tek/src/space/cond.rs

use crate::*;

/// Conditional rendering, in unary and binary forms.
pub struct Cond;

impl Cond {
    /// Render `item` when `cond` is true.
    pub fn when <E: Engine, A: Render<E>> (cond: bool, item: A) -> When<E, A> {
        When(cond, item, Default::default())
    }
    /// Render `item` if `cond` is true, otherwise render `other`.
    pub fn either <E: Engine, A: Render<E>, B: Render<E>> (cond: bool, item: A, other: B) -> Either<E, A, B> {
        Either(cond, item, other, Default::default())
    }
}

/// Renders `self.1` when `self.0` is true.
pub struct When<E: Engine, A: Render<E>>(bool, A, PhantomData<E>);

/// Renders `self.1` when `self.0` is true, otherwise renders `self.2`
pub struct Either<E: Engine, A: Render<E>, B: Render<E>>(bool, A, B, PhantomData<E>);

impl<E: Engine, A: Render<E>> Render<E> for When<E, A> {
    fn min_size (&self, to: E::Size) -> Perhaps<E::Size> {
        let Self(cond, item, ..) = self;
        if *cond { item.min_size(to) } else { Ok(Some([0.into(), 0.into()].into())) }
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        let Self(cond, item, ..) = self;
        if *cond { item.render(to) } else { Ok(()) }
    }
}

impl<E: Engine, A: Render<E>, B: Render<E>> Render<E> for Either<E, A, B> {
    fn min_size (&self, to: E::Size) -> Perhaps<E::Size> {
        let Self(cond, item, other, ..) = self;
        if *cond { item.min_size(to) } else { other.min_size(to) }
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        let Self(cond, item, other, ..) = self;
        if *cond { item.render(to) } else { other.render(to) }
    }
}