everything I've read about layout problems makes me think that LP or MIP is a better fit for the domain. Solving s

I'm a huge fan of constraint programming, but everything I've read about layout problems makes me think that LP or MIP is a better fit for the domain. Solving systems of linear equalities and inequalities is what LP solvers do, and they've got decades of research into how to do it extremely fast. Most commercial solvers can solve systems with tens of millions of constraints in mere seconds. Is there a reason why cassowary took the constraint solver approach other than the fact that it is called constraint-based layout?

ridiculous_fish 11y ago
(I worked on autolayout at Apple) Cassowary is LP - it's just the Simplex method. Constraints become equalities and inequalities in the tableau, and the objective function is constructed from non-required ("best effort") constraints. The novel contribution from Cassowary is its incrementalism technique. It can add, remove, and edit constraints without having to re-solve the whole LP problem over again. So it's well suited for interactive applications. An example is dragging. You establish a constraint that sets the position of the dragged object, and then modify that constraint during the drag. Cassowary can re-use the old computation to solve the new one very efficiently.
- saosebastiao 11y ago
  That's interesting! Is it using Lagrangian duality to do any sort of shadow pricing for incremental changes? I guess I should probably read the paper by now.
- junkor 11y ago
  @ridiculous_fish: Hi, I just found that you are worked on autolayout, I have a little question about autolayout. There is a simple view as follow, a subView with some Constraints, they are all UILayoutPriorityRequired,when superview frame change, system will break the subView's height Constraint(300), but the top||bottom Constraint(30). they are all UILayoutPriorityRequired, why the system did things like this, and how does AutoLayout work (what's the calculate order? superView->subView->subviews'subview?),and where can I find the docs about this? :) |-----------------------------| | | | | 30 | | | | |-60-[subView(height:300)]-60-| | | | | 30 | | | | |-----------------------------| (maybe this case is not much suitable,because I should remove or change some constraints, I just want to use this case to explain the question)
  - junkor 11y ago
    shit, I use characters draw the constraints, but the Comment removed the \n characters, Add the "\n" by yourself:\n |-----------------------------|\n | | |\n | 30 |\n | | |\n |-60-[subView(height:300)]-60-|\n | | |\n | 30 |\n | | |\n |-----------------------------|\n

ridiculous_fish 11y ago

(I worked on autolayout at Apple) Cassowary is LP - it's just the Simplex method. Constraints become equalities and inequalities in the tableau, and the objective function is constructed from non-required ("best effort") constraints. The novel contribution from Cassowary is its incrementalism technique. It can add, remove, and edit constraints without having to re-solve the whole LP problem over again. So it's well suited for interactive applications. An example is dragging. You establish a constraint that sets the position of the dragged object, and then modify that constraint during the drag. Cassowary can re-use the old computation to solve the new one very efficiently.

saosebastiao 11y ago
That's interesting! Is it using Lagrangian duality to do any sort of shadow pricing for incremental changes? I guess I should probably read the paper by now.
junkor 11y ago
@ridiculous_fish: Hi, I just found that you are worked on autolayout, I have a little question about autolayout. There is a simple view as follow, a subView with some Constraints, they are all UILayoutPriorityRequired,when superview frame change, system will break the subView's height Constraint(300), but the top||bottom Constraint(30). they are all UILayoutPriorityRequired, why the system did things like this, and how does AutoLayout work (what's the calculate order? superView->subView->subviews'subview?),and where can I find the docs about this? :) |-----------------------------| | | | | 30 | | | | |-60-[subView(height:300)]-60-| | | | | 30 | | | | |-----------------------------| (maybe this case is not much suitable,because I should remove or change some constraints, I just want to use this case to explain the question)
- junkor 11y ago
  shit, I use characters draw the constraints, but the Comment removed the \n characters, Add the "\n" by yourself:\n |-----------------------------|\n | | |\n | 30 |\n | | |\n |-60-[subView(height:300)]-60-|\n | | |\n | 30 |\n | | |\n |-----------------------------|\n