＜Rust＞egui学习之小部件（四）：如何在窗口中添加滚动条Scroll部件？

前言
本专栏是关于Rust的GUI库egui的部件讲解及应用实例分析，主要讲解egui的源代码、部件属性、如何应用。

环境配置
系统：windows
平台：visual studio code
语言：rust
库：egui、eframe

概述
本文是本专栏的第四篇博文，主要讲述滑动条部件的使用。

事实上，类似于iced，egui都提供了示例程序，本专栏的博文都是建立在官方示例程序以及源代码的基础上，进行的实例讲解。
即，本专栏的文章并非只是简单的翻译egui的官方示例与文档，而是针对于官方代码进行的实际使用，会在官方的代码上进行修改，包括解决一些问题。

系列博客链接：
1、＜Rust＞egui学习之小部件（一）：如何在窗口及部件显示中文字符？
2、＜Rust＞egui学习之小部件（二）：如何在egui窗口中添加按钮button以及标签label部件？
3、＜Rust＞egui学习之小部件（三）：如何为窗口UI元件设置布局（间隔、水平、垂直排列）？

部件属性

有时候我们会需要在窗口设置滚动条，或者滑动条这样的部件，来实现滚动操作。在egui中，如果要添加滚动条，则使用ScrollArea部件，它表示的是创造一块区域，这个区域内显示滚动条操作，有两个方向，一个是水平滚动，一个是垂直滚动，可以单独设置，也可以都选择。
ScrollArea的官方定义：

#[derive(Clone, Debug)]     
#[must_use = "You should call .show()"]
pub struct ScrollArea {
    /// Do we have horizontal/vertical scrolling enabled?
    scroll_enabled: Vec2b,

    auto_shrink: Vec2b,
    max_size: Vec2,
    min_scrolled_size: Vec2,
    scroll_bar_visibility: ScrollBarVisibility,
    id_source: Option<Id>,
    offset_x: Option<f32>,
    offset_y: Option<f32>,

    /// If false, we ignore scroll events.
    scrolling_enabled: bool,
    drag_to_scroll: bool,

    /// If true for vertical or horizontal the scroll wheel will stick to the
    /// end position until user manually changes position. It will become true
    /// again once scroll handle makes contact with end.
    stick_to_end: Vec2b,

    /// If false, `scroll_to_*` functions will not be animated
    animated: bool,
}

添加滚动条区域

egui::ScrollArea::both().enable_scrolling(true)  
            .scroll_bar_visibility(egui::scroll_area::ScrollBarVisibility::AlwaysVisible)
            .show_rows(ui,row_height,total_rows,|ui,rowrange|{
                ui.horizontal(|ui|{
                    for i in 0..8{
                        ui.label(format!("label{}",i));
                    }
                });
                ui.vertical(|ui|{
                    for i in 0..10{
                        ui.label(format!("label{}",i));
                    }
                })
            });

在这里插入图片描述

如上图，滚动条在窗口中作为部件显示，其显示区域可以通过布局调整。正常来说，我们只需要滚动条的拖动功能，即无需其状态反馈。
但如果需要也可以设置，ScrollArea有State属性，用于接受对ScrollArea操作时的状态反馈：

let sc1=egui::ScrollArea::both().enable_scrolling(true)  
            .scroll_bar_visibility(egui::scroll_area::ScrollBarVisibility::VisibleWhenNeeded)
            .max_height(200.0)
            .show_rows(ui,row_height,total_rows,|ui,rowrange|{
                ui.set_height(200.0);
                ui.set_width(100.0);
                ui.horizontal(|ui|{
                    for i in 0..8{
                        ui.label(format!("label{}",i));
                    }
                });
                ui.vertical(|ui|{
                    for i in 0..10{
                        ui.label(format!("label{}",i));
                    }
                })
            }); 
            ui.label(format!("scroll area:{}",sc1.state.offset.x));
            ui.label(format!("scroll area:{}",sc1.state.offset.y));

如上，我们添加两个标签，用于显示滚动条滑动时滑块的移动量：
在这里插入图片描述
State参数如下：

#[derive(Clone, Copy, Debug)]          
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
#[cfg_attr(feature = "serde", serde(default))]
pub struct State {
    /// Positive offset means scrolling down/right
    pub offset: Vec2,

    /// If set, quickly but smoothly scroll to this target offset.
    offset_target: [Option<ScrollTarget>; 2],

    /// Were the scroll bars visible last frame?
    show_scroll: Vec2b,

    /// The content were to large to fit large frame.
    content_is_too_large: Vec2b,

    /// Did the user interact (hover or drag) the scroll bars last frame?
    scroll_bar_interaction: Vec2b,

    /// Momentum, used for kinetic scrolling
    #[cfg_attr(feature = "serde", serde(skip))]
    vel: Vec2,

    /// Mouse offset relative to the top of the handle when started moving the handle.
    scroll_start_offset_from_top_left: [Option<f32>; 2],

