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199 lines
9.0 KiB
Markdown
199 lines
9.0 KiB
Markdown
# Base object (lv_obj)
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## Overview
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The 'Base Object' implements the basic properties of widgets on a screen, such as:
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- coordinates
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- parent object
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- children
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- contains the styles
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- attributes like *Clickable*, *Scrollable*, etc.
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In object-oriented thinking, it is the base class from which all other objects in LVGL are inherited.
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The functions and functionalities of the Base object can be used with other widgets too. For example `lv_obj_set_width(slider, 100)`
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The Base object can be directly used as a simple widget: it's nothing more than a rectangle. In HTML terms, think of it as a `<div>`.
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### Coordinates
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Only a small subset of coordinate settings is described here. To see all the features of LVGL (padding, coordinates in styles, layouts, etc) visit the [Coordinates](/overview/coords) page.
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#### Size
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The object size can be modified on individual axes with `lv_obj_set_width(obj, new_width)` and `lv_obj_set_height(obj, new_height)`, or both axes can be modified at the same time with `lv_obj_set_size(obj, new_width, new_height)`.
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#### Position
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You can set the position relative to the parent with `lv_obj_set_x(obj, new_x)` and `lv_obj_set_y(obj, new_y)`, or both axes at the same time with `lv_obj_set_pos(obj, new_x, new_y)`.
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#### Alignment
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You can align the object on its parent with `lv_obj_set_align(obj, LV_ALIGN_...)`. After this every x and y setting will be relative to the set alignment mode.
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For example, this will shift the object by 10;20 px from the center of its parent:
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```c
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lv_obj_set_align(obj, LV_ALIGN_CENTER);
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lv_obj_set_pos(obj, 10, 20);
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//Or in one function
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lv_obj_align(obj, LV_ALIGN_CENTER, 10, 20);
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```
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To align one object to another use: `lv_obj_align_to(obj_to_align, obj_referece, LV_ALIGN_..., x, y)`
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For example, to align a text below an image: `lv_obj_align_to(text, image, LV_ALIGN_OUT_BOTTOM_MID, 0, 10)`.
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The following align types exist:
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### Parents and children
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You can set a new parent for an object with `lv_obj_set_parent(obj, new_parent)`. To get the current parent, use `lv_obj_get_parent(obj)`.
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To get a specific child of a parent use `lv_obj_get_child(parent, idx)`. Some examples for `idx`:
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- `0` get the child created first
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- `1` get the child created second
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- `-1` get the child created last
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The children can be iterated lke this:
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```c
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uint32_t i;
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for(i = 0; i < lv_obj_get_child_cnt(parent); i++) {
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lv_obj_t * child = lv_obj_get_child(parent, i);
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/*Do something with child*/
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}
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```
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`lv_obj_get_index(obj)` returns the index of the object in its parent. It is equivalent to the number of younger children in the parent.
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You can bring an object to the foreground or send it to the background with `lv_obj_move_foreground(obj)` and `lv_obj_move_background(obj)`.
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You can change the index of an object in its parent using `lv_obj_move_to_index(obj, index)`.
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You can swap the position of two objects with `lv_obj_swap(obj1, obj2)`.
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### Display and Screens
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At the highest level of the LVGL object hierarchy is the *display* which represents the driver for a display device (physical display or simulator). A display can have one or more screens associated with it. Each screen contains a hierarchy of objects for graphical widgets representing a layout that covers the entire display.
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When you have created a screen like `lv_obj_t * screen = lv_obj_create(NULL)`, you can make it active with `lv_scr_load(screen)`. The `lv_scr_act()` function gives you a pointer to the active screen.
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If you have multiple displays, it's important to know that the screen functions operate on the most recently created display or the one explicitly selected with `lv_disp_set_default`.
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To get an object's screen use the `lv_obj_get_screen(obj)` function.
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### Events
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To set an event callback for an object, use `lv_obj_add_event_cb(obj, event_cb, LV_EVENT_..., user_data)`,
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To manually send an event to an object, use `lv_event_send(obj, LV_EVENT_..., param)`
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Read the [Event overview](/overview/event) to learn more about events.
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### Styles
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Be sure to read the [Style overview](/overview/style). Here only the most essential functions are described.
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A new style can be added to an object with the `lv_obj_add_style(obj, &new_style, selector)` function.
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`selector` is an ORed combination of part and state(s). E.g. `LV_PART_SCROLLBAR | LV_STATE_PRESSED`.
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The base objects use `LV_PART_MAIN` style properties and `LV_PART_SCROLLBAR` with the typical background style properties.
