1use crate::{
 2    AnyElement, App, Bounds, Element, GlobalElementId, IntoElement, LayoutId, Pixels, Window,
 3};
 4
 5/// Builds a `Deferred` element, which delays the layout and paint of its child.
 6pub fn deferred(child: impl IntoElement) -> Deferred {
 7    Deferred {
 8        child: Some(child.into_any_element()),
 9        priority: 0,
10    }
11}
12
13/// An element which delays the painting of its child until after all of
14/// its ancestors, while keeping its layout as part of the current element tree.
15pub struct Deferred {
16    child: Option<AnyElement>,
17    priority: usize,
18}
19
20impl Deferred {
21    /// Sets the `priority` value of the `deferred` element, which
22    /// determines the drawing order relative to other deferred elements,
23    /// with higher values being drawn on top.
24    pub fn with_priority(mut self, priority: usize) -> Self {
25        self.priority = priority;
26        self
27    }
28}
29
30impl Element for Deferred {
31    type RequestLayoutState = ();
32    type PrepaintState = ();
33
34    fn id(&self) -> Option<crate::ElementId> {
35        None
36    }
37
38    fn request_layout(
39        &mut self,
40        _id: Option<&GlobalElementId>,
41        window: &mut Window,
42        cx: &mut App,
43    ) -> (LayoutId, ()) {
44        let layout_id = self.child.as_mut().unwrap().request_layout(window, cx);
45        (layout_id, ())
46    }
47
48    fn prepaint(
49        &mut self,
50        _id: Option<&GlobalElementId>,
51        _bounds: Bounds<Pixels>,
52        _request_layout: &mut Self::RequestLayoutState,
53        window: &mut Window,
54        _cx: &mut App,
55    ) {
56        let child = self.child.take().unwrap();
57        let element_offset = window.element_offset();
58        window.defer_draw(child, element_offset, self.priority)
59    }
60
61    fn paint(
62        &mut self,
63        _id: Option<&GlobalElementId>,
64        _bounds: Bounds<Pixels>,
65        _request_layout: &mut Self::RequestLayoutState,
66        _prepaint: &mut Self::PrepaintState,
67        _window: &mut Window,
68        _cx: &mut App,
69    ) {
70    }
71}
72
73impl IntoElement for Deferred {
74    type Element = Self;
75
76    fn into_element(self) -> Self::Element {
77        self
78    }
79}
80
81impl Deferred {
82    /// Sets a priority for the element. A higher priority conceptually means painting the element
83    /// on top of deferred draws with a lower priority (i.e. closer to the viewer).
84    pub fn priority(mut self, priority: usize) -> Self {
85        self.priority = priority;
86        self
87    }
88}