CalloutWidgetBuilder typedef
Signature for rendering custom callout content.
The returned widget is laid out using the active CalloutStyle width
constraints before the callout is shown. When
CalloutStyle.autoAdjustWidth is true, the widget is measured between
CalloutStyle.minWidth and CalloutStyle.maxWidth. When it is false,
the widget is measured at a fixed width of CalloutStyle.maxWidth.
For example, a SizedBox(width: 200) with CalloutStyle(maxWidth: 100)
is measured at 100 logical pixels wide.
The geoElement argument is non-null only when used from
Callout.showCalloutForGeoElement.
For Callout.showAt and Callout.showForLocationDisplay, geoElement
is always null.