add readable and writable traits for signal-like structs

packages/signals/src/global/memo.rs

@@ -0,0 +1,128 @@
+use crate::read::Readable;
+use dioxus_core::prelude::{IntoAttributeValue, ScopeId};
+use generational_box::{AnyStorage, UnsyncStorage};
+use std::{mem::MaybeUninit, ops::Deref};
+
+use crate::{ReadOnlySignal, Signal};
+
+use super::get_global_context;
+
+/// A signal that can be accessed from anywhere in the application and created in a static
+pub struct GlobalMemo<T: 'static> {
+    selector: fn() -> T,
+}
+
+impl<T: PartialEq + 'static> GlobalMemo<T> {
+    /// Create a new global signal
+    pub const fn new(selector: fn() -> T) -> GlobalMemo<T>
+    where
+        T: PartialEq,
+    {
+        GlobalMemo { selector }
+    }
+
+    /// Get the signal that backs this global.
+    pub fn signal(&self) -> ReadOnlySignal<T> {
+        let key = self as *const _ as *const ();
+
+        let context = get_global_context();
+
+        let read = context.signal.borrow();
+        match read.get(&key) {
+            Some(signal) => *signal.downcast_ref::<ReadOnlySignal<T>>().unwrap(),
+            None => {
+                drop(read);
+                // Constructors are always run in the root scope
+                let signal = ScopeId::ROOT.in_runtime(|| Signal::selector(self.selector));
+                context.signal.borrow_mut().insert(key, Box::new(signal));
+                signal
+            }
+        }
+    }
+
+    /// Get the scope the signal was created in.
+    pub fn origin_scope(&self) -> ScopeId {
+        ScopeId::ROOT
+    }
+
+    /// Get the generational id of the signal.
+    pub fn id(&self) -> generational_box::GenerationalBoxId {
+        self.signal().id()
+    }
+}
+
+impl<T: PartialEq + 'static> Readable<T> for GlobalMemo<T> {
+    type Ref<R: ?Sized + 'static> = generational_box::GenerationalRef<std::cell::Ref<'static, R>>;
+
+    fn map_ref<I, U: ?Sized, F: FnOnce(&I) -> &U>(ref_: Self::Ref<I>, f: F) -> Self::Ref<U> {
+        <UnsyncStorage as AnyStorage>::map(ref_, f)
+    }
+
+    fn try_map_ref<I, U: ?Sized, F: FnOnce(&I) -> Option<&U>>(
+        ref_: Self::Ref<I>,
+        f: F,
+    ) -> Option<Self::Ref<U>> {
+        <UnsyncStorage as AnyStorage>::try_map(ref_, f)
+    }
+
+    #[track_caller]
+    fn read(&self) -> Self::Ref<T> {
+        self.signal().read()
+    }
+
+    #[track_caller]
+    fn peek(&self) -> Self::Ref<T> {
+        self.signal().peek()
+    }
+}
+
+impl<T: PartialEq + 'static> IntoAttributeValue for GlobalMemo<T>
+where
+    T: Clone + IntoAttributeValue,
+{
+    fn into_value(self) -> dioxus_core::AttributeValue {
+        self.signal().into_value()
+    }
+}
+
+impl<T: PartialEq + 'static> PartialEq for GlobalMemo<T> {
+    fn eq(&self, other: &Self) -> bool {
+        std::ptr::eq(self, other)
+    }
+}
+
+/// Allow calling a signal with signal() syntax
+///
+/// Currently only limited to copy types, though could probably specialize for string/arc/rc
+impl<T: PartialEq + Clone + 'static> Deref for GlobalMemo<T> {
+    type Target = dyn Fn() -> T;
+
+    fn deref(&self) -> &Self::Target {
+        // https://github.com/dtolnay/case-studies/tree/master/callable-types
+
+        // First we create a closure that captures something with the Same in memory layout as Self (MaybeUninit<Self>).
+        let uninit_callable = MaybeUninit::<Self>::uninit();
+        // Then move that value into the closure. We assume that the closure now has a in memory layout of Self.
+        let uninit_closure = move || Self::read(unsafe { &*uninit_callable.as_ptr() }).clone();
+
+        // Check that the size of the closure is the same as the size of Self in case the compiler changed the layout of the closure.
+        let size_of_closure = std::mem::size_of_val(&uninit_closure);
+        assert_eq!(size_of_closure, std::mem::size_of::<Self>());
+
+        // Then cast the lifetime of the closure to the lifetime of &self.
+        fn cast_lifetime<'a, T>(_a: &T, b: &'a T) -> &'a T {
+            b
+        }
+        let reference_to_closure = cast_lifetime(
+            {
+                // The real closure that we will never use.
+                &uninit_closure
+            },
+            // We transmute self into a reference to the closure. This is safe because we know that the closure has the same memory layout as Self so &Closure == &self.
+            unsafe { std::mem::transmute(self) },
+        );
+
+        // Cast the closure to a trait object.
+        reference_to_closure as &Self::Target
+    }
+}