add validation for background_rgba_color option

Author: Mifrill

README.md

@@ -335,6 +335,7 @@ Saves screenshot on a disk or returns it as base64.
   * :full `Boolean` whether you need full page screenshot or a viewport
   * :selector `String` css selector for given element
   * :scale `Float` zoom in/out
+  * :background_rgba_color `Array` like `[R,G,B,A]` to have specific background color
 
 ```ruby
 browser.go_to("https://google.com/")
@@ -344,11 +345,8 @@ browser.screenshot(path: "google.png") # => 134660
 browser.screenshot(path: "google.jpg") # => 30902
 # Save to Base64 the whole page not only viewport and reduce quality
 browser.screenshot(full: true, quality: 60) # "iVBORw0KGgoAAAANSUhEUgAABAAAAAMACAYAAAC6uhUNAAAAAXNSR0IArs4c6Q...
-
-# To have transparent background for PNGs run before screenshotting
-browser.command('Emulation.setDefaultBackgroundColorOverride', color: { r: 0, g: 0, b: 0, a: 0 }) 
-# To restore background run
-browser.command('Emulation.setDefaultBackgroundColorOverride') 
+# Save with specific background color
+browser.screenshot(background_rgba_color: [0, 0, 0, 0]) 
 ```
 
 #### pdf(\*\*options) : `String` | `Integer`

lib/ferrum/page/screenshot.rb

@@ -123,9 +123,21 @@ def screenshot_options(path = nil, format: nil, scale: 1.0, **opts)
           options[:clip].merge!(scale: scale)
         end
 
+        if validate_background_rgba_color(opts[:background_rgba_color])
+          raise ArgumentError, "Specify :background_rgba_color as [R,G,B,A] array"
+        end
+
         options
       end
 
+      def validate_background_rgba_color(option)
+        option && !(
+          option.is_a?(Array) &&
+            option.size == 4 &&
+            option.all? { |value| value.is_a?(Integer) }
+        )
+      end
+
       def get_bounding_rect(selector)
         rect = evaluate_async(%Q(
           const rect = document

spec/screenshot_spec.rb

@@ -176,6 +176,32 @@ def create_screenshot(**options)
           end
         end
 
+        context 'background_rgba_color option' do
+          shared_examples 'background_rgba_color ArgumentError' do
+            it 'raises ArgumentError with message' do
+              browser.go_to
+              expect {
+                browser.screenshot(path: file, background_rgba_color: background_rgba_color)
+              }.to raise_exception(ArgumentError, 'Specify :background_rgba_color as [R,G,B,A] array')
+            end
+          end
+
+          include_examples 'background_rgba_color ArgumentError' do
+            let(:background_rgba_color) do
+              r, g, b = 0, 0, 0
+              [r, g , b]
+            end
+          end
+
+          include_examples 'background_rgba_color ArgumentError' do
+            let(:background_rgba_color)  { '#FFF' }
+          end
+
+          include_examples 'background_rgba_color ArgumentError' do
+            let(:background_rgba_color)  { %w[0 0 0 0] }
+          end
+        end
+
         shared_examples "when scale is set" do
           it "changes image dimensions" do
             browser.go_to("/ferrum/zoom_test")