Adding a C callback to ToughGFX Events

TouchGFX automatically generates GUI C++ code for STM32 based applications that use a display and touch screen. Here is a demonstration on how to attach a C function callback to a TouchGFX entity (a button).

When a button is clicked, a C function in a custom C file has to be called. Here is how.

Create a Default TouchGFX Page and Button

  • Create a new project (we call this one MyApplication_1) and select a Discovery or similar STM32 display-capable kit.
  • A blank TouchGFX screen is created by default (note that the default name is Screen1, this will be relevant later).
  • Drag and drop a button, in this case, a toggle button named ToggleButton1.
create stm32 touchgfx button and screen
Create a Default Screen with Toggle Button

Add a Custom Click Event to Toggle Button

  • Select the ToggleButton1 object
  • On the Interactions tab, add Interaction1 with the following properties
add click callback interaction to touchgfx button
Add a Virtual Function Callback to Button Click Event
  • When ToggleButton1 is clicked, the virtual function called btn1_pressed_vfn is called by the TouchGFX core.

Create Target Callback C Code

When ToggleButton1 is clicked, the goal is to execute a C function placed in a custom C file. The C file is placed in the application code folder (Code/Src).

  • Create a source file gfx_callbacks.c in MyApplication_1\Core\Src with the following function. This function will be called when ToggleButton1 is clicked.
// File:    gfx_callbacks.c

#include "main.h"
#include "gfx_callbacks.h"

void btn11_pressed_callback (void)
	static uint8_t btn1_ctr = 0;

  • Create a header file gfx_callbacks.h in MyApplication_1\Core\Inc with the following content.
// File:    gfx_callbacks.h


void btn11_pressed_callback (void);


Attach TouchGFX Button Callback with C Code

The generated TouchGFX code that you can modify is placed in Application/User/gui in the CubeIDE project explorer.

  • Open Screen1View.cpp
  • Add your custom definition of button click event. In this case, we simply call our callback C code in gfx_callbacks.c
add virtual function callback definition for touchgfx button
Add Custom Definition of Callback Function in Screen1View.cpp
  • Add a declaration of btn1_pressed_vfn in Screen1View.hpp
add button callback declaration in Screen1View hpp
Declare btn1_pressed_vfn() in Screen1View.hpp

C Callback is Triggered when Toggle Button is Clicked

  • When you build and run this application, a blank screen with the toggle button appears on the Discovery Kit display.
  • Clicking the button calls ScreenView::btn1_pressed_vfn() in Screen1View.cpp.
  • This function, in turn, calls btn1_pressed_callback() in our custom C file called gfx_callbacks.c.
Change Log
  • Initial Release: 23 December 2021

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