Hello world

First open source release of Campfire 🎉

app/javascript/models/scroll_manager.js

@@ -0,0 +1,57 @@
+const AUTO_SCROLL_THRESHOLD = 100
+
+export default class ScrollManager {
+  static #pendingOperations = Promise.resolve()
+
+  #container
+
+  constructor(container) {
+    this.#container = container
+  }
+
+  async autoscroll(forceScroll, render = () => {}) {
+    return this.#appendOperation(async () => {
+      const wasNearEnd = this.#scrolledNearEnd
+
+      await render()
+
+      if (wasNearEnd || forceScroll) {
+        this.#container.scrollTop = this.#container.scrollHeight
+        return true
+      } else {
+        return false
+      }
+    })
+  }
+
+  async keepScroll(top, render) {
+    return this.#appendOperation(async () => {
+      const scrollTop = this.#container.scrollTop
+      const scrollHeight = this.#container.scrollHeight
+
+      await render()
+
+      if (top) {
+        this.#container.scrollTop = scrollTop + (this.#container.scrollHeight - scrollHeight)
+      } else {
+        this.#container.scrollTop = scrollTop
+      }
+    })
+  }
+
+  // Private
+
+  #appendOperation(operation) {
+    ScrollManager.#pendingOperations =
+      ScrollManager.#pendingOperations.then(operation)
+    return ScrollManager.#pendingOperations
+  }
+
+  get #scrolledNearEnd() {
+    return this.#distanceScrolledFromEnd <= AUTO_SCROLL_THRESHOLD
+  }
+
+  get #distanceScrolledFromEnd() {
+    return this.#container.scrollHeight - this.#container.scrollTop - this.#container.clientHeight
+  }
+}