Flex

The Flexbox (or Flex for short) is a subset of CSS Flexbox behaviors.

Overview

The Flexbox (or Flex for short) is a subset of CSS Flexbox behaviors.

It can arrange items (child Widgets) into rows or columns (tracks), handle wrapping, adjust the spacing between items and tracks, handle grow to make item(s) fill remaining space with respect to min/max width and height.

To make a Widget a Flex container call lv_obj_set_layout(widget, LV_LAYOUT_FLEX).

Note that the Flex layout feature of LVGL needs to be globally enabled with LV_USE_FLEX in lv_conf.h.

Terms

• tracks: rows or columns
• main direction: row or column, the direction in which multiple items are placed first
• cross direction: the direction perpendicular to the main direction
• wrap: if there is no more space in the track, a new track is started
• grow: if set on an item it will "grow" to fill the remaining space in the track. The available space will be distributed among items respective to their grow value (larger value means more space)
• gap: the space between rows and columns or the items on a track

See CSS Flexbox for illustrations showing the meanings of these terms.

Simple Interface

Use the following functions to set and control the Flex layout on any parent Widget.

Flex flow

lv_obj_set_flex_flow(widget, flex_flow)

The possible values for flex_flow are:

• LV_FLEX_FLOW_ROW: Place the children in a row without wrapping
• LV_FLEX_FLOW_COLUMN: Place the children in a column without wrapping
• LV_FLEX_FLOW_ROW_WRAP: Place the children in a row with wrapping
• LV_FLEX_FLOW_COLUMN_WRAP: Place the children in a column with wrapping
• LV_FLEX_FLOW_ROW_REVERSE: Place the children in a row without wrapping but in reversed order
• LV_FLEX_FLOW_COLUMN_REVERSE: Place the children in a column without wrapping but in reversed order
• LV_FLEX_FLOW_ROW_WRAP_REVERSE: Place the children in a row with wrapping but in reversed order
• LV_FLEX_FLOW_COLUMN_WRAP_REVERSE: Place the children in a column with wrapping but in reversed order

These values cause the Widget's layout behavior to model CSS Flexbox behavior by combining flex-direction_ and flex-wrap_ as defined under flex-flow_.

Flex align

To manage placement of children use

lv_obj_set_flex_align(widget, main_place, cross_place, track_cross_place)

which makes the parent Widget model the Flex-container behavior defined here.

• main_place determines how to distribute the items in their track on the main axis. E.g. flush the items to the right on LV_FLEX_FLOW_ROW_WRAP. (It's called justify-content_ in CSS.)
• cross_place determines how to distribute the items in their track on the cross axis. E.g. if the items have different height, align them against the bottom of the track. (It's called align-items_ in CSS.)
• track_cross_place determines how to distribute the tracks (It's called align-content_ in CSS.)

The possible values are:

• LV_FLEX_ALIGN_START: means left when direction is horizontal, top when vertical (default)
• LV_FLEX_ALIGN_END: means right when direction is horizontal, bottom when vertical
• LV_FLEX_ALIGN_CENTER: simply center with respect to direction
• LV_FLEX_ALIGN_SPACE_EVENLY: items are distributed so that the spacing between any two items (and the space to the edges) is equal. Does not apply to track_cross_place.
• LV_FLEX_ALIGN_SPACE_AROUND: items are evenly distributed in the track with equal space around them. Note that visually the spaces are not equal since all the items have equal space on both sides. This makes the space between items double the space between edge items and the container's edge. Does not apply to track_cross_place.
• LV_FLEX_ALIGN_SPACE_BETWEEN: items are evenly distributed in the track with no space before and after first and last items. Does not apply to track_cross_place.

See justify-content_, align-items_ and align-content_ for illustrations of these values.

Flex grow

Flex grow can be used to make one or more children fill available space in the track. When more than one child Widget have non-zero grow values, all available space will be distributed in proportion to their respective grow values. For example, if there is 400 px space remaining and 3 child Widgets with non-zero grow values:

• A with grow = 1
• B with grow = 1
• C with grow = 2

A and B will occupy 100 px, and C will occupy 200 px.

Flex grow can be set on a child Widget with lv_obj_set_flex_grow(child, value). value needs to be >= 1 or 0 to disable grow on the child.

See flex-grow_ for an illustration of this behavior.

Style Interface

All Flex-related values are style properties under the hood so you can use them as you would any other style property.

The following flex related style properties exist:

• FLEX_FLOW
• FLEX_MAIN_PLACE
• FLEX_CROSS_PLACE
• FLEX_TRACK_PLACE
• FLEX_GROW

Internal padding

To modify the minimum space flexbox inserts between Widgets, the following functions can be used to set the flex container padding style:

• lv_style_set_pad_row sets padding between rows.
• lv_style_set_pad_column sets padding between columns.

These can, for example, be used if you do not want any padding between Widgets: lv_style_set_pad_column(&row_container_style, 0)

Other Features

RTL

If the base direction of the container is set the LV_BASE_DIR_RTL the meaning of LV_FLEX_ALIGN_START and LV_FLEX_ALIGN_END is swapped on ROW layouts. I.e. START will mean right.

The items on ROW layouts, and tracks of COLUMN layouts will be placed from right to left.

New track

You can force Flex to put an item into a new line with lv_obj_add_flag(child, LV_OBJ_FLAG_FLEX_IN_NEW_TRACK).

Edge cases and notes

The following behaviors clarify interactions between flex grow, wrapping and size constraints:

• Grow with LV_SIZE_CONTENT parent: If a flex container's size on the main axis is LV_SIZE_CONTENT and it contains items with non-zero grow, the grown items initially use their own minimum size when determining the container's content size. If the container has min/max constraints that force a concrete size, the remaining space is then distributed among grow items (respecting each item's min/max).
• Wrapping with grow: For LV_FLEX_FLOW_ROW_WRAP and LV_FLEX_FLOW_COLUMN_WRAP, wrapping is used as expected even when the object is grown in the same direction and its size in that direction is LV_SIZE_CONTENT. In this case the LV_SIZE_CONTENT constraint is ignored to be consistent with how the object size is calculated, and the grown size is used.
• Min/max as content or percent: Item min/max width/height can be set to LV_SIZE_CONTENT or LV_PCT(x). These constraints are respected during grow and wrapping and can clamp size of grown items.
• Gaps with zero-size grow: Padding/gap is still applied even if a grown item ends up with zero size on the main axis; this matches CSS flex behavior.

Examples

A simple row and a column layout with flexbox

Arrange items in rows with wrap and even spacing

Demonstrate flex grow

Demonstrate flex grow

Demonstrate column and row gap style properties

RTL base direction changes order of the items

API