From a4ae92c083ae4b195b7e4d39e957e0a4becdeaa8 Mon Sep 17 00:00:00 2001
From: torkleyy <torkleyy@gmail.com>
Date: Sat, 2 Sep 2017 11:57:27 +0200
Subject: [PATCH] Add optional fetching of resources

---
 Cargo.toml            |  6 +++---
 examples/fetch_opt.rs | 45 +++++++++++++++++++++++++++++++++++++++++++
 src/res.rs            | 32 ++++++++++++++++++++++++++++++
 3 files changed, 80 insertions(+), 3 deletions(-)
 create mode 100644 examples/fetch_opt.rs

diff --git a/Cargo.toml b/Cargo.toml
index 412068b..82e8078 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "shred"
-version = "0.4.3"
-authors = ["torkleyy"]
+version = "0.4.4"
+authors = ["torkleyy <torkleyy@gmail.com>"]
 description = """
 Dispatches systems in parallel which need read access to some resources,
 and write access to others.
@@ -27,4 +27,4 @@ shred-derive = { path = "shred-derive", version = "0.3" }
 smallvec = "0.4"
 
 [dev-dependencies]
-cgmath = "0.14"
+cgmath = "0.15"
diff --git a/examples/fetch_opt.rs b/examples/fetch_opt.rs
new file mode 100644
index 0000000..57eb4e2
--- /dev/null
+++ b/examples/fetch_opt.rs
@@ -0,0 +1,45 @@
+extern crate shred;
+
+use shred::{DispatcherBuilder, Fetch, FetchMut, Resources, System};
+
+#[derive(Debug)]
+struct ResA;
+
+#[derive(Debug)]
+struct ResB;
+
+struct PrintSystem;
+
+impl<'a> System<'a> for PrintSystem {
+    // We can simply use `Option<Fetch>` or `Option<FetchMut>` if a resource
+    // isn't strictly required.
+    type SystemData = (Fetch<'a, ResA>, Option<FetchMut<'a, ResB>>);
+
+    fn run(&mut self, data: Self::SystemData) {
+        let (a, mut b) = data;
+
+        println!("{:?}", &*a);
+
+        if let Some(ref mut x) = b {
+            println!("{:?}", &**x);
+
+            **x = ResB;
+        }
+    }
+}
+
+fn main() {
+    let mut resources = Resources::new();
+    let mut dispatcher = DispatcherBuilder::new()
+        .add(PrintSystem, "print", &[]) // Adds a system "print" without dependencies
+        .build();
+    resources.add(ResA);
+
+    // `ResB` is not in resources, but `PrintSystem` still works.
+    dispatcher.dispatch(&mut resources);
+
+    resources.add(ResB);
+
+    // Now `ResB` can be printed, too.
+    dispatcher.dispatch(&mut resources);
+}
diff --git a/src/res.rs b/src/res.rs
index 7de2cae..7b7f565 100644
--- a/src/res.rs
+++ b/src/res.rs
@@ -124,6 +124,38 @@ impl<'a, T> SystemData<'a> for FetchMut<'a, T>
     }
 }
 
+impl<'a, T> SystemData<'a> for Option<Fetch<'a, T>>
+    where T: Resource
+{
+    fn fetch(res: &'a Resources, id: usize) -> Self {
+        res.try_fetch(id)
+    }
+
+    fn reads(id: usize) -> Vec<ResourceId> {
+        vec![ResourceId::new_with_id::<T>(id)]
+    }
+
+    fn writes(_: usize) -> Vec<ResourceId> {
+        vec![]
+    }
+}
+
+impl<'a, T> SystemData<'a> for Option<FetchMut<'a, T>>
+    where T: Resource
+{
+    fn fetch(res: &'a Resources, id: usize) -> Self {
+        res.try_fetch_mut(id)
+    }
+
+    fn reads(_: usize) -> Vec<ResourceId> {
+        vec![]
+    }
+
+    fn writes(id: usize) -> Vec<ResourceId> {
+        vec![ResourceId::new_with_id::<T>(id)]
+    }
+}
+
 /// A resource defines a set of data
 /// which can only be accessed according
 /// to Rust's typical borrowing model (one writer xor multiple readers).