From bfed67e2552c44c5bc1a58d4168f099fee5d758a Mon Sep 17 00:00:00 2001
From: schmelczerandras <schmelczer.andras@simonyi.bme.hu>
Date: Mon, 19 Oct 2020 11:41:47 +0200
Subject: [PATCH] Add delta time calculator

---
 src/helper/delta-time-calculator.ts | 44 +++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 src/helper/delta-time-calculator.ts

diff --git a/src/helper/delta-time-calculator.ts b/src/helper/delta-time-calculator.ts
new file mode 100644
index 0000000..87fd4c3
--- /dev/null
+++ b/src/helper/delta-time-calculator.ts
@@ -0,0 +1,44 @@
+/**
+ * A helper class for calculating the elapsed time between frames.
+ *
+ * Handles the case, where the browser tab is not in focus and `requestAnimationFrame`
+ * does not get called for performance reasons. In this case, the return deltaTime won't be
+ * an unreasonably large value.
+ */
+export class DeltaTimeCalculator {
+  private previousTime: DOMHighResTimeStamp | null = null;
+  private visibilityChangeHandler = this.handleVisibilityChange.bind(this);
+
+  constructor() {
+    document.addEventListener('visibilitychange', this.visibilityChangeHandler);
+  }
+
+  /**
+   * Return the elapsed time in **milliseconds** since the previous call of this
+   * method by subtracting the previously given `currentTime` argument from the
+   * current `currentTime` argument.
+   *
+   * Takes visibilitychange events into account.
+   *
+   * @param currentTime Current time acquired e.g. with `requestAnimationFrame`.
+   */
+  public getNextDeltaTime(currentTime: DOMHighResTimeStamp): DOMHighResTimeStamp {
+    if (this.previousTime === null) {
+      this.previousTime = currentTime;
+    }
+
+    const delta = currentTime - this.previousTime;
+    this.previousTime = currentTime;
+    return delta;
+  }
+
+  public destroy() {
+    document.removeEventListener('visibilitychange', this.visibilityChangeHandler);
+  }
+
+  private handleVisibilityChange() {
+    if (!document.hidden) {
+      this.previousTime = null;
+    }
+  }
+}