Add a Channel-like type

CHANGELOG.md

@@ -117,6 +117,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   application to create deadlocks with relatively simple data flows like the new
   `7guis-timer` example.
 - The type `cushy::Graphics` is now available at `cushy::graphics::Graphics`.
+- `Source::for_each_cloned_*` now invoke the callback with the current contents
+  of of the source before attaching the callback. New functions beginning with
+  `for_each_subsequent_cloned` have been added with the original behavior. This
+  change was done to make `for_each_cloned` and `for_each` have the same
+  semantics.
 
 ### Changed
 

examples/channels.rs

@@ -0,0 +1,58 @@
+use std::time::{Duration, Instant};
+
+use cushy::value::{Destination, Dynamic, Source};
+use cushy::widget::MakeWidget;
+use cushy::widgets::label::Displayable;
+use cushy::Run;
+
+fn main() -> cushy::Result {
+    let channel_counter = Dynamic::new(0_usize);
+    let channel_counter_label = channel_counter.to_label();
+    let sender = cushy::channel::build()
+        .on_receive({
+            move |_| {
+                std::thread::sleep(Duration::from_secs(1));
+                *channel_counter.lock() += 1;
+            }
+        })
+        .finish();
+
+    let dynamic_counter = Dynamic::new(0_usize);
+    let dynamic_counter_label = dynamic_counter.to_label();
+    let dynamic_value = Dynamic::new(Instant::now());
+    // We use a `for_each_subsequent_cloned` to only execute the callback after
+    // the current value changes, and the `cloned` version ensures the dynamic
+    // isn't locked while the callback is being executed.
+    dynamic_value
+        .for_each_subsequent_cloned(move |_| {
+            std::thread::sleep(Duration::from_secs(1));
+            *dynamic_counter.lock() += 1;
+        })
+        .persist();
+
+    "Channels ensure every value sent is received. Try \
+        clicking the button quickly and seeing how the \
+        channel version increments for every click while \
+        the dynamic version increments at most once every \
+        second."
+        .and(
+            "Click Me"
+                .into_button()
+                .on_click(move |_| {
+                    let now = Instant::now();
+                    sender.send(now).expect("value to be received");
+                    dynamic_value.set(now);
+                })
+                .centered(),
+        )
+        .and(
+            "Channel Counter"
+                .and(channel_counter_label)
+                .into_rows()
+                .and("Dynamic Counter".and(dynamic_counter_label).into_rows())
+                .into_columns()
+                .centered(),
+        )
+        .into_rows()
+        .run()
+}

src/lib.rs

@@ -20,11 +20,12 @@ mod names;
 #[macro_use]
 pub mod styles;
 mod app;
+mod reactive;
+pub use reactive::{channel, value};
 pub mod debug;
 pub mod fonts;
 mod tick;
 mod tree;
-pub mod value;
 pub mod widget;
 pub mod widgets;
 pub mod window;