Base Widget (lv_obj)

The root widget that every other widget inherits from. Provides position, size, layout, styles, events, and more.

The most fundamental of all widgets is the Base Widget, on which all other widgets are based. From an object-oriented perspective, think of the Base Widget as the widget class from which all other widgets inherit.

By this mechanism, all widgets carry the features of the Base Widget. Therefore, the functions and functionalities of the Base Widget can be used with other widgets as well. For example: lv_obj_set_width(slider, 100).

Although it's just a simple rectangle, the Base Widget is the most powerful widget, as it supports all the features that other widgets can use.

The Base Widget can have:

• Children
• Position and Size
• Layouts
• Styles
• Events
• Data bindings
• Flags
• Parts and States
• And many more

A Slider widget, for example, uses these features to realize slider-like behavior, but it’s all based on the same style, event, layout, and other concepts supported by the Base Widget.

All these common and versatile features are introduced in the Common Widget Features chapter.

In HTML terms, think of it as a <div>, a very versatile building block that, with some CSS and JS, can become almost anything.

Data binding

Every widget inherits the Base Widget's binding helpers, which let a Subject drive styles, flags, or states declaratively. The most common one is bind_style: attach a named style to a widget only while a Subject equals a reference value — perfect for theme switches and "mode" toggles. See the Observer page for an introduction to the underlying mechanism.

• lv_obj_bind_style(obj, &style, selector, subject, ref_value)
• lv_obj_bind_bool(obj, subject, set_flag_cb) — drive a flag from a boolean Subject; a per-flag setter such as lv_obj_set_hidden can be passed directly
• lv_subject_add_observer_obj(subject, observer_cb, obj, user_data) — for a state, or when the Subject must be compared against a reference value

Examples

Make an object draggable

Interactive drag-to-move is driven from an LV_EVENT_PRESSING callback, which reads the input device's motion vector and feeds it back through lv_obj_set_pos. There's no XML attribute for "drag with the pointer", so this lives in C.

Transform an object with a 3x3 matrix

A timer ticks the rotation and scale of the object via lv_obj_set_transform. This needs LV_DRAW_TRANSFORM_USE_MATRIX and an animation loop, neither of which is reachable from XML attributes — the C example shows the full setup.