1 2# Flex 3 4## Overview 5 6The Flexbox (or Flex for short) is a subset of [CSS Flexbox](https://css-tricks.com/snippets/css/a-guide-to-flexbox/). 7 8It can arrange items into rows or columns (tracks), handle wrapping, adjust the spacing between the items and tracks, handle *grow* to make the item(s) fill the remaining space with respect to min/max width and height. 9 10To make an object flex container call `lv_obj_set_layout(obj, LV_LAYOUT_FLEX)`. 11 12Note that the flex layout feature of LVGL needs to be globally enabled with `LV_USE_FLEX` in `lv_conf.h`. 13 14## Terms 15- tracks: the rows or columns 16- main direction: row or column, the direction in which the items are placed 17- cross direction: perpendicular to the main direction 18- wrap: if there is no more space in the track a new track is started 19- grow: if set on an item it will grow to fill the remaining space on the track. 20The available space will be distributed among items respective to their grow value (larger value means more space) 21- gap: the space between the rows and columns or the items on a track 22 23## Simple interface 24 25With the following functions you can set a Flex layout on any parent. 26 27### Flex flow 28 29`lv_obj_set_flex_flow(obj, flex_flow)` 30 31The possible values for `flex_flow` are: 32- `LV_FLEX_FLOW_ROW` Place the children in a row without wrapping 33- `LV_FLEX_FLOW_COLUMN` Place the children in a column without wrapping 34- `LV_FLEX_FLOW_ROW_WRAP` Place the children in a row with wrapping 35- `LV_FLEX_FLOW_COLUMN_WRAP` Place the children in a column with wrapping 36- `LV_FLEX_FLOW_ROW_REVERSE` Place the children in a row without wrapping but in reversed order 37- `LV_FLEX_FLOW_COLUMN_REVERSE` Place the children in a column without wrapping but in reversed order 38- `LV_FLEX_FLOW_ROW_WRAP_REVERSE` Place the children in a row with wrapping but in reversed order 39- `LV_FLEX_FLOW_COLUMN_WRAP_REVERSE` Place the children in a column with wrapping but in reversed order 40 41### Flex align 42To manage the placement of the children use `lv_obj_set_flex_align(obj, main_place, cross_place, track_cross_place)` 43 44- `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) 45- `cross_place` determines how to distribute the items in their track on the cross axis. E.g. if the items have different height place them to the bottom of the track. (It's called `align-items` in CSS) 46- `track_cross_place` determines how to distribute the tracks (It's called `align-content` in CSS) 47 48The possible values are: 49- `LV_FLEX_ALIGN_START` means left on a horizontally and top vertically. (default) 50- `LV_FLEX_ALIGN_END` means right on a horizontally and bottom vertically 51- `LV_FLEX_ALIGN_CENTER` simply center 52- `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`. 53- `LV_FLEX_ALIGN_SPACE_AROUND` items are evenly distributed in the track with equal space around them. 54Note that visually the spaces aren’t equal, since all the items have equal space on both sides. 55The first item will have one unit of space against the container edge, but two units of space between the next item because that next item has its own spacing that applies. Not applies to `track_cross_place`. 56- `LV_FLEX_ALIGN_SPACE_BETWEEN` items are evenly distributed in the track: first item is on the start line, last item on the end line. Not applies to `track_cross_place`. 57 58 59### Flex grow 60 61Flex grow can be used to make one or more children fill the available space on the track. When more children have grow parameters, the available space will be distributed proportionally to the grow values. 62For example, there is 400 px remaining space and 4 objects with grow: 63- `A` with grow = 1 64- `B` with grow = 1 65- `C` with grow = 2 66 67`A` and `B` will have 100 px size, and `C` will have 200 px size. 68 69Flex grow can be set on a child with `lv_obj_set_flex_grow(child, value)`. `value` needs to be > 1 or 0 to disable grow on the child. 70 71 72## Style interface 73 74All the Flex-related values are style properties under the hood and you can use them similarly to any other style property. The following flex related style properties exist: 75 76- `FLEX_FLOW` 77- `FLEX_MAIN_PLACE` 78- `FLEX_CROSS_PLACE` 79- `FLEX_TRACK_PLACE` 80- `FLEX_GROW` 81 82### Internal padding 83 84To modify the minimum space flexbox inserts between objects, the following properties can be set on the flex container style: 85 86- `pad_row` Sets the padding between the rows. 87 88- `pad_column` Sets the padding between the columns. 89 90These can for example be used if you don't want any padding between your objects: `lv_style_set_pad_column(&row_container_style,0)` 91 92## Other features 93 94### RTL 95If 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. 96 97The items on `ROW` layouts, and tracks of `COLUMN` layouts will be placed from right to left. 98 99### New track 100 101You can force Flex to put an item into a new line with `lv_obj_add_flag(child, LV_OBJ_FLAG_FLEX_IN_NEW_TRACK)`. 102 103 104## Example 105 106```eval_rst 107 108.. include:: ../../examples/layouts/flex/index.rst 109 110``` 111 112## API 113 114```eval_rst 115 116.. doxygenfile:: lv_flex.h 117 :project: lvgl 118 119``` 120
