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Positions, Sizes and Layouts

Overview

Similar to many other parts of LVGL, the concept of setting the coordinates was inspired by CSS. LVGL has by no means a complete implementation of CSS but a comparable subset is implemented (sometimes with minor adjustments).

In short this means:

• Explicitly set coordinates are stored in styles (position, size, layouts, etc.)

• support min-width, max-width, min-height, max-height

• have pixel, percentage, and "content" units

• x=0; y=0 coordinate means the top-left corner of the parent plus the left/top padding plus border width

• width/height means the full size, the "content area" is smaller with padding and border width

• a subset of flexbox and grid layouts are supported

Units

• pixel: Simply a position in pixels. An integer always means pixels. E.g. lv_obj_set_x(btn, 10)

• percentage: The percentage of the size of the Widget or its parent (depending on the property). lv_pct(value) converts a value to percentage. E.g. lv_obj_set_width(btn, lv_pct(50))

• LV_SIZE_CONTENT: Special value to set the width/height of an Widget to involve all the children. It's similar to auto in CSS. E.g. lv_obj_set_width(btn, LV_SIZE_CONTENT).

Boxing Model

LVGL follows CSS's border-box model. A Widget's "box" is built from the following parts:

• bounding box: the width/height of the elements.

• border width: the width of the border.

• padding: space between the sides of the Widget and its children.

• margin: space outside of the Widget (considered only by some layouts)

• content: the content area which is the size of the bounding box reduced by the border width and padding.

The border is drawn inside the bounding box. Inside the border LVGL keeps a "padding margin" when placing a Widget's children.

The outline is drawn outside the bounding box.

Important Notes

This section describes special cases in which LVGL's behavior might be unexpected.

Postponed coordinate calculation

LVGL doesn't recalculate all the coordinate changes immediately. This is done to improve performance. Instead, the Widgets are marked as "dirty" and before redrawing the screen LVGL checks if there are any "dirty" Widgets. If so it refreshes their position, size and layout.

In other words, if you need to get the coordinate of a Widget and the coordinates were just changed, LVGL needs to be forced to recalculate the coordinates. To do this call lv_obj_update_layout().

The size and position might depend on the parent or layout. Therefore lv_obj_update_layout() recalculates the coordinates of all Widgets on the screen of obj.

Removing styles

As it's described in the Using styles section, coordinates can also be set via style properties. To be more precise, under the hood every style coordinate related property is stored as a style property. If you use lv_obj_set_x(widget, 20) LVGL saves x=20 in the local style of the Widget.

This is an internal mechanism and doesn't matter much as you use LVGL. However, there is one case in which you need to be aware of the implementation. If the style(s) of a Widget are removed by

lv_obj_remove_style_all(widget)

or

lv_obj_remove_style(widget, NULL, LV_PART_MAIN);

the earlier set coordinates will be removed as well.

For example:

/* The size of obj1 will be set back to the default in the end */
lv_obj_set_size(widget1, 200, 100);  /* Now obj1 has 200;100 size */
lv_obj_remove_style_all(widget1);    /* It removes the set sizes */


/* widget2 will have 200;100 size in the end */
lv_obj_remove_style_all(widget2);
lv_obj_set_size(widget2, 200, 100);

Positioning Widgets

Direct

To simply set the x and y coordinates of a Widget use:

lv_obj_set_x(widget, 10);          /* Separate... */
lv_obj_set_y(widget, 20);
lv_obj_set_pos(widget, 10, 20);    /* Or in one function */

By default, the x and y coordinates are measured from the top left corner of the parent's content area. For example if the parent has five pixels of padding on every side the above code will place obj at (15, 25) because the content area starts after the padding.

