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ui / text_debug - See this example in Bevy github repo
use bevy::{
    diagnostic::{Diagnostics, FrameTimeDiagnosticsPlugin},
    prelude::*,
};

/// This example is for debugging text layout
fn main() {
    App::new()
        .insert_resource(WindowDescriptor {
            vsync: false,
            ..Default::default()
        })
        .add_plugins(DefaultPlugins)
        .add_plugin(FrameTimeDiagnosticsPlugin)
        .add_startup_system(infotext_system)
        .add_system(change_text_system)
        .run();
}

#[derive(Component)]
struct TextChanges;

fn infotext_system(mut commands: Commands, asset_server: Res<AssetServer>) {
    let font = asset_server.load("fonts/FiraSans-Bold.ttf");
    commands.spawn_bundle(UiCameraBundle::default());
    commands.spawn_bundle(TextBundle {
        style: Style {
            align_self: AlignSelf::FlexEnd,
            position_type: PositionType::Absolute,
            position: Rect {
                top: Val::Px(5.0),
                left: Val::Px(15.0),
                ..Default::default()
            },
            ..Default::default()
        },
        text: Text::with_section(
            "This is\ntext with\nline breaks\nin the top left",
            TextStyle {
                font: font.clone(),
                font_size: 50.0,
                color: Color::WHITE,
            },
            Default::default(),
        ),
        ..Default::default()
    });
    commands.spawn_bundle(TextBundle {
        style: Style {
            align_self: AlignSelf::FlexEnd,
            position_type: PositionType::Absolute,
            position: Rect {
                top: Val::Px(5.0),
                right: Val::Px(15.0),
                ..Default::default()
            },
            max_size: Size {
                width: Val::Px(400.),
                height: Val::Undefined,
            },
            ..Default::default()
        },
        text: Text::with_section(
                    "This text is very long, has a limited width, is centred, is positioned in the top right and is also coloured pink.",
                        TextStyle {
                    font: font.clone(),
                    font_size: 50.0,
                    color: Color::rgb(0.8, 0.2, 0.7),
                },
            TextAlignment {
                horizontal: HorizontalAlign::Center,
                vertical: VerticalAlign::Center,
            },
        ),
        ..Default::default()
    });
    commands
        .spawn_bundle(TextBundle {
            style: Style {
                align_self: AlignSelf::FlexEnd,
                position_type: PositionType::Absolute,
                position: Rect {
                    bottom: Val::Px(5.0),
                    right: Val::Px(15.0),
                    ..Default::default()
                },
                ..Default::default()
            },
            text: Text {
                sections: vec![
                    TextSection {
                        value: "This text changes in the bottom right".to_string(),
                        style: TextStyle {
                            font: font.clone(),
                            font_size: 30.0,
                            color: Color::WHITE,
                        },
                    },
                    TextSection {
                        value: "\nThis text changes in the bottom right - ".to_string(),
                        style: TextStyle {
                            font: font.clone(),
                            font_size: 30.0,
                            color: Color::RED,
                        },
                    },
                    TextSection {
                        value: "".to_string(),
                        style: TextStyle {
                            font: font.clone(),
                            font_size: 30.0,
                            color: Color::ORANGE_RED,
                        },
                    },
                    TextSection {
                        value: " fps, ".to_string(),
                        style: TextStyle {
                            font: font.clone(),
                            font_size: 30.0,
                            color: Color::YELLOW,
                        },
                    },
                    TextSection {
                        value: "".to_string(),
                        style: TextStyle {
                            font: font.clone(),
                            font_size: 30.0,
                            color: Color::GREEN,
                        },
                    },
                    TextSection {
                        value: " ms/frame".to_string(),
                        style: TextStyle {
                            font: font.clone(),
                            font_size: 30.0,
                            color: Color::BLUE,
                        },
                    },
                ],
                alignment: Default::default(),
            },
            ..Default::default()
        })
        .insert(TextChanges);
    commands.spawn_bundle(TextBundle {
        style: Style {
            align_self: AlignSelf::FlexEnd,
            position_type: PositionType::Absolute,
            position: Rect {
                bottom: Val::Px(5.0),
                left: Val::Px(15.0),
                ..Default::default()
            },
            size: Size {
                width: Val::Px(200.0),
                ..Default::default()
            },
            ..Default::default()
        },
        text: Text::with_section(
            "This\ntext has\nline breaks and also a set width in the bottom left".to_string(),
            TextStyle {
                font,
                font_size: 50.0,
                color: Color::WHITE,
            },
            Default::default(),
        ),
        ..Default::default()
    });
}

fn change_text_system(
    time: Res<Time>,
    diagnostics: Res<Diagnostics>,
    mut query: Query<&mut Text, With<TextChanges>>,
) {
    for mut text in query.iter_mut() {
        let mut fps = 0.0;
        if let Some(fps_diagnostic) = diagnostics.get(FrameTimeDiagnosticsPlugin::FPS) {
            if let Some(fps_avg) = fps_diagnostic.average() {
                fps = fps_avg;
            }
        }

        let mut frame_time = time.delta_seconds_f64();
        if let Some(frame_time_diagnostic) = diagnostics.get(FrameTimeDiagnosticsPlugin::FRAME_TIME)
        {
            if let Some(frame_time_avg) = frame_time_diagnostic.average() {
                frame_time = frame_time_avg;
            }
        }

        text.sections[0].value = format!(
            "This text changes in the bottom right - {:.1} fps, {:.3} ms/frame",
            fps,
            frame_time * 1000.0,
        );

        text.sections[2].value = format!("{:.1}", fps);

        text.sections[4].value = format!("{:.3}", frame_time * 1000.0);
    }
}