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Widget JSON

Widget layout can be loaded from a JSON description. The application can resolve widgets by unique id and event handlers are resolved from the application binary at runtime.

Function to load an application JSON layout
gp_widget *gp_app_layout_load(const char *app_name, void **uids);

Attempts to load an application layout based on app_name. The library first looks for a layout into a user directory and if there is no layout there system directory is used instead. This means that user can override system application layout by creating one in the user home directory.

The uids is a pointer to store the hash table of mappings between widget unique ids and pointers to.

The function returns NULL if no layout has been found or if it couldn’t be parsed.

Function to load an JSON layout
gp_widget *gp_widget_from_json_str(const char *str, void **uids);

gp_widget *gp_widget_layout_json(const char *fname, void **uids);

JSON layout can be also loaded from a file or C string.

Example JSON layout
 "version": 1,
 "widgets": [
   "type": "button",
   "label": "Button",
   "on_event": "button_event"

A JSON layout always starts with a version, currently version is set to 1 and the number will be increased if there are incompatible changes done in the format in a subsequent releases.

The widget type defaults to grid and cols and rows default to 1 for a grid widget so the outer widget is a grid with exactly one cell.

The inner widget type is a button and as such it should have either btype or label, in this case we have a button with a label.

The button on_event callback is set to button_event which means that, at runtime, fuction with this name will be resolved by the dynamic linker and called when the button has been pressed.

Table 1. Generic widget JSON attributes
Attribute Type Default Description



Widget universal id. Must be unique.




Widget type, e.g. button.




Sets both haling and valign {center, fill}




Horizontal alignment {center, left, right, fill}




Vertical alignment {center, top, bottom, fill}



Widget event handler function name.

The on_event function is a widget event handler.