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### Flags
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There are some attributes which can be enabled/disabled by `lv_obj_add/clear_flag(obj, LV_OBJ_FLAG_...)`:
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- `LV_OBJ_FLAG_HIDDEN` Make the object hidden. (Like it wasn't there at all)
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- `LV_OBJ_FLAG_CLICKABLE` Make the object clickable by input devices
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- `LV_OBJ_FLAG_CLICK_FOCUSABLE` Add focused state to the object when clicked
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- `LV_OBJ_FLAG_CHECKABLE` Toggle checked state when the object is clicked
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- `LV_OBJ_FLAG_SCROLLABLE` Make the object scrollable
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- `LV_OBJ_FLAG_SCROLL_ELASTIC` Allow scrolling inside but with slower speed
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- `LV_OBJ_FLAG_SCROLL_MOMENTUM` Make the object scroll further when "thrown"
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- `LV_OBJ_FLAG_SCROLL_ONE` Allow scrolling only one snappable children
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- `LV_OBJ_FLAG_SCROLL_CHAIN_HOR` Allow propagating the horizontal scroll to a parent
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- `LV_OBJ_FLAG_SCROLL_CHAIN_VER` Allow propagating the vertical scroll to a parent
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- `LV_OBJ_FLAG_SCROLL_CHAIN` Simple packaging for (`LV_OBJ_FLAG_SCROLL_CHAIN_HOR | LV_OBJ_FLAG_SCROLL_CHAIN_VER`)
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- `LV_OBJ_FLAG_SCROLL_ON_FOCUS` Automatically scroll object to make it visible when focused
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- `LV_OBJ_FLAG_SCROLL_WITH_ARROW` Allow scrolling the focused object with arrow keys
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- `LV_OBJ_FLAG_SNAPPABLE` If scroll snap is enabled on the parent it can snap to this object
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- `LV_OBJ_FLAG_PRESS_LOCK` Keep the object pressed even if the press slid from the object
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- `LV_OBJ_FLAG_EVENT_BUBBLE` Propagate the events to the parent too
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- `LV_OBJ_FLAG_GESTURE_BUBBLE` Propagate the gestures to the parent
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- `LV_OBJ_FLAG_ADV_HITTEST` Allow performing more accurate hit (click) test. E.g. accounting for rounded corners
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- `LV_OBJ_FLAG_IGNORE_LAYOUT` Make the object positionable by the layouts
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- `LV_OBJ_FLAG_FLOATING` Do not scroll the object when the parent scrolls and ignore layout
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- `LV_OBJ_FLAG_OVERFLOW_VISIBLE` Do not clip the children's content to the parent's boundary
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- `LV_OBJ_FLAG_LAYOUT_1` Custom flag, free to use by layouts
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- `LV_OBJ_FLAG_LAYOUT_2` Custom flag, free to use by layouts
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- `LV_OBJ_FLAG_WIDGET_1` Custom flag, free to use by widget
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- `LV_OBJ_FLAG_WIDGET_2` Custom flag, free to use by widget
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- `LV_OBJ_FLAG_USER_1` Custom flag, free to use by user
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- `LV_OBJ_FLAG_USER_2` Custom flag, free to use by user
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- `LV_OBJ_FLAG_USER_3` Custom flag, free to use by user
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- `LV_OBJ_FLAG_USER_4` Custom flag, free to use by user
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Some examples:
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```c
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/*Hide on object*/
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lv_obj_add_flag(obj, LV_OBJ_FLAG_HIDDEN);
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/*Make an object non-clickable*/
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lv_obj_clear_flag(obj, LV_OBJ_FLAG_CLICKABLE);
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```
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### Groups
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Read the [Input devices overview](/overview/indev) to learn more about *Groups*.
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Objects are added to a *group* with `lv_group_add_obj(group, obj)`, and you can use `lv_obj_get_group(obj)` to see which group an object belongs to.
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`lv_obj_is_focused(obj)` returns if the object is currently focused on its group or not. If the object is not added to a group, `false` will be returned.
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### Extended click area
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By default, the objects can be clicked only within their bounding area. However, this can be extended with `lv_obj_set_ext_click_area(obj, size)`.
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## Events
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- `LV_EVENT_VALUE_CHANGED` when the `LV_OBJ_FLAG_CHECKABLE` flag is enabled and the object clicked (on transition to/from the checked state)
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- `LV_EVENT_DRAW_PART_BEGIN` and `LV_EVENT_DRAW_PART_END` is sent for the following types:
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- `LV_OBJ_DRAW_PART_RECTANGLE` The main rectangle
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- `part`: `LV_PART_MAIN`
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- `rect_dsc`
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- `draw_area`: the area of the rectangle
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- `LV_OBJ_DRAW_PART_BORDER_POST` The border if the `border_post` style property is `true`
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- `part`: `LV_PART_MAIN`
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- `rect_dsc`
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- `draw_area`: the area of the rectangle
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- `LV_OBJ_DRAW_PART_SCROLLBAR` the scrollbars
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- `part`: `LV_PART_SCROLLBAR`
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- `rect_dsc`
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- `draw_area`: the area of the rectangle
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Learn more about [Events](/overview/event).
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## Keys
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If `LV_OBJ_FLAG_CHECKABLE` is enabled, `LV_KEY_RIGHT` and `LV_KEY_UP` make the object checked, and `LV_KEY_LEFT` and `LV_KEY_DOWN` make it unchecked.
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If `LV_OBJ_FLAG_SCROLLABLE` is enabled, but the object is not editable (as declared by the widget class), the arrow keys (`LV_KEY_UP`, `LV_KEY_DOWN`, `LV_KEY_LEFT`, `LV_KEY_RIGHT`) scroll the object. If the object can only scroll vertically, `LV_KEY_LEFT` and `LV_KEY_RIGHT` will scroll up/down instead, making it compatible with an encoder input device. See [Input devices overview](/overview/indev) for more on encoder behaviors and the edit mode.
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Learn more about [Keys](/overview/indev).
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## Example
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```eval_rst
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.. include:: ../../examples/widgets/obj/index.rst
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```
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## API
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```eval_rst
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.. doxygenfile:: lv_obj.h
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:project: lvgl
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```
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