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//! Write your own renderer.
//!
//! You will need to implement the `Renderer` trait first. It simply contains
//! an `Output` associated type.
//!
//! There is no common trait to draw all the widgets. Instead, every [`Widget`]
//! constrains its generic `Renderer` type as necessary.
//!
//! This approach is flexible and composable. For instance, the
//! [`Text`] widget only needs a [`text::Renderer`] while a [`Checkbox`] widget
//! needs both a [`text::Renderer`] and a [`checkbox::Renderer`], reusing logic.
//!
//! In the end, a __renderer__ satisfying all the constraints is
//! needed to build a [`UserInterface`].
//!
//! [`Widget`]: crate::Widget
//! [`UserInterface`]: crate::UserInterface
//! [`Text`]: crate::widget::Text
//! [`text::Renderer`]: crate::widget::text::Renderer
//! [`Checkbox`]: crate::widget::Checkbox
//! [`checkbox::Renderer`]: crate::widget::checkbox::Renderer

#[cfg(debug_assertions)]
mod null;
#[cfg(debug_assertions)]
pub use null::Null;

use crate::{layout, Element, Rectangle};

/// A component that can take the state of a user interface and produce an
/// output for its users.
pub trait Renderer: Sized {
    /// The type of output of the [`Renderer`].
    ///
    /// If you are implementing a graphical renderer, your output will most
    /// likely be a tree of visual primitives.
    type Output;

    /// The default styling attributes of the [`Renderer`].
    ///
    /// This type can be leveraged to implement style inheritance.
    type Defaults: Default;

    /// Lays out the elements of a user interface.
    ///
    /// You should override this if you need to perform any operations before or
    /// after layouting. For instance, trimming the measurements cache.
    fn layout<'a, Message>(
        &mut self,
        element: &Element<'a, Message, Self>,
        limits: &layout::Limits,
    ) -> layout::Node {
        element.layout(self, limits)
    }

    /// Overlays the `overlay` output with the given bounds on top of the `base`
    /// output.
    fn overlay(
        &mut self,
        base: Self::Output,
        overlay: Self::Output,
        overlay_bounds: Rectangle,
    ) -> Self::Output;
}