Percentage values are calculated from the parent's content area size.

lv_obj_set_x(btn, lv_pct(10)); //x = 10 % of parent content area width

Alignment

Inside parent widget

In many cases it is more convenient to tell LVGL to align your object relative to an "anchor" in its parent other than its upper left corner. To establish that "anchor", call lv_obj_set_align(widget, LV_ALIGN_...). After that call, that "anchor" will be remembered until another one is established. In other words, every futire x and y setting for that Widget will be relative to the that "anchor".

Example: Position Widget (10,20) px relative to the center of its parent:

lv_obj_set_align(widget, LV_ALIGN_CENTER);
lv_obj_set_pos(widget, 10, 20);

/* Or combine the above in one function... */
lv_obj_align(widget, LV_ALIGN_CENTER, 10, 20);

9 convenient "anchors" can be used with these functions:

• LV_ALIGN_TOP_LEFT

• LV_ALIGN_TOP_MID

• LV_ALIGN_TOP_RIGHT

• LV_ALIGN_BOTTOM_LEFT

• LV_ALIGN_BOTTOM_MID

• LV_ALIGN_BOTTOM_RIGHT

• LV_ALIGN_LEFT_MID

• LV_ALIGN_RIGHT_MID

• LV_ALIGN_CENTER

See illustration below to visualize what these mean.

It's quite common to align a child to the center of its parent, therefore a dedicated function exists:

lv_obj_center(widget);

//Has the same effect
lv_obj_align(widget, LV_ALIGN_CENTER, 0, 0);

If the parent's size changes, the set alignment and position of the children is updated automatically.

Relative to another Widget

Alternately, you can choose an "anchor" on another Widget.

lv_obj_align_to(widget, reference_widget, align, x, y);

where align can be done of the following:

• LV_ALIGN_OUT_TOP_LEFT

• LV_ALIGN_OUT_TOP_MID

• LV_ALIGN_OUT_TOP_RIGHT

• LV_ALIGN_OUT_BOTTOM_LEFT

• LV_ALIGN_OUT_BOTTOM_MID

• LV_ALIGN_OUT_BOTTOM_RIGHT

• LV_ALIGN_OUT_LEFT_TOP

• LV_ALIGN_OUT_LEFT_MID

• LV_ALIGN_OUT_LEFT_BOTTOM

• LV_ALIGN_OUT_RIGHT_TOP

• LV_ALIGN_OUT_RIGHT_MID

• LV_ALIGN_OUT_RIGHT_BOTTOM

Example: to horizontally center a label 10 pixels above a button:

lv_obj_align_to(label, btn, LV_ALIGN_OUT_TOP_MID, 0, -10);

Note that, unlike with lv_obj_align(), lv_obj_align_to() does not remember the "anchor" used, and so will not automatically reposition the aligned widget if the reference widget later moves.

The following illustration shows the meaning of each "anchor" mentioned above.

Size

Sizing the simple way

The width and the height of a Widget can be set easily as well:

lv_obj_set_width(widget, 200);         /* Separate... */
lv_obj_set_height(widget, 100);
lv_obj_set_size(widget, 200, 100);     /* Or in one function */

Percentage values are calculated based on the parent's content area size. For example to set the Widget's height to the screen height:

lv_obj_set_height(widget, lv_pct(100));

The size settings support a special value: LV_SIZE_CONTENT. It means the Widget's size in the respective direction will be set to the size of its children. Note that only children on the right and bottom sides will be considered and children on the top and left remain cropped. This limitation makes the behavior more predictable.

Widgets with LV_OBJ_FLAG_HIDDEN or LV_OBJ_FLAG_FLOATING will be ignored by the LV_SIZE_CONTENT calculation.

The above functions set the size of a Widget's bounding box but the size of the content area can be set as well. This means a Widget's bounding box will be enlarged with the addition of padding.

lv_obj_set_content_width(widget, 50);  /* The actual width: padding left + 50 + padding right */
lv_obj_set_content_height(widget, 30); /* The actual width: padding top + 30 + padding bottom */

The size of the bounding box and the content area can be retrieved with the following functions:

int32_t w = lv_obj_get_width(widget);
int32_t h = lv_obj_get_height(widget);
int32_t content_w = lv_obj_get_content_width(widget);
int32_t content_h = lv_obj_get_content_height(widget);

Extending the click area

By default, Widgets can be clicked only within their bounding area. However, especially with small Widgets, it can be helpful to make a Widget's "clickable" area larger. You can do this with lv_obj_set_ext_click_area(widget, size).

