Add CrossFlow component

- Queues children to make sure children aren't forcefully removed.
- Uses TransitionPresence and thus works just like TransitionPresence

packages/react-transition-presence/src/CrossFlow/CrossFlow.mdx

@@ -0,0 +1,57 @@
+import { Meta, Canvas, Controls } from '@storybook/blocks';
+import * as stories from './CrossFlow.stories';
+
+<Meta title="components/CrossFlow" of={stories} />
+
+# CrossFlow
+
+`<CrossFlow />` is a component that allows you to defer the unmount lifecycle of a component.
+
+The callback function used in `useBeforeUnmount` in components that are rendered in the context of
+`<CrossFlow />` will be called just before a component is unmounted. The promises that are created
+using the callback are awaited before unmounting a component. In the CrossFlow component new
+children are immediately rendered.
+
+The most common use-case for `<CrossFlow />` is animations, but it's not limited to this use case.
+
+## Props
+
+### Children
+
+`<CrossFlow />` accepts a single child, use a `Fragment` when you want to render multiple components
+on the root level. New children are rendered when the component type for the children (`<p />` to
+`<div />`) change, or when the key of the children changes (`<p key="one" />` to `null`).
+
+```tsx
+// Transition between two instances
+<CrossFlow>
+  {myBoolean ? <MyComponent key="first-instance" /> : <MyComponent key="second-instance" />}
+</CrossFlow>
+
+// Transition between component types (new instances are automatically created)
+<CrossFlow>
+  {myBoolean ? <FirstComponent /> : <SecondComponent />}
+</CrossFlow>
+```
+
+### onChildrenMounted
+
+The `onChildrenMounted` is called when the new children are mounted.
+
+```tsx
+onChildrenMounted?: () => void;
+
+<CrossFlow onChildrenMounted={() => console.log('onChildrenMounted')}>
+  ...
+</CrossFlow>
+```
+
+## Demo
+
+### Basic
+
+<Canvas of={stories.CrossFlowExample} />
+
+### Basic with Fragments
+
+<Canvas of={stories.CrossFlowFragmentExample} />

