add iter example

examples/custom_iter.rs

@@ -0,0 +1,57 @@
+use std::{
+    thread::sleep,
+    time::Duration,
+};
+
+struct List {
+    arr: Vec<u8>,
+    msg: String,
+    i: usize,
+}
+
+impl List {
+    fn new() -> Self {
+        Self {
+            arr: (0..10).collect(),
+            msg: "fist init".to_string(),
+            i: 0,
+        }
+    }
+
+    fn fill(&mut self, val: String) {
+        println!("{val}");
+        self.msg = "new fill".to_string();
+    }
+}
+
+impl Iterator for List {
+    type Item = u8;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.i == 0 {
+            println!("{}", self.msg);
+        }
+        if self.i < self.arr.len() {
+            let current = self.arr[self.i];
+            self.i += 1;
+
+            Some(current)
+        } else {
+            self.i = 1;
+            let current = self.arr[0];
+            self.fill("pass to function".to_string());
+            println!("{}", self.msg);
+
+            Some(current)
+        }
+    }
+}
+
+fn main() {
+    let list = List::new();
+
+    for i in list {
+        println!("{i}");
+        sleep(Duration::from_millis(300));
+    }
+}