Creating a widget
It's possible to implement custom widgets. A custom widget can be declared and implemented outside of the µGFX library. This means that it can be part of the actual project and that it's not necessary to do any modification of the µGFX library itself. This article will explain and show how to create a custom widget. As an example we'll create a statusbar widget. The statusbar consists of a rectangular area, the current system time, two icons to indicate USB and SD-Card state and a button to open the settings dialog.
It's vital that you've read the GWIN article and the corresponding sub-articles about windows, widgets and containers before following this guide.
Note: If you just want to change the look of an existing widget, please have a look at WidgetStyles and custom rendering functions.
Architecture
A widget consists of two main parts: The object structure and the VMT.
Object structure
The C language is not object oriented - yet it's possible to write object oriented software without any problems. The philosophy is to create a struct which is the object and to pass that struct as the first parameter to any function that has to have access to the object. A widget is an object and hence it needs a struct that defines the object. The only thing that makes the widget object struct become a widget struct is that the very first field is a GWidgetObject:
typedef struct StatusbarObject_t { GWidgetObject w; // Base Class } StatusbarObject;
Note that it is absolutely mandatory that the GWidgetObject is the very first field in the struct. This is needed to implement inheritance and polymorphism. After that first field you can add any field you want to the widget object struct.
The GWidgetObject itself is based on a GWindowObject as explained in the article about widgets. Therefore, the following attributes are already part of our StatusbarObject and don't need to be added manually:
- Position relative to parent (x, y)
- Dimensions (width, height)
- Parent
- Font
- Text
- WidgetStyle
- Widget tag
