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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;
}
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