Using styles

Under the hood the position, size and alignment properties are style properties. The above described "simple functions" hide the style related code for the sake of simplicity and set the position, size, and alignment properties in the local styles of the Widget.

However, using styles to set the coordinates has some great advantages:

• It makes it easy to set the width/height/etc. for several Widgets together. E.g. make all the sliders 100x10 pixels sized.

• It also makes possible to modify the values in one place.

• The values can be partially overwritten by other styles. For example style_btn makes the Widget 100x50 by default but adding style_full_width overwrites only the width of the Widget.

• The Widget can have different position or size depending on state. E.g. 100 px wide in LV_STATE_DEFAULT but 120 px in LV_STATE_PRESSED.

• Style transitions can be used to make the coordinate changes smooth.

Here are some examples to set a Widget's size using a style:

static lv_style_t style;
lv_style_init(&style);
lv_style_set_width(&style, 100);

lv_obj_t * btn = lv_button_create(lv_screen_active());
lv_obj_add_style(btn, &style, LV_PART_MAIN);

As you will see below there are some other great features of size and position setting. However, to keep the LVGL API lean, only the most common coordinate setting features have a "simple" version and the more complex features can be used via styles.

Translation

Let's say the there are 3 buttons next to each other. Their position is set as described above. Now you want to move a button up a little when it's pressed.

One way to achieve this is by setting a new Y coordinate for the pressed state:

static lv_style_t style_normal;
lv_style_init(&style_normal);
lv_style_set_y(&style_normal, 100);

static lv_style_t style_pressed;
lv_style_init(&style_pressed);
lv_style_set_y(&style_pressed, 80);

lv_obj_add_style(btn1, &style_normal, LV_STATE_DEFAULT);
lv_obj_add_style(btn1, &style_pressed, LV_STATE_PRESSED);

lv_obj_add_style(btn2, &style_normal, LV_STATE_DEFAULT);
lv_obj_add_style(btn2, &style_pressed, LV_STATE_PRESSED);

lv_obj_add_style(btn3, &style_normal, LV_STATE_DEFAULT);
lv_obj_add_style(btn3, &style_pressed, LV_STATE_PRESSED);

This works, but it's not really flexible because the pressed coordinate is hard-coded. If the buttons are not at y=100, style_pressed won't work as expected. Translations can be used to solve this:

static lv_style_t style_normal;
lv_style_init(&style_normal);
lv_style_set_y(&style_normal, 100);

static lv_style_t style_pressed;
lv_style_init(&style_pressed);
lv_style_set_translate_y(&style_pressed, -20);

lv_obj_add_style(btn1, &style_normal, LV_STATE_DEFAULT);
lv_obj_add_style(btn1, &style_pressed, LV_STATE_PRESSED);

lv_obj_add_style(btn2, &style_normal, LV_STATE_DEFAULT);
lv_obj_add_style(btn2, &style_pressed, LV_STATE_PRESSED);

lv_obj_add_style(btn3, &style_normal, LV_STATE_DEFAULT);
lv_obj_add_style(btn3, &style_pressed, LV_STATE_PRESSED);

Translation is applied from the current position of the Widget.

Percentage values can be used in translations as well. The percentage is relative to the size of the Widget (and not to the size of the parent). For example lv_pct(50) will move the Widget with half of its width/height.

The translation is applied after the layouts are calculated. Therefore, even laid out Widgets' position can be translated.

The translation actually moves the Widget. That means it makes the scrollbars and LV_SIZE_CONTENT sized Widgets react to the position change.