packages/react-transition-presence/src/CrossFlow/CrossFlow.stories.tsx

@@ -0,0 +1,144 @@
+/* eslint-disable react/no-multi-comp, react/jsx-no-literals,no-console */
+// eslint-disable-next-line import/no-extraneous-dependencies
+import { useAnimation } from '@mediamonks/react-animation';
+import type { Meta } from '@storybook/react';
+import gsap from 'gsap';
+import { Fragment, useEffect, useRef, useState, type ReactElement } from 'react';
+import { useBeforeUnmount } from '../useBeforeUnmount/useBeforeUnmount.js';
+import { CrossFlow } from './CrossFlow.js';
+
+export default {
+  title: 'components/CrossFlow',
+  argTypes: {
+    children: {
+      description: 'ReactElement | null',
+      type: 'string',
+    },
+    onChildrenMounted: {
+      description: '() => void | undefined',
+      type: 'string',
+    },
+  },
+} satisfies Meta;
+
+type ChildProps = {
+  background: string;
+  duration?: number;
+  delayIn?: number;
+  onClick?(): void;
+};
+
+function Child({ background, onClick, duration = 1, delayIn = 0 }: ChildProps): ReactElement {
+  const ref = useRef<HTMLButtonElement>(null);
+
+  // show when mounted/unmounted
+  useEffect(() => {
+    console.log('mounted', background);
+
+    return () => {
+      console.log('unmounted', background);
+    };
+  }, [background]);
+
+  useAnimation(() => {
+    console.log('animate-in', background);
+    return gsap.fromTo(ref.current, { opacity: 0 }, { opacity: 1, duration, delay: delayIn });
+  }, []);
+
+  // show visible animation during "before unmount" lifecycle
+  useBeforeUnmount(async (abortSignal) => {
+    console.log('animate-out', background);
+
+    const animation = gsap.fromTo(ref.current, { opacity: 1 }, { opacity: 0, duration });
+
+    abortSignal.addEventListener('abort', () => {
+      animation.pause(0);
+    });
+
+    return animation;
+  });
+
+  return (
+    <button
+      ref={ref}
+      aria-label="Click to change color"
+      type="button"
+      style={{
+        background,
+        border: 'none',
+        width: 200,
+        height: 200,
+      }}
+      onClick={onClick}
+    />
+  );
+}
+
+export function CrossFlowExample(): ReactElement {
+  const [hue, setHue] = useState(0);
+
+  return (
+    <>
+      <CrossFlow>
+        <Child
+          // Changing a key will create a new component instance and thus animates out the previous child
+          key={hue}
+          background={`repeating-linear-gradient(
+            -45deg,
+            hsl(${hue}, 100%, 50%),
+            hsl(${hue}, 100%, 50%) 10px,
+            hsl(${(hue + 180) % 360}, 100%, 50%) 10px,
+            hsl(${(hue + 180) % 360}, 100%, 50%) 20px
+          )`}
+          // eslint-disable-next-line react/jsx-no-bind
+          onClick={(): void => {
+            setHue(hue + ((90 + Math.random() * 270) % 360));
+          }}
+        />
+      </CrossFlow>
+
+      <div style={{ marginTop: 24 }}>Click the square to create a new element</div>
+    </>
+  );
+}
+
+export function CrossFlowFragmentExample(): ReactElement {
+  const [hue, setHue] = useState(0);
+
+  return (
+    <>
+      <CrossFlow>
+        <Fragment key={hue}>
+          <Child
+            background={`repeating-linear-gradient(
+            -45deg,
+            hsl(${hue}, 100%, 50%),
+            hsl(${hue}, 100%, 50%) 10px,
+            hsl(${(hue + 180) % 360}, 100%, 50%) 10px,
+            hsl(${(hue + 180) % 360}, 100%, 50%) 20px
+          )`}
+            // eslint-disable-next-line react/jsx-no-bind
+            onClick={(): void => {
+              setHue(hue + ((90 + Math.random() * 270) % 360));
+            }}
+          />
+          <Child
+            background={`repeating-linear-gradient(
+            -45deg,
+            hsl(${hue + (90 % 360)}, 100%, 50%),
+            hsl(${hue + (90 % 360)}, 100%, 50%) 10px,
+            hsl(${(hue + 270) % 360}, 100%, 50%) 10px,
+            hsl(${(hue + 270) % 360}, 100%, 50%) 20px
+          )`}
+            // eslint-disable-next-line react/jsx-no-bind
+            onClick={(): void => {
+              setHue(hue + ((90 + Math.random() * 270) % 360));
+            }}
+          />
+        </Fragment>
+      </CrossFlow>
+
+      <div style={{ marginTop: 24 }}>Click the square to create a new element</div>
+    </>
+  );
+}

