[api] Add "step" option to mouse.move method (#601)

This patch adds "step" option to the mouse.move method, that optionally tweens mouse movement over multiple steps.

References #423.

docs/api.md

@@ -79,7 +79,7 @@
   * [class: Mouse](#class-mouse)
     + [mouse.click(x, y, [options])](#mouseclickx-y-options)
     + [mouse.down([options])](#mousedownoptions)
-    + [mouse.move(x, y)](#mousemovex-y)
+    + [mouse.move(x, y, [options])](#mousemovex-y-options)
     + [mouse.up([options])](#mouseupoptions)
   * [class: Tracing](#class-tracing)
     + [tracing.start(options)](#tracingstartoptions)
@@ -933,9 +933,11 @@ Shortcut for [`mouse.move`](#mousemovex-y), [`mouse.down`](#mousedownoptions) an
 
 Dispatches a `mousedown` event.
 
-#### mouse.move(x, y)
+#### mouse.move(x, y, [options])
 - `x` <[number]>
 - `y` <[number]>
+- `options` <[Object]>
+  - `steps` <[number]> defaults to 1. Sends intermediate `mousemove` events.
 - returns: <[Promise]>
 
 Dispatches a `mousemove` event.

lib/Input.js

@@ -109,17 +109,23 @@ class Mouse {
   /**
    * @param {number} x
    * @param {number} y
+   * @param {Object=} options
    * @return {!Promise}
    */
-  async move(x, y) {
+  async move(x, y, options = {}) {
+    const fromX = this._x, fromY = this._y;
     this._x = x;
     this._y = y;
-    await this._client.send('Input.dispatchMouseEvent', {
+    const steps = options.steps || 1;
-      type: 'mouseMoved',
+    for (let i = 1; i <= steps; i++) {
-      button: this._button,
+      await this._client.send('Input.dispatchMouseEvent', {
-      x, y,
+        type: 'mouseMoved',
-      modifiers: this._keyboard._modifiers
+        button: this._button,
-    });
+        x: fromX + (this._x - fromX) * (i / steps),
+        y: fromY + (this._y - fromY) * (i / steps),
+        modifiers: this._keyboard._modifiers
+      });
+    }
   }
 
   /**

test/test.js

@@ -1472,6 +1472,24 @@ describe('Page', function() {
       // This await should not hang.
       await page.click('a');
     }));
+    it('should tween mouse movement', SX(async function() {
+      await page.evaluate(() => {
+        window.result = [];
+        document.addEventListener('mousemove', event => {
+          window.result.push([event.clientX, event.clientY]);
+        });
+      });
+      await page.mouse.move(100, 100);
+      await page.mouse.move(200, 300, {steps: 5});
+      expect(await page.evaluate('result')).toEqual([
+        [100, 100],
+        [120, 140],
+        [140, 180],
+        [160, 220],
+        [180, 260],
+        [200, 300]
+      ]);
+    }));
     function dimensions() {
       const rect = document.querySelector('textarea').getBoundingClientRect();
       return {