LibWeb: Implement screenshot painting inside Web::WebDriver

This will allow for implementing the screenshot action closer to the
spec, as we can now use HTMLCanvasElement to encode the bitmap, and
capture the screenshot on the animation frame loop.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -442,6 +442,7 @@ set(SOURCES
     WebDriver/Error.cpp
     WebDriver/ExecuteScript.cpp
     WebDriver/Response.cpp
+    WebDriver/Screenshot.cpp
     WebGL/WebGLContextAttributes.cpp
     WebGL/WebGLContextEvent.cpp
     WebGL/WebGLRenderingContext.cpp

Userland/Libraries/LibWeb/WebDriver/Screenshot.cpp

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Optional.h>
+#include <LibGfx/Bitmap.h>
+#include <LibGfx/Rect.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/ElementFactory.h>
+#include <LibWeb/HTML/AnimationFrameCallbackDriver.h>
+#include <LibWeb/HTML/BrowsingContext.h>
+#include <LibWeb/HTML/HTMLCanvasElement.h>
+#include <LibWeb/HTML/TagNames.h>
+#include <LibWeb/HTML/Window.h>
+#include <LibWeb/Namespace.h>
+#include <LibWeb/Page/Page.h>
+#include <LibWeb/Platform/EventLoopPlugin.h>
+#include <LibWeb/WebDriver/Error.h>
+#include <LibWeb/WebDriver/Screenshot.h>
+
+namespace Web::WebDriver {
+
+// https://w3c.github.io/webdriver/#dfn-encoding-a-canvas-as-base64
+static Response encode_canvas_element(HTML::HTMLCanvasElement const& canvas)
+{
+    // FIXME: 1. If the canvas element’s bitmap’s origin-clean flag is set to false, return error with error code unable to capture screen.
+
+    // 2. If the canvas element’s bitmap has no pixels (i.e. either its horizontal dimension or vertical dimension is zero) then return error with error code unable to capture screen.
+    if (canvas.bitmap()->width() == 0 || canvas.bitmap()->height() == 0)
+        return Error::from_code(ErrorCode::UnableToCaptureScreen, "Captured screenshot is empty"sv);
+
+    // 3. Let file be a serialization of the canvas element’s bitmap as a file, using "image/png" as an argument.
+    // 4. Let data url be a data: URL representing file. [RFC2397]
+    auto data_url = canvas.to_data_url("image/png"sv, {});
+
+    // 5. Let index be the index of "," in data url.
+    auto index = data_url.find(',');
+    VERIFY(index.has_value());
+
+    // 6. Let encoded string be a substring of data url using (index + 1) as the start argument.
+    auto encoded_string = data_url.substring(*index + 1);
+
+    // 7. Return success with data encoded string.
+    return JsonValue { move(encoded_string) };
+}
+
+// Common animation callback steps between:
+// https://w3c.github.io/webdriver/#take-screenshot
+// https://w3c.github.io/webdriver/#take-element-screenshot
+Response capture_element_screenshot(Painter const& painter, Page& page, DOM::Element& element, Gfx::IntRect& rect)
+{
+    Optional<Response> encoded_string_or_error;
+
+    element.document().window().animation_frame_callback_driver().add([&](auto) {
+        auto viewport_rect = page.top_level_browsing_context().viewport_rect();
+        rect.intersect(viewport_rect);
+
+        auto canvas_element = DOM::create_element(element.document(), HTML::TagNames::canvas, Namespace::HTML);
+        auto& canvas = verify_cast<HTML::HTMLCanvasElement>(*canvas_element);
+
+        if (!canvas.create_bitmap(rect.width(), rect.height())) {
+            encoded_string_or_error = Error::from_code(ErrorCode::UnableToCaptureScreen, "Unable to create a screenshot bitmap"sv);
+            return;
+        }
+
+        painter(rect, *canvas.bitmap());
+        encoded_string_or_error = encode_canvas_element(canvas);
+    });
+
+    Platform::EventLoopPlugin::the().spin_until([&]() { return encoded_string_or_error.has_value(); });
+    return encoded_string_or_error.release_value();
+}
+
+}

Userland/Libraries/LibWeb/WebDriver/Screenshot.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <LibGfx/Forward.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/WebDriver/Response.h>
+
+namespace Web::WebDriver {
+
+using Painter = Function<void(Gfx::IntRect const&, Gfx::Bitmap&)>;
+Response capture_element_screenshot(Painter const& painter, Page& page, DOM::Element& element, Gfx::IntRect& rect);
+
+}