2 files changed, 132 insertions, 0 deletions
diff --git a/src/lib/animations/blink.test.ts b/src/lib/animations/blink.test.ts
new file mode 100644
index 0000000..32dc872
--- /dev/null
+++ b/src/lib/animations/blink.test.ts
@@ -0,0 +1,107 @@
+/**
+ * @jest-environment jsdom
+ */
+
+import { blink } from './blink';
+import { sineOut } from 'svelte/easing';
+
+const internals = {
+  response: null
+};
+
+describe('blink', () => {
+
+  test('it has a default delay of 0ms', () => {
+
+    const response = blink(document.createElement('div'), {});
+
+    expect(response.delay).toBe(0);
+  });
+
+  test('it allows delay to be overridden', () => {
+
+    const response = blink(document.createElement('div'), {
+      delay: 300
+    });
+
+    expect(response.delay).toBe(300);
+  });
+
+  test('it has a default duration of 400ms', () => {
+
+    const response = blink(document.createElement('div'), {});
+
+    expect(response.duration).toBe(400);
+  });
+
+  test('it allows delay to be overridden', () => {
+
+    const response = blink(document.createElement('div'), {
+      duration: 999
+    });
+
+    expect(response.duration).toBe(999);
+  });
+
+  test('it uses sineOut as the default easing function', () => {
+
+    const response = blink(document.createElement('div'), {});
+
+    expect(response.easing).toBe(sineOut);
+  });
+
+  test('it allows easing function to be overridden', () => {
+
+    const response = blink(document.createElement('div'), {
+      easing: () => 666
+    });
+
+    expect(response.easing(0)).toBe(666);
+  });
+
+  describe('css animation function', () => {
+
+    beforeEach(() => {
+
+      const div = document.createElement('div');
+      div.style.width = '100px';
+      div.style.height = '200px';
+      internals.response = blink(div, {});
+    });
+
+    test('It starts with with zeroed width and height', () => {
+
+      const css = internals.response.css(0, 1);
+      expect(css).toContain('width: 0px');
+      expect(css).toContain('height: 0px');
+    });
+
+    test('It grows to full height and 0 width in first 20%', () => {
+
+      const css = internals.response.css(0.2, 0.8);
+      expect(css).toContain('width: 0px');
+      expect(css).toContain('height: 200px');
+    });
+
+    test('It expands to full height by the end', () => {
+
+      const css = internals.response.css(1, 0);
+      expect(css).toContain('width: 100px');
+      expect(css).toContain('height: 200px');
+    });
+
+    test('It keeps element vertically centered by adjusting the margin', () => {
+
+      const css = internals.response.css(0.1, 0.9);
+      expect(css).toContain('margin: 50px 50px');
+      expect(css).toContain('height: 100px');
+    });
+
+    test('It keeps element horizontally centered by adjusting the margin', () => {
+
+      const css = internals.response.css(0.6, 0.4);
+      expect(css).toContain('margin: 0px 25px');
+      expect(css).toContain('width: 50px');
+    });
+  });
+});
diff --git a/src/lib/animations/blink.ts b/src/lib/animations/blink.ts
new file mode 100644
index 0000000..c291b06
--- /dev/null
+++ b/src/lib/animations/blink.ts
@@ -0,0 +1,25 @@
+import { sineOut } from 'svelte/easing';
+import type { AnimationConfig } from 'svelte/animate';
+
+export const blink = function blink(node: HTMLElement, params: AnimationConfig): AnimationConfig{
+
+  const originalWidth = parseFloat(getComputedStyle(node).width);
+  const originalHeight = parseFloat(getComputedStyle(node).height);
+
+  return {
+    delay: params.delay || 0,
+    duration: params.duration || 400,
+    easing: params.easing || sineOut,
+    css: (t: number): string => {
+
+      const halfWidth = originalWidth / 2;
+      const halfHeight = originalHeight / 2;
+      const height = Math.round(t <= 0.2 ? (originalHeight * t) / 0.2 : originalHeight);
+      const marginY = Math.round(t <= 0.2 ? halfHeight * (1 - t / 0.2) : 0);
+      const width = Math.round(t > 0.2 ? ((t - 0.2) / 0.8) * originalWidth : 0);
+      const marginX = Math.round(t > 0.2 ? (1 - (t - 0.2) / 0.8) * halfWidth : halfWidth);
+
+      return `width: ${width}px; height: ${height}px; margin: ${marginY}px ${marginX}px`;
+    }
+  };
+};