Creating a widget

From uGFX Wiki
Revision as of 12:11, 13 April 2016 by Tectu (Talk | contribs) (→‎Available functions)

Jump to: navigation, search

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. The first half of this article will explain how widgets are implemented an whats needed to create a custom widget. The second half of the article is a step-by-step guide that will show how to actually implement a custom widget based on an example.

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.

Theory of operation

A widget consists of three main parts: The object structure, the VMT and the rendering routine.

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. For example, a slider would add fields to hold the minimum, maximum and current value. A list widget would hold a list of items here. A text edit (line edit) would hold the current cursor position and so on. In terms of an object oriented language this is the private area of the class. 

typedef struct widgetObject_t {
    GWidgetObject w;  // Base Class
 
    uint8_t  myVariable1;
    char*    someOtherStuff;
    uint16_t currentValue;
} WidgetObject;

The GWidgetObject itself is based on a GWindowObject as explained in the article about widgets. Therefore, the following attributes are already part of our WidgetObject and don't need to be added manually:

Flags

The flags need some explanation: The GWindowObject contains a field of the type uint32_t and can therefore represent 32 flags. Some of these flags are used by the GWIN module internally to store certain information like the enable state, the visibility, whether the widget was allocated dynamically and so on. However, the first 8 bits are reserved for widget internal use. For example, the pushbutton widget uses one of these bits to keep record of the pressed/release state. The label widget uses the flags to keep the information whether the a border around the text should be rendered and so on.

VMT

The VMT (virtual method table) is a mechanism to implement dynamic function binding. In case of C it's a simple struct containing function pointers. The VMT technique allows to implement virtual methods in our C-classes. The VMT is defined by the GWIN module. The custom widget simply creates an instance of that struct and fills in the functions pointers. The GWIN module defines a VMT for each type of GWIN element: Window, widget and container. As a widget inherits from the window the widget VMT actually contains the window VMT. The VMTs declarations look like this (taken from /src/gwin/gwin_class.h): 

/**
 * @brief   The Virtual Method Table for a GWIN window
 * @{
 */
typedef struct gwinVMT {
    const char *        classname;                      /**< The GWIN classname (mandatory) */
    size_t              size;                           /**< The size of the class object */
    void (*Destroy)     (GWindowObject *gh);            /**< The GWIN destroy function (optional) */
    void (*Redraw)      (GWindowObject *gh);            /**< The GWIN redraw routine (optional) */
    void (*AfterClear)  (GWindowObject *gh);            /**< The GWIN after-clear function (optional) */
} gwinVMT;
/** @} */
 
 
/**
 * @brief   The Virtual Method Table for a widget
 * @note    A widget must have a destroy function. Either use @p _gwidgetDestroy() or use your own function
 *          which internally calls @p _gwidgetDestroy().
 * @note    A widget must have a redraw function. Use @p _gwidgetRedraw().
 * @note    If toggleroles != 0, ToggleAssign(), ToggleGet() and one or both of ToggleOff() and ToggleOn() must be specified.
 * @note    If dialroles != 0, DialAssign(), DialGet() and DialMove() must be specified.
 * @{
 */
typedef struct gwidgetVMT {
    struct gwinVMT                  g;                                                                  /**< This is still a GWIN */
    void (*DefaultDraw)             (GWidgetObject *gw, void *param);                                   /**< The default drawing routine (mandatory) */
    #if GINPUT_NEED_MOUSE
        struct {
            void (*MouseDown)       (GWidgetObject *gw, coord_t x, coord_t y);                          /**< Process mouse down events (optional) */
            void (*MouseUp)         (GWidgetObject *gw, coord_t x, coord_t y);                          /**< Process mouse up events (optional) */
            void (*MouseMove)       (GWidgetObject *gw, coord_t x, coord_t y);                          /**< Process mouse move events (optional) */
        };
    #endif
    #if GINPUT_NEED_KEYBOARD || GWIN_NEED_KEYBOARD
        struct {
            void (*KeyboardEvent)   (GWidgetObject *gw, GEventKeyboard *pke);                           /**< Process keyboard events (optional) */
        };
    #endif
    #if GINPUT_NEED_TOGGLE
        struct {
            uint16_t                toggleroles;                                                        /**< The roles supported for toggles (0->toggleroles-1) */
            void (*ToggleAssign)    (GWidgetObject *gw, uint16_t role, uint16_t instance);              /**< Assign a toggle to a role (optional) */
            uint16_t (*ToggleGet)   (GWidgetObject *gw, uint16_t role);                                 /**< Return the instance for a particular role (optional) */
            void (*ToggleOff)       (GWidgetObject *gw, uint16_t role);                                 /**< Process toggle off events (optional) */
            void (*ToggleOn)        (GWidgetObject *gw, uint16_t role);                                 /**< Process toggle on events (optional) */
        };
    #endif
    #if GINPUT_NEED_DIAL
        struct {
            uint16_t                dialroles;                                                          /**< The roles supported for dials (0->dialroles-1) */
            void (*DialAssign)      (GWidgetObject *gw, uint16_t role, uint16_t instance);              /**< Test the role and save the dial instance handle (optional) */
            uint16_t (*DialGet)     (GWidgetObject *gw, uint16_t role);                                 /**< Return the instance for a particular role (optional) */
            void (*DialMove)        (GWidgetObject *gw, uint16_t role, uint16_t value, uint16_t max);   /**< Process dial move events (optional) */
        };
    #endif
} gwidgetVMT;
/** @} */
 