    /// Is the scroll sticky. This is true while scroll handle is in the end position
    /// and remains that way until the user moves the scroll_handle. Once unstuck (false)
    /// it remains false until the scroll touches the end position, which reenables stickiness.
    scroll_stuck_to_end: Vec2b,

    /// Area that can be dragged. This is the size of the content from the last frame.
    interact_rect: Option<Rect>,
}

如果要设置其样式，那么我们需要设置其Style。
但ScrollArea并没有单独的Style设置，不过可以通用的Style进行设置，可以修改字体尺寸和字体样式：

#[derive(Clone, Debug, PartialEq)]     
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
#[cfg_attr(feature = "serde", serde(default))]
pub struct Style {
    /// If set this will change the default [`TextStyle`] for all widgets.
    ///
    /// On most widgets you can also set an explicit text style,
    /// which will take precedence over this.
    pub override_text_style: Option<TextStyle>,

    /// If set this will change the font family and size for all widgets.
    ///
    /// On most widgets you can also set an explicit text style,
    /// which will take precedence over this.
    pub override_font_id: Option<FontId>,

    /// The [`FontFamily`] and size you want to use for a specific [`TextStyle`].
    ///
    /// The most convenient way to look something up in this is to use [`TextStyle::resolve`].
    ///
    /// If you would like to overwrite app `text_styles`
    ///
    /// ```
    /// # let mut ctx = egui::Context::default();
    /// use egui::FontFamily::Proportional;
    /// use egui::FontId;
    /// use egui::TextStyle::*;
    ///
    /// // Get current context style
    /// let mut style = (*ctx.style()).clone();
    ///
    /// // Redefine text_styles
    /// style.text_styles = [
    ///   (Heading, FontId::new(30.0, Proportional)),
    ///   (Name("Heading2".into()), FontId::new(25.0, Proportional)),
    ///   (Name("Context".into()), FontId::new(23.0, Proportional)),
    ///   (Body, FontId::new(18.0, Proportional)),
    ///   (Monospace, FontId::new(14.0, Proportional)),
    ///   (Button, FontId::new(14.0, Proportional)),
    ///   (Small, FontId::new(10.0, Proportional)),
    /// ].into();
    ///
    /// // Mutate global style with above changes
    /// ctx.set_style(style);
    /// ```
    pub text_styles: BTreeMap<TextStyle, FontId>,

    /// The style to use for [`DragValue`] text.
    pub drag_value_text_style: TextStyle,

    /// How to format numbers as strings, e.g. in a [`crate::DragValue`].
    ///
    /// You can override this to e.g. add thousands separators.
    #[cfg_attr(feature = "serde", serde(skip))]
    pub number_formatter: NumberFormatter,

    /// If set, labels, buttons, etc. will use this to determine whether to wrap the text at the
    /// right edge of the [`Ui`] they are in. By default, this is `None`.
    ///
    /// **Note**: this API is deprecated, use `wrap_mode` instead.
    ///
    /// * `None`: use `wrap_mode` instead
    /// * `Some(true)`: wrap mode defaults to [`crate::TextWrapMode::Wrap`]
    /// * `Some(false)`: wrap mode defaults to [`crate::TextWrapMode::Extend`]
    #[deprecated = "Use wrap_mode instead"]
    pub wrap: Option<bool>,

    /// If set, labels, buttons, etc. will use this to determine whether to wrap or truncate the
    /// text at the right edge of the [`Ui`] they are in, or to extend it. By default, this is
    /// `None`.
    ///
    /// * `None`: follow layout (with may wrap)
    /// * `Some(mode)`: use the specified mode as default
    pub wrap_mode: Option<crate::TextWrapMode>,

    /// Sizes and distances between widgets
    pub spacing: Spacing,

    /// How and when interaction happens.
    pub interaction: Interaction,

    /// Colors etc.
    pub visuals: Visuals,

    /// How many seconds a typical animation should last.
    pub animation_time: f32,

    /// Options to help debug why egui behaves strangely.
    ///
    /// Only available in debug builds.
    #[cfg(debug_assertions)]
    pub debug: DebugOptions,

    /// Show tooltips explaining [`DragValue`]:s etc when hovered.
    ///
    /// This only affects a few egui widgets.
    pub explanation_tooltips: bool,

    /// Show the URL of hyperlinks in a tooltip when hovered.
    pub url_in_tooltip: bool,

    /// If true and scrolling is enabled for only one direction, allow horizontal scrolling without pressing shift
    pub always_scroll_the_only_direction: bool,
}

但好像没有修改背景颜色、修改滑块样式这些选项，可自定义的还是比较少。但是如果只想要修改文字样式，那么可以针对label部件单独设置，可以适应Richtext来创建标签文本，那么就可以设置颜色、背景色、下划线等多种自定义样式：

ui.label(RichText::new(format!("标签{}",i))
                        .color(Color32::from_rgb(0, 0, 0)).underline()
                        .strikethrough()
                        .background_color(Color32::from_rgb(255, 0, 255)));