From dd64226dc11bdacfd60de28b3cee67d7591c20be Mon Sep 17 00:00:00 2001
From: Simon Hausmann <simon.hausmann@sixtyfps.io>
Date: Wed, 2 Dec 2020 14:59:56 +0100
Subject: [PATCH] Implement sixtyfps::Weak

This is the counter-part to the strongly referencing type the compiler
generates for components.
---
 api/sixtyfps-rs/lib.rs              | 42 +++++++++++++++++++++++++++++
 sixtyfps_compiler/generator/rust.rs | 11 ++++++++
 2 files changed, 53 insertions(+)

diff --git a/api/sixtyfps-rs/lib.rs b/api/sixtyfps-rs/lib.rs
index 3b952c324..d6a7a18f3 100644
--- a/api/sixtyfps-rs/lib.rs
+++ b/api/sixtyfps-rs/lib.rs
@@ -305,6 +305,48 @@ impl<T> ComponentWeakHandle<T> {
     }
 }
 
+/// This trait describes the conversion of a strongly referenced SixtyFPS component,
+/// held by a [vtable::VRc] into a weak reference.
+pub trait IntoWeak {
+    /// The type of the generated component.
+    type Inner;
+    /// Returns a new weak pointer.
+    fn as_weak(&self) -> Weak<Self>
+    where
+        Self: Sized;
+
+    /// Internal function used when upgrading a weak reference to a strong one.
+    fn from_inner(_: vtable::VRc<re_exports::ComponentVTable, Self::Inner>) -> Self;
+}
+
+/// Struct that's used to hold weak references for SixtyFPS components.
+pub struct Weak<T: IntoWeak> {
+    inner: vtable::VWeak<re_exports::ComponentVTable, T::Inner>,
+}
+
+impl<T: IntoWeak> Weak<T> {
+    #[doc(hidden)]
+    pub fn new(rc: &vtable::VRc<re_exports::ComponentVTable, T::Inner>) -> Self {
+        Self { inner: vtable::VRc::downgrade(&rc) }
+    }
+
+    /// Returns a new strongly referenced component if some other instance still
+    /// holds a strong reference. Otherwise, returns None.
+    pub fn upgrade(&self) -> Option<T>
+    where
+        T: IntoWeak,
+    {
+        self.inner.upgrade().map(|inner| T::from_inner(inner))
+    }
+
+    /// Convenience function that returns a new stronlyg referenced component if
+    /// some other instance still holds a strong reference. Otherwise, this function
+    /// panics.
+    pub fn unwrap(&self) -> T {
+        self.upgrade().unwrap()
+    }
+}
+
 /// This module contains functions useful for unit tests
 pub mod testing {
     /// This trait gives access to the underyling Window of a component for the
diff --git a/sixtyfps_compiler/generator/rust.rs b/sixtyfps_compiler/generator/rust.rs
index 3c8fad6c2..c5f33e3a2 100644
--- a/sixtyfps_compiler/generator/rust.rs
+++ b/sixtyfps_compiler/generator/rust.rs
@@ -107,6 +107,7 @@ pub fn generate(doc: &Document, diag: &mut BuildDiagnostics) -> Option<TokenStre
             const _THE_SAME_VERSION_MUST_BE_USED_FOR_THE_COMPILER_AND_THE_RUNTIME : sixtyfps::#version_check = sixtyfps::#version_check;
         }
         pub use #compo_module::{#compo_id, #public_compo_id #(,#structs_ids)* };
+        pub use sixtyfps::IntoWeak;
     })
 }
 
@@ -810,6 +811,16 @@ fn generate_component(
                 #run_fun
             }
 
+            impl sixtyfps::IntoWeak for #public_struct {
+                type Inner = #component_id;
+                fn as_weak(&self) -> sixtyfps::Weak<Self> {
+                    sixtyfps::Weak::new(&self.0)
+                }
+
+                fn from_inner(inner: vtable::VRc<sixtyfps::re_exports::ComponentVTable, #component_id>) -> Self {
+                    Self(inner)
+                }
+            }
         ))
     } else {
         None