Add useInstance (#6)

packages/reacord/test/use-instance.test.tsx

@@ -0,0 +1,72 @@
+import React from "react"
+import { describe, expect, it } from "vitest"
+import type { ReacordInstance } from "../library/main"
+import { Button, useInstance } from "../library/main"
+import type { MessageSample } from "./test-adapter"
+import { ReacordTester } from "./test-adapter"
+
+describe("useInstance", () => {
+  it("returns the instance of itself", async () => {
+    let instanceFromHook: ReacordInstance | undefined
+
+    function TestComponent({ name }: { name: string }) {
+      const instance = useInstance()
+      instanceFromHook ??= instance
+      return (
+        <>
+          <Button
+            label={`create ${name}`}
+            onClick={(event) => {
+              event.reply(<TestComponent name="child" />)
+            }}
+          />
+          <Button
+            label={`destroy ${name}`}
+            onClick={() => instance.destroy()}
+          />
+        </>
+      )
+    }
+
+    function messageOutput(name: string): MessageSample {
+      return {
+        content: "",
+        embeds: [],
+        actionRows: [
+          [
+            {
+              type: "button",
+              label: `create ${name}`,
+              style: "secondary",
+            },
+            {
+              type: "button",
+              label: `destroy ${name}`,
+              style: "secondary",
+            },
+          ],
+        ],
+      }
+    }
+
+    const tester = new ReacordTester()
+    const instance = tester.send(<TestComponent name="parent" />)
+
+    await tester.assertMessages([messageOutput("parent")])
+    expect(instanceFromHook).toBe(instance)
+
+    tester.findButtonByLabel("create parent").click()
+    await tester.assertMessages([
+      messageOutput("parent"),
+      messageOutput("child"),
+    ])
+
+    // this test ensures that the only the child instance is destroyed,
+    // and not the parent instance
+    tester.findButtonByLabel("destroy child").click()
+    await tester.assertMessages([messageOutput("parent")])
+
+    tester.findButtonByLabel("destroy parent").click()
+    await tester.assertMessages([])
+  })
+})