agg_gui/
persistence.rs

1//! Small persistence helpers shared across platform harnesses.
2//!
3//! Both the native (disk) and wasm (localStorage) harnesses follow the
4//! same auto-save policy: each frame, serialize the current session
5//! state, compare to the last-persisted blob, and write only when they
6//! differ AND no mouse button is currently held.  The "no mouse held"
7//! guard keeps us from hammering the backend while the user is
8//! mid-drag / mid-resize.
9//!
10//! This module owns the diff-and-save policy so hosts don't reimplement
11//! it.  Callers plug in their own serializer and persist backend via
12//! closures — [`AutoSave::tick`] takes care of the rest.
13
14/// Tracks the last-persisted serialized blob so the platform harness
15/// can avoid writing unchanged state every frame.
16///
17/// # Example (pseudo-Rust)
18/// ```ignore
19/// let mut auto_save = AutoSave::new();
20/// loop {
21///     paint_frame();
22///     auto_save.tick(
23///         mouse_buttons_held == 0,
24///         || state.serialize(),
25///         |blob| std::fs::write("state.json", blob).ok(),
26///     );
27/// }
28/// ```
29#[derive(Default)]
30pub struct AutoSave {
31    last_saved: String,
32}
33
34impl AutoSave {
35    /// Create an empty auto-save tracker.  First `tick` call will always
36    /// persist (because the last-saved blob is empty).
37    pub const fn new() -> Self {
38        Self {
39            last_saved: String::new(),
40        }
41    }
42
43    /// Consider persisting the current state.
44    ///
45    /// `idle` — guard that must be `true` for the save to happen.
46    /// Typically `mouse_buttons_held == 0`; callers can layer other
47    /// conditions (e.g. "no in-flight drag").
48    ///
49    /// `serialize_now` — closure that produces the current serialized
50    /// blob.  Only invoked when `idle` is `true`.
51    ///
52    /// `persist` — closure that writes the blob to the platform backend
53    /// (file / localStorage / network).  Only invoked when the blob
54    /// differs from the last persisted version.
55    ///
56    /// Returns `true` iff the persist closure was called.
57    pub fn tick<S, P>(&mut self, idle: bool, serialize_now: S, persist: P) -> bool
58    where
59        S: FnOnce() -> String,
60        P: FnOnce(&str),
61    {
62        if !idle {
63            return false;
64        }
65        let blob = serialize_now();
66        if blob == self.last_saved {
67            return false;
68        }
69        persist(&blob);
70        self.last_saved = blob;
71        true
72    }
73
74    /// Seed the tracker with a blob that's already on disk (on startup).
75    /// Prevents the first `tick` from writing the same content back to
76    /// the backend unnecessarily.
77    pub fn seed(&mut self, last_saved: String) {
78        self.last_saved = last_saved;
79    }
80}
agg_gui/persistence.rs

agg_gui/
persistence.rs