 
/**
 * @brief   The Virtual Method Table for a container
 * @note    A container must have a destroy function. Either use @p _gcontainerDestroy() or use your own function
 *          which internally calls @p _gcontainerDestroy().
 * @note    A container must have a gwin redraw function. Use @p _containerRedraw().
 * @note    If toggleroles != 0, ToggleAssign(), ToggleGet() and one or both of ToggleOff() and ToggleOn() must be specified.
 * @note    If dialroles != 0, DialAssign(), DialGet() and DialMove() must be specified.
 * @{
 */
typedef struct gcontainerVMT {
    gwidgetVMT  gw;
    coord_t (*LeftBorder)       (GHandle gh);                           /**< The size of the left border (mandatory) */
    coord_t (*TopBorder)        (GHandle gh);                           /**< The size of the top border (mandatory) */
    coord_t (*RightBorder)      (GHandle gh);                           /**< The size of the right border (mandatory) */
    coord_t (*BottomBorder)     (GHandle gh);                           /**< The size of the bottom border (mandatory) */
    void (*NotifyAdd)           (GHandle gh, GHandle ghChild);          /**< Notification that a child has been added (optional) */
    void (*NotifyDelete)        (GHandle gh, GHandle ghChild);          /**< Notification that a child has been deleted (optional) */
} gcontainerVMT;
/** @} */

Note how the VMTs reflect the inheritance of the GWIN elements: Windows, widgets and containers. If you want to implement a widget, you don't have to implement the container VMT.

Function pointers

The function pointer declarations in the VMT specify the signature of a function (the return type and the parameters the function takes). In order to be able to register a function in the VMT, the function must match that exact signature. Let's have a look at the function to render/draw the widget: 

void (*DefaultDraw) (GWidgetObject* gw, void* param);

This means that our own function that we will write to render the widget must look like this: 

void myRenderingFunction(GWidgetObject* gw, void* param) { ... }

Note that the function pointer in the VMT doesn't specify the name of the function. The name DefaultDraw is only used internally by the GWIN module to call the function. You can assign a function with any name as long as the function signature matches the one from the function pointer declaration. Once a function has been declared it can be added to the VMT by simply typing the actual function name in the corresponding field in the VMT: 

void myRenderingFunction(GWidgetObject* gw, void* param){
    ...
}
 
...
 
static const gwidgetVMT mywidgetVMT = {
    {
        "MyWideget",                // The classname
        sizeof(MyWidgetObject),     // The object size
        _gwidgetDestroy,            // The destroy routine
        _gwidgetRedraw,             // The redraw routine
        0,                          // The after-clear routine
    },
    myRenderingFunction,            // The default drawing routine    #if GINPUT_NEED_MOUSE
        {
            0,                      // Process mouse down events
            0,                      // Process mouse up events
    ...
    ...
    ...
};

Available functions

The following table contains a short explanation for each field in the VMT:

Name Required Dependency Description
GFX_USE_OS_RAW32 Use the bare metal port without any underlying OS.
See BareMetal. 		GFX_USE_OS_RAW32 Use the bare metal port without any underlying OS.
See BareMetal.
INTERRUPTS_ON() Optionally set code that is executed when interrupts are turned on.
See Interrupts. 		GFX_USE_OS_RAW32 Use the bare metal port without any underlying OS.
See BareMetal.

Rendering routine

This is the part that usually takes the most time when writing a custom widget: The rendering routine. A widget must have at least one built-in rendering routine which is registered as the default rendering routine in the VMT. The rendering routine is the function which draws/paints the widget on the screen. This function is called by the GWIN module whenever the widget needs to be redrawn (eg. when the visibility changed).

For further information about rendering routines it's recommended to read the article about custom rendering routines.

Example

This part of the article will show how to actually implement a widget. As an example we'll implement a statusbar. The statusbar is very application specific. It consists of a time label, a button to open a settigs dialog and two icons that show whether a USB connection and an SD-Card are present.

Retrieved from "https://wiki.ugfx.io/index.php?title=Creating_a_widget&oldid=1573"