Expose widget frame count (#827)

Saw this issue asking for a way to get the frame count for an Animation or Marquee widget #823

Looks like that's defined for all widgets (and is 1 in case it's not animated). This just updates the template for the render module to expose it as a readonly method, following the way size() is available for Text widgets.

Added test cases. Also checked that it gives a sensible answer for marquees in particular, which gets longer as the text gets longer.

docs/widgets.md

@@ -14,6 +14,11 @@ A quick note about colors.  When specifying colors, use a CSS-like
 hexdecimal color specification.  Pixlet supports "#rgb", "#rrggbb",
 "#rgba", and "#rrggbbaa" color specifications.
 
+For animated widgets like Marquee, you can also call `frame_count()`
+to work out how many frames are required to display the whole
+animation. You can also call `size()` on dynamically-sized widgets
+like Text to get the width and height.
+
 
 ## Animation
 Animations turns a list of children into an animation, where each

runtime/gen/docs/render.tmpl

@@ -14,6 +14,11 @@ A quick note about colors.  When specifying colors, use a CSS-like
 hexdecimal color specification.  Pixlet supports "#rgb", "#rrggbb",
 "#rgba", and "#rrggbbaa" color specifications.
 
+For animated widgets like Marquee, you can also call `frame_count()`
+to work out how many frames are required to display the whole
+animation. You can also call `size()` on dynamically-sized widgets
+like Text to get the width and height.
+
 {{range .}}{{if .Documentation}}{{$name := .GoName}}
 ## {{.GoName}}
 {{.Documentation}}

runtime/gen/type.tmpl

@@ -11,6 +11,9 @@ type {{.GoName}} struct {
 {{if .HasSize}}
 	size *starlark.Builtin
 {{end}}
+{{if .GoWidgetName}}
+	frame_count *starlark.Builtin
+{{end}}
 }
 
 func new{{.GoName}}(
@@ -44,6 +47,9 @@ func new{{.GoName}}(
 {{if .HasSize}}
 	w.size = starlark.NewBuiltin("size", {{.GoName|ToLower}}Size)
 {{end}}
+{{if .GoWidgetName}}
+	w.frame_count = starlark.NewBuiltin("frame_count", {{.GoName|ToLower}}FrameCount)
+{{end}}
 
 {{if .HasInit}}
 	if err := w.Init(); err != nil {
@@ -93,6 +99,10 @@ func (w *{{.GoName}}) Attr(name string) (starlark.Value, error) {
 {{if .HasSize}}
 	case "size":
 		return w.size.BindReceiver(w), nil
+{{end}}
+{{if .GoWidgetName}}
+	case "frame_count":
+		return w.frame_count.BindReceiver(w), nil
 {{end}}
 	default:
 		return nil, nil
@@ -126,3 +136,17 @@ func {{.GoName|ToLower}}Size(
 }
 
 {{end}}
+{{if .GoWidgetName}}
+func {{.GoName|ToLower}}FrameCount(
+	thread *starlark.Thread,
+	b *starlark.Builtin,
+	args starlark.Tuple,
+	kwargs []starlark.Tuple) (starlark.Value, error) {
+
+	w := b.Receiver().(*{{.GoName}})
+	count := w.FrameCount()
+
+	return starlark.MakeInt(count), nil
+}
+
+{{end}}