There’s no way to run designer code right now, that includes bindings. This is an intentional choice as it would have side effects. Any binding sans a direct property property could corrupt the pristine version you set at design time. I’ve considered allowing the direct bindings to work, but as far as the widget code goes, it doesn’t know the difference when we init the widget between a binding that’s accessing a property directly through the special native direct binders v.s. some arbitrary blueprint or native function, running arbitrary game code. Also only the simple case is solved by this, most people would need to init a bogus mock object containing real data for their bindings to work. So just fixing direct bindings v.s. function bindings isn’t really worth pursuing I think.
I’ve got a few ideas for the future, this issue has been brought up internally as well, so we’ll eventually do something about it, because it does suck with a complex enough layering of widgets. My current thinking is that there needs to be a another tab (Sandbox Mode). That inits the widget for real in a sandbox world like Blueprints do when you click Simulate. Might even have a special function on widgets that gets called when they are created in a sandbox world, to allow a UI author to initialize mock objects, data, whatever. There’s complexity there as well, but it’s a lot more a complete solution.
Simplest work around now is have UI testing world you can play in to quickly test out these things.