如何在nannou中处理触摸板滚动？解决PhysicalPosition类型不匹配问题

问题 #

笔记本触摸板滚动触发的事件格式为：

MouseWheel(PixelDelta(PhysicalPosition(x, y)), Moved)

在nannou中找不到PhysicalPosition结构体，尝试引入winit::dpi::PhysicalPosition时出现类型不匹配错误：

expected `PhysicalPosition<f64>`, found a different `PhysicalPosition<f64>` 

最小示例代码 #

use nannou::prelude::{
    App, Frame, WindowEvent, WindowId, BLACK,
    MouseScrollDelta::LineDelta, MouseScrollDelta::PixelDelta, MouseWheel,
};

use winit::dpi::PhysicalPosition;

fn main() { nannou::app(model).run(); }

struct Model { _window: WindowId }

fn model(app: &App) -> Model {
    let _window = app.new_window().view(view).event(event).build().unwrap();
    Model { _window }
}

fn view(app: &App, _model: &Model, frame: Frame) {
    frame.clear(BLACK);
    let draw = app.draw();
    draw.to_frame(app, &frame).unwrap();
}

fn event(_app: &App, _model: &mut Model, event: WindowEvent) {
    //println!("{:?}", event);
    match event {
        MouseWheel(LineDelta(x, y), ..) => { 
            println!("{event:?} - x,y: {x}, {y}");
        }
        MouseWheel(PixelDelta(PhysicalPosition::<f64>{ x, y }), ..) => {
            println!("{event:?} - x,y: {x}, {y}");
        }
        _ => (),
    }
}

编译错误 #

error[E0308]: mismatched types
   --> src/main.rs:28:24
    |
24  |     match event {
    |           ----- this expression has type `nannou::event::WindowEvent`
...
28  |         MouseWheel(PixelDelta(PhysicalPosition::<f64>{ x, y }), ..) => {
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `PhysicalPosition<f64>`, found a different `PhysicalPosition<f64>`

错误原因 #

本质是crate版本不一致导致的类型不兼容：你直接导入的winit::dpi::PhysicalPosition和nannou内部依赖的winit版本导出的PhysicalPosition是两个完全独立的类型，即使名字和结构一致，Rust也会判定为不同类型。

解决方法 #

不要直接引入外部winit的类型，改用nannou重新导出的PhysicalPosition：

• 直接从nannou::prelude中导入，或者通过nannou::winit::dpi::PhysicalPosition引用。

修改后的代码 #

use nannou::prelude::{
    App, Frame, WindowEvent, WindowId, BLACK,
    MouseScrollDelta::LineDelta, MouseScrollDelta::PixelDelta, MouseWheel,
    PhysicalPosition, // 从nannou的prelude导入对应类型
};

fn main() { nannou::app(model).run(); }

struct Model { _window: WindowId }

fn model(app: &App) -> Model {
    let _window = app.new_window().view(view).event(event).build().unwrap();
    Model { _window }
}

fn view(app: &App, _model: &Model, frame: Frame) {
    frame.clear(BLACK);
    let draw = app.draw();
    draw.to_frame(app, &frame).unwrap();
}

fn event(_app: &App, _model: &mut Model, event: WindowEvent) {
    match event {
        MouseWheel(LineDelta(x, y), ..) => { 
            println!("{event:?} - x,y: {x}, {y}");
        }
        MouseWheel(PixelDelta(PhysicalPosition::<f64>{ x, y }), ..) => {
            println!("{event:?} - x,y: {x}, {y}");
        }
        _ => (),
    }
}

内容的提问来源于stack exchange，提问作者Raf