Screens

Overview

Screens work very similarly to Components. Both can be:

• Loaded from XML

• Contain Widgets and Components as children

Screens are wrapped in the <screen> XML root element and used to organize the content of the UI.

Screens can have only the following child XML tags:

• <consts>

• <styles>

• <view>

That is, Screens cannot have:

• <api>: Screens are always created as they are, with no parameters.

• <preview>: In LVGL's UI Editor, <preview> is used to set the style, size, and other parameters of the previewing "screen". For Screens, the preview options are defined in project.xml. Learn more in Screen preview

Usage

Each XML file describes a Screen. The name of the XML file will also be the name of the Screen.

This example illustrates a screen in XML. In the example, a <my_header> and a <my_main_cont> component are used to keep the screen simple.

<screen>
    <consts>
        <string name="title" value="Main menu"/>
    </consts>

    <styles>
        <style name="dark" bg_color="0x333"/>
    </styles>

    <view>
        <my_header label="#title"/>
        <my_main_cont>
            <style name="dark"/>

            <button text="Weather" icon="cloudy"/>
            <button text="Messages" icon="envelope"/>
            <button text="Settings" icon="cogwheel"/>
            <button text="About" icon="questionmark"/>
        </my_main_cont>
    </view>
</screen>

Code export

LVGL's UI Editor can export C code for Screens as well. It generates screen_name_gen.c/h files, containing only a single lv_obj_t * screen_name_create(void) create function.

By using this function, any number of screen instances can be created and loaded as needed.

Permanent Screens

The only possible property of the <screen> tag is permanent, which can be true or false (default).

This property affects the screen load and create events. If a screen is permanent, then when a new screen is loaded, the permanent screen is not deleted so that it keeps its state. Non-permanent screens will be deleted and created automatically when they are opened and closed.

It's also assumed that permanent screens are created when the UI is initialized. When code is exported from the UI Editor, the permanent screens are actually created and stored in global pointers.

In light of that, permanent screens only need to be loaded by <screen_load_event screen="my_permanent_screen"/> and non-permanent screens need to be created by <screen_create_event screen="my_non_permanent_screen"/>.

Preview

Screens don't support the <preview> tag because it doesn't make sense to preview each screen in different resolutions.

As Screens are related to the target hardware in the project.xml file, multiple <display> elements can be defined. In the UI Editor, when a Screen is being developed, the user can select from all the defined displays in the Preview, and the Screen will be shown with the given resolution and color depth.

This is useful for verifying responsive designs.

Events

It's very common to load or create Screens on a button click or other events.

Both are supported by adding special XML tags as children of Components or Widgets:

<view>
    <lv_button>
        <lv_label text="Click or Long press me"/>

        <!-- Create an instance of "screen2" and load it. -->
        <screen_load_event screen="first" trigger="clicked" anim_type="fade"/>

         <!-- Load an already created instance of screen1.-->
        <screen_create_event screen="about" trigger="long_pressed"/>
    </lv_button>
</view>

Learn more on the XML Events page.