Transformation

Similarly to position, a Widget's size can be changed relative to the current size as well. The transformed width and height are added on both sides of the Widget. This means a 10 px transformed width makes the Widget 2x10 pixels wider.

Unlike position translation, the size transformation doesn't make the Widget "really" larger. In other words scrollbars, layouts, and LV_SIZE_CONTENT will not react to the transformed size. Hence, size transformation is "only" a visual effect.

This code enlarges a button when it's pressed:

static lv_style_t style_pressed;
lv_style_init(&style_pressed);
lv_style_set_transform_width(&style_pressed, 10);
lv_style_set_transform_height(&style_pressed, 10);

lv_obj_add_style(btn, &style_pressed, LV_STATE_PRESSED);

Min and Max size

Similarly to CSS, LVGL also supports min-width, max-width, min-height and max-height. These are limits preventing a Widget's size from becoming smaller/larger than these values. They are especially useful if the size is set by percentage or LV_SIZE_CONTENT.

static lv_style_t style_max_height;
lv_style_init(&style_max_height);
lv_style_set_y(&style_max_height, 200);

lv_obj_set_height(widget, lv_pct(100));
lv_obj_add_style(widget, &style_max_height, LV_STATE_DEFAULT); //Limit the  height to 200 px

Percentage values can be used as well which are relative to the size of the parent's content area.

static lv_style_t style_max_height;
lv_style_init(&style_max_height);
lv_style_set_y(&style_max_height, lv_pct(50));

lv_obj_set_height(widget, lv_pct(100));
lv_obj_add_style(widget, &style_max_height, LV_STATE_DEFAULT); //Limit the height to half parent height

Layout

Layout overview

Layouts can update the position and size of a Widget's children. They can be used to automatically arrange the children into a line or column, or in much more complicated forms.

The position and size set by the layout overwrites the "normal" x, y, width, and height settings.

There is only one function that is the same for every layout: lv_obj_set_layout() (widget, <LAYOUT_NAME>) sets the layout on a Widget. For further settings of the parent and children see the documentation of the given layout.

Built-in layouts

LVGL comes with two very powerful layouts:

• Flexbox: arrange Widgets into rows or columns, with support for wrapping and expanding items.

• Grid: arrange Widgets into fixed positions in 2D table.

Both are heavily inspired by the CSS layouts with the same name. Layouts are described in detail in their own section of documentation.

Flags

There are some flags that can be used on Widgets to affect how they behave with layouts:

• LV_OBJ_FLAG_HIDDEN Hidden Widgets are ignored in layout calculations.

• LV_OBJ_FLAG_IGNORE_LAYOUT The Widget is simply ignored by the layouts. Its coordinates can be set as usual.

• LV_OBJ_FLAG_FLOATING Same as LV_OBJ_FLAG_IGNORE_LAYOUT but the Widget with LV_OBJ_FLAG_FLOATING will be ignored in LV_SIZE_CONTENT calculations.

These flags can be added/removed with lv_obj_add_flag(widget, FLAG) and lv_obj_remove_flag(widget, FLAG)

Adding new layouts

LVGL can be freely extended by a custom layout like this:

uint32_t MY_LAYOUT;

...

MY_LAYOUT = lv_layout_register(my_layout_update, &user_data);

...

void my_layout_update(lv_obj_t * widget, void * user_data)
{
    /* Will be called automatically if it's required to reposition/resize the children of "obj" */
}

Custom style properties can be added which can be retrieved and used in the update callback. For example:

uint32_t MY_PROP;
...

LV_STYLE_MY_PROP = lv_style_register_prop();

...
static inline void lv_style_set_my_prop(lv_style_t * style, uint32_t value)
{
    lv_style_value_t v = {
        .num = (int32_t)value
    };
    lv_style_set_prop(style, LV_STYLE_MY_PROP, v);
}

Examples

API

lv_api_map_v8.h