packages/react-transition-presence/src/CrossFlow/CrossFlow.tsx

@@ -0,0 +1,90 @@
+import { cloneElement, useCallback, useEffect, useRef, useState, type ReactElement } from 'react';
+import {
+  TransitionPresence,
+  type TransitionPresenceProps,
+} from '../TransitionPresence/TransitionPresence.js';
+import { useIsMounted } from '../_hooks/useIsMounted.js';
+import { childrenAreEqual } from '../_utils/childrenAreEqual.js';
+import { getId } from '../_utils/getId.js';
+
+type CrossFlowQueue = Record<string, TransitionPresenceProps['children']>;
+
+type CrossFlowProps = TransitionPresenceProps;
+
+export function CrossFlow({
+  children,
+  onChildrenMounted,
+  ...props
+}: TransitionPresenceProps): ReactElement {
+  const previousChildren = useRef<typeof children>(children);
+  const [queue, setQueue] = useState<CrossFlowQueue>(() =>
+    children === null
+      ? {}
+      : {
+          [getId()]: children,
+        },
+  );
+
+  const isMounted = useIsMounted();
+
+  useEffect(() => {
+    // Don't do anything during the first render
+    if (!isMounted.current) {
+      return;
+    }
+
+    // If the children are equal we don't want to do anything
+    const areChildrenEqual = childrenAreEqual(children, previousChildren.current);
+    previousChildren.current = children;
+
+    if (areChildrenEqual) {
+      return;
+    }
+
+    // Add new children to the queue and remove the old children instance for
+    // its key to start the  out transition
+    setQueue((value) => {
+      const emptyQueue: Record<string, null> = {};
+
+      // eslint-disable-next-line guard-for-in
+      for (const key in value) {
+        emptyQueue[key] = null;
+      }
+
+      if (children === null) {
+        return emptyQueue;
+      }
+
+      return {
+        ...emptyQueue,
+        [getId()]: children,
+      };
+    });
+  }, [children, isMounted, previousChildren]);
+
+  const onChildrenMountedCleanup = useCallback(
+    (_children: ReactElement) => {
+      setQueue((value) => {
+        if ('key' in _children && _children.key !== null) {
+          // eslint-disable-next-line @typescript-eslint/no-dynamic-delete
+          delete value[_children.key];
+        }
+
+        return { ...value };
+      });
+
+      onChildrenMounted?.(_children, null);
+    },
+    [onChildrenMounted],
+  );
+
+  return (
+    <>
+      {Object.entries(queue).map(([key, queuedChildren]) => (
+        <TransitionPresence onChildrenMounted={onChildrenMountedCleanup} {...props} key={key}>
+          {queuedChildren && cloneElement(queuedChildren, { key })}
+        </TransitionPresence>
+      ))}
+    </>
+  );
+}

packages/react-transition-presence/src/TransitionPresence/TransitionPresence.mdx

@@ -5,11 +5,13 @@ import * as stories from './TransitionPresence.stories';
 
 # TransitionPresence
 
-`<TransitionPresence />` is a component that allows you to defer unmount of a component.
+`<TransitionPresence />` is a component that allows you to defer the unmount lifecycle of a
+component.
 
 The callback function used in `useBeforeUnmount` in components that are rendered in the context of
 `<TransitionPresence />` will be called just before a component is unmounted. The promises that are
-created using the callback are await before unmounting a component.
+created using the callback are awaited before unmounting a component. New children are rendered
+after old children are unmounted.
 
 The most common use-case for `<TransitionPresence />` is animations, but it's not limited to this
 use case.

packages/react-transition-presence/src/TransitionPresence/TransitionPresence.tsx

@@ -25,7 +25,10 @@ export type TransitionPresenceProps = {
     previousChildren: ReactElement | null,
     children: ReactElement | null,
   ): void | Promise<void>;
-  onChildrenMounted?(children: ReactElement | null): void | Promise<void>;
+  onChildrenMounted?(
+    previousChildren: ReactElement | null,
+    children: ReactElement | null,
+  ): void | Promise<void>;
 };
 
 /**
@@ -84,7 +87,7 @@ export function TransitionPresence({
       setPreviousChildren(children);
       await tick();
 
-      onChildrenMountedRef.current?.(children);
+      onChildrenMountedRef.current?.(previousChildren, children);
     })();
 
     return () => {

packages/react-transition-presence/src/_hooks/useIsMounted.ts

@@ -0,0 +1,17 @@
+import { useEffect, useRef, type RefObject } from 'react';
+
+export function useIsMounted(): RefObject<boolean> {
+  const isMounted = useRef(false);
+
+  useEffect(() => {
+    queueMicrotask(() => {
+      isMounted.current = true;
+    });
+
+    return () => {
+      isMounted.current = false;
+    };
+  }, []);
+
+  return isMounted;
+}

packages/react-transition-presence/src/_utils/getId.ts

@@ -0,0 +1,5 @@
+let id = 0;
+
+export function getId(): string {
+  return (id++ % Number.MAX_SAFE_INTEGER).toString();
+}