Add useTimeout snippet

snippets/useInterval.md

@@ -8,7 +8,7 @@ A hook that implements `setInterval` in a declarative manner.
 - Create a custom hook that takes a `callback` and a `delay`.
 - Use the `React.useRef()` hook to create a `ref` for the callback function.
 - Use the `React.useEffect()` hook to remember the latest callback.
-- Use the `Rect.useEffect()` hook to set up the interval.
+- Use the `Rect.useEffect()` hook to set up the interval and clean up.
 
 
 ```jsx

snippets/useTimeout.md

@@ -0,0 +1,50 @@
+---
+title: useTimeout
+tags: hooks,effect,intermediate
+---
+
+A hook that implements `setTimeout` in a declarative manner.
+
+- Create a custom hook that takes a `callback` and a `delay`.
+- Use the `React.useRef()` hook to create a `ref` for the callback function.
+- Use the `React.useEffect()` hook to remember the latest callback.
+- Use the `Rect.useEffect()` hook to set up the timeout and clean up.
+
+
+```jsx
+const useTimeout = (callback, delay) => {
+  const savedCallback = React.useRef();
+
+  React.useEffect(() => {
+    savedCallback.current = callback;
+  }, [callback]);
+
+  React.useEffect(() => {
+    function tick () {
+      savedCallback.current();
+    }
+    if (delay !== null) {
+      let id = setTimeout(tick, delay);
+      return () => clearTimeout(id);
+    }
+  }, [delay]);
+};
+```
+
+```jsx
+const OneSecondTimer = (props) => {
+  const [seconds,setSeconds] = React.useState(0);
+  useTimeout(() => {
+    setSeconds(seconds + 1);
+  }, 1000);
+
+  return (
+    <p>{seconds}</p>
+  );
+}
+
+ReactDOM.render(
+  <OneSecondTimer />,
+  document.getElementById('root')
+);
+```