Add option to update when releasing mouse button from slide

docs/examples/blocks/slider.md

@@ -9,6 +9,12 @@ The currently selected value is in the attribute `value`.
 Don't change this value manually, but use the function `set_close_to!(slider, value)`.
 This is necessary to ensure the value is actually present in the `range` attribute.
 
+If the slider value is used for a relatively slow task, it may be more effective to
+use the attribute `value_dragstop` instead of `value`.  The `value_dragstop` is only
+updated when the mouse is released to conclude the slider drag operation.  It is 
+is synchronized with `value` when calling `set_close_to!(slider, value)` or one a mouse
+click or double click event.
+
 You can double-click the slider to reset it (approximately) to the value present in `startvalue`.
 
 If you set the attribute `snap = false`, the slider will move continously while dragging and only jump to the closest available value when releasing the mouse.
@@ -37,6 +43,55 @@ fig
 \end{examplefigure}
 
 
+This slightly more complicated example compares the behavior of the `value`
+and `value_dragstop` slide attributes.  There are two text fields displaying
+the x value selected with the horizontal slider.  However, one updates while the slider
+is ddragged to show continuous updates but the other text only updates on mouse
+release.  The vertical slider and associated text fields behave the same way.
+A more realistic application could use the the `value_dragstop` attribute to
+trigger a slow computation only on the mouse release event.
+
+\begin{examplefigure}{}
+```julia
+using GLMakie
+GLMakie.activate!() # hide
+fig = Figure()
+
+ax = Axis(fig[1, 1])
+
+sl_x = Slider(fig[2, 1], range = 0:0.5:10, startvalue = 3)
+sl_y = Slider(fig[1, 2], range = 0:0.5:10, horizontal = false, startvalue = 6)
+
+x_text = lift(sl_x.value) do x
+   "x slider value=$(x)"
+end
+x_text_stop = lift(sl_x.value_dragstop) do x
+   "mouse release x slider value=$(x)"
+end
+y_text = lift(sl_y.value) do y
+    "y slider value=$(y)"
+end
+y_text_stop = lift(sl_y.value_dragstop) do y
+   "mouse release y slider value=$(y)"
+end
+
+point = lift(sl_x.value, sl_y.value) do x, y
+    Point2f(x, y)
+end
+
+scatter!(point, color = :red, markersize = 20)
+text!(2.5,9.5, text = x_text, align = (:center, :center))
+text!(2.5,9.0, text = x_text_stop, align = (:center, :center))
+text!(7.5,9.5, text = y_text, align = (:center, :center))
+text!(7.5,9.0, text = y_text_stop, align = (:center, :center))
+
+limits!(ax, 0, 10, 0, 10)
+
+fig
+```
+\end{examplefigure}
+
+
 ## Labelled sliders and grids
 
 The functions \apilink{labelslider!} and \apilink{labelslidergrid!} are deprecated, use \apilink{SliderGrid} instead.

src/makielayout/blocks/slider.jl

@@ -61,6 +61,9 @@ function initialize_block!(sl::Slider)
 
     on(topscene, selected_index) do i
         sl.value[] = sliderrange[][i]
+        if !dragging[]
+            sl.value_dragstop[] = sliderrange[][i]
+        end
     end
 
     # initialize slider value with closest from range
@@ -93,19 +96,16 @@ function initialize_block!(sl::Slider)
 
     onmouseleftdrag(mouseevents) do event
         dragging[] = true
-        dif = event.px - event.prev_px
-        fraction = clamp(if sl.horizontal[]
-            (event.px[1] - endpoints[][1][1]) / (endpoints[][2][1] - endpoints[][1][1])
-        else
-            (event.px[2] - endpoints[][1][2]) / (endpoints[][2][2] - endpoints[][1][2])
-        end, 0, 1)
+
+        fraction = current_sliderfraction(sl, event, endpoints, sliderrange)
+        displayed_sliderfraction[] = fraction
 
         newindex = closest_fractionindex(sliderrange[], fraction)
         if sl.snap[]
             fraction = (newindex - 1) / (length(sliderrange[]) - 1)
         end
-        displayed_sliderfraction[] = fraction
 
+        newindex = closest_fractionindex(sliderrange[], fraction)
         if selected_index[] != newindex
             selected_index[] = newindex
         end
@@ -115,8 +115,11 @@ function initialize_block!(sl::Slider)
 
     onmouseleftdragstop(mouseevents) do event
         dragging[] = false
+        # selected_index should be set correctly in onmouseleftdrag
+        sl.value_dragstop = sliderrange[][selected_index[]]
         # adjust slider to closest legal value
-        sliderfraction[] = sliderfraction[]
+        # This line is not necessary?
+        # sliderfraction[] = sliderfraction[]
         linecolors[] = [sl.color_active_dimmed[], sl.color_inactive[]]
         return Consume(true)
     end
@@ -126,6 +129,7 @@ function initialize_block!(sl::Slider)
         dim = sl.horizontal[] ? 1 : 2
         frac = (pos[dim] - endpoints[][1][dim]) / (endpoints[][2][dim] - endpoints[][1][dim])
         selected_index[] = closest_fractionindex(sliderrange[], frac)
+        sl.value_dragstop = sliderrange[][selected_index[]]
         # linecolors[] = [color_active[], color_inactive[]]
         return Consume(true)
     end
@@ -151,6 +155,23 @@ function initialize_block!(sl::Slider)
     sl
 end
 
+
+function current_sliderfraction(sl, event, endpoints, sliderrange)
+    fraction = clamp(if sl.horizontal[]
+        (event.px[1] - endpoints[][1][1]) / (endpoints[][2][1] - endpoints[][1][1])
+        else
+             (event.px[2] - endpoints[][1][2]) / (endpoints[][2][2] - endpoints[][1][2])
+              end, 0, 1
+    )
+
+    newindex = closest_fractionindex(sliderrange[], fraction)
+    if sl.snap[]
+        fraction = (newindex - 1) / (length(sliderrange[]) - 1)
+    end
+
+    return fraction
+end
+
 function valueindex(sliderrange, value)
     for (i, val) in enumerate(sliderrange)
         if val == value
@@ -201,4 +222,5 @@ function set_close_to!(slider::Slider, value)
     closest = closest_index(slider.range[], value)
     slider.selected_index = closest
     slider.range[][closest]
+    slider.value_dragstop = slider.range[][closest]
 end

src/makielayout/types.jl

@@ -878,6 +878,8 @@ end
         startvalue = 0
         "The current value of the slider. Don't set this manually, use the function `set_close_to!`."
         value = 0
+        "Value of slider when mouse is released after dragging."
+        value_dragstop = 0
         "The width of the slider line"
         linewidth::Float32 = 15
         "The color of the slider when the mouse hovers over it."