Gtk::GestureDrag and retrieving x,y coordinates

[gummybears]

Working on interactive constraints and trying to call methods
to get the point where the drag started

def start_point(x : Float64 | Nil, y : Float64| Nil) : Bool
end

and to get the offset from the start point.

def offset(x : Float64 | Nil, y : Float64 | Nil ) : Bool

If the gesture is active, this function returns true and fills in x and y with the drag start coordinates, in surface-relative coordinates

These x,y parameters are passed by value and not by reference, so not sure how these x,y values are returned.

The compiler complains with error

In lib/gi-crystal/src/auto/gtk-4.0/gesture_drag.cr:115:21

 115 | Float64.null

[gummybears]

The code with the interactive constraints is found at

https://github.com/gummybears/gtk4_with_crystal/blob/master/lesson008/interactive_grid.cr

The compiler error message got truncated

In lib/gi-crystal/src/auto/gtk-4.0/gesture_drag.cr:141:21

 141 | Float64.null
               ^---
Error: undefined method 'null' for Float64.class

[gummybears]

It seems one has to use callback functions to retrieve the (x,y) coordinates when a widget is dragged

drag = Gtk::GestureDrag.new
drag.drag_begin_signal.connect(->drag_begin_callback(Float64,Float64))
drag.drag_update_signal.connect(->drag_update_callback(Float64,Float64))
drag.drag_end_signal.connect(->drag_begin_callback(Float64,Float64))

[hugopl]

I see, it's another issue from the fact that GI-Crystal just ignore the out parameters.

It's on my queue to touch this on GI-Crystal and let every method that have out parameters to return a Tuple {return_value, out_param1, out_param2, ...}

[gummybears]

I am also seeing methods where a parameter of type Pointer(Void) has to be specified e.g

class Gdk::Clipboard
  ....
  def read_value_async(type : UInt64, io_priority : Int32, cancellable : Gio::Cancellable?, callback : Gio::AsyncReadyCallback?, user_data : Pointer(Void)?) : Nil
   ....
end

but reading the Gi-Crystal doc, it says I should ignore user_data ??

[hugopl]

Yes, what the binding generator uses this user_data pointer to pass a Crystal ::Box object with the closure, so you can pass a Crystal callback for it.

All these _async methodw from GTK follow the same pattern, so I think will be possible to even create better a crystallized API for them.

[gummybears]

For the moment I will skip some of the Gtk4 examples as the API is still work in progress.

PS I am not familiar with ::Box object with closures.