Added channel example

Also added missing for_each_subsequent variants.

Author: ecton

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 \
+        500 milliseconds."
+        .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/reactive/value.rs

@@ -299,6 +299,9 @@ pub trait Source<T> {
 
     /// Invokes `for_each` with the current contents and each time this source's
     /// contents are updated.
+    ///
+    /// Returning `Err(CallbackDisconnected)` will prevent the callback from
+    /// being invoked again.
     fn for_each_cloned_try<F>(&self, mut for_each: F) -> CallbackHandle
     where
         T: Clone + Send + 'static,
@@ -307,6 +310,30 @@ pub trait Source<T> {
         self.for_each_generational_cloned_try(move |gen| for_each(gen.value))
     }
 
+    /// Invokes `for_each` each time this source's contents are updated.
+    ///
+    /// Returning `Err(CallbackDisconnected)` will prevent the callback from
+    /// being invoked again.
+    fn for_each_subsequent_cloned_try<F>(&self, mut for_each: F) -> CallbackHandle
+    where
+        T: Clone + Send + 'static,
+        F: FnMut(T) -> Result<(), CallbackDisconnected> + Send + 'static,
+    {
+        self.for_each_subsequent_generational_cloned_try(move |gen| for_each(gen.value))
+    }
+
+    /// Invokes `for_each` each time this source's contents are updated.
+    fn for_each_subsequent_cloned<F>(&self, mut for_each: F) -> CallbackHandle
+    where
+        T: Clone + Send + 'static,
+        F: FnMut(T) + Send + 'static,
+    {
+        self.for_each_subsequent_cloned_try(move |value| {
+            for_each(value);
+            Ok(())
+        })
+    }
+
     /// Invokes `for_each` with the current contents and each time this source's
     /// contents are updated.
     fn for_each_cloned<F>(&self, mut for_each: F) -> CallbackHandle