1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108

//! Query or update internal widget state.
pub mod focusable;
pub mod scrollable;
pub mod text_input;

pub use focusable::Focusable;
pub use scrollable::Scrollable;
pub use text_input::TextInput;

use crate::widget::Id;

use std::fmt;

/// A piece of logic that can traverse the widget tree of an application in
/// order to query or update some widget state.
pub trait Operation<T> {
    /// Operates on a widget that contains other widgets.
    ///
    /// The `operate_on_children` function can be called to return control to
    /// the widget tree and keep traversing it.
    fn container(
        &mut self,
        id: Option<&Id>,
        operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
    );

    /// Operates on a widget that can be focused.
    fn focusable(&mut self, _state: &mut dyn Focusable, _id: Option<&Id>) {}

    /// Operates on a widget that can be scrolled.
    fn scrollable(&mut self, _state: &mut dyn Scrollable, _id: Option<&Id>) {}

    /// Operates on a widget that has text input.
    fn text_input(&mut self, _state: &mut dyn TextInput, _id: Option<&Id>) {}

    /// Finishes the [`Operation`] and returns its [`Outcome`].
    fn finish(&self) -> Outcome<T> {
        Outcome::None
    }
}

/// The result of an [`Operation`].
pub enum Outcome<T> {
    /// The [`Operation`] produced no result.
    None,

    /// The [`Operation`] produced some result.
    Some(T),

    /// The [`Operation`] needs to be followed by another [`Operation`].
    Chain(Box<dyn Operation<T>>),
}

impl<T> fmt::Debug for Outcome<T>
where
    T: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::None => write!(f, "Outcome::None"),
            Self::Some(output) => write!(f, "Outcome::Some({:?})", output),
            Self::Chain(_) => write!(f, "Outcome::Chain(...)"),
        }
    }
}

/// Produces an [`Operation`] that applies the given [`Operation`] to the
/// children of a container with the given [`Id`].
pub fn scoped<T: 'static>(
    target: Id,
    operation: impl Operation<T> + 'static,
) -> impl Operation<T> {
    struct ScopedOperation<Message> {
        target: Id,
        operation: Box<dyn Operation<Message>>,
    }

    impl<Message: 'static> Operation<Message> for ScopedOperation<Message> {
        fn container(
            &mut self,
            id: Option<&Id>,
            operate_on_children: &mut dyn FnMut(&mut dyn Operation<Message>),
        ) {
            if id == Some(&self.target) {
                operate_on_children(self.operation.as_mut());
            } else {
                operate_on_children(self);
            }
        }

        fn finish(&self) -> Outcome<Message> {
            match self.operation.finish() {
                Outcome::Chain(next) => {
                    Outcome::Chain(Box::new(ScopedOperation {
                        target: self.target.clone(),
                        operation: next,
                    }))
                }
                outcome => outcome,
            }
        }
    }

    ScopedOperation {
        target,
        operation: Box::new(operation),
    }
}