add Ferrum::Mouse#scroll_by

Author: toy

lib/ferrum/mouse.rb

@@ -18,6 +18,23 @@ def initialize(page)
       @buttons = 0
     end
 
+    #
+    # Scroll page by the given amount x, y.
+    #
+    # @param [Integer] x
+    #  The horizontal pixel value that you want to scroll by.
+    #
+    # @param [Integer] y
+    #   The vertical pixel value that you want to scroll by.
+    #
+    # @example
+    #   browser.go_to("https://www.google.com/search?q=Ruby+headless+driver+for+Capybara")
+    #   browser.mouse.scroll_by(0, 400)
+    #
+    def scroll_by(x, y)
+      tap { @page.execute("window.scrollBy(#{x}, #{y})") }
+    end
+
     #
     # Scroll page to a given x, y coordinates.
     #

spec/mouse_spec.rb

@@ -32,6 +32,19 @@
     end
   end
 
+  describe "#scroll_by" do
+    it "allows the page to be scrolled" do
+      browser.go_to("/ferrum/long_page")
+      browser.resize(width: 10, height: 10)
+      browser.mouse.scroll_by(30, 70)
+      browser.mouse.scroll_by(40, -50)
+      browser.mouse.scroll_by(-60, 0)
+      expect(
+        browser.evaluate("[window.scrollX, window.scrollY]")
+      ).to eq([10, 20])
+    end
+  end
+
   describe "#scroll_to" do
     it "allows the page to be scrolled" do
       browser.go_to("/ferrum/long_page")