[NaCl SDK] Screen capture extension for use with SDK visual testing.

native_client_sdk/src/build_tools/screenshot_extension/screenshot.js

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var screenshot = (function() {
+  /** A map of id to pending callback. */
+  var callbackMap = {};
+
+  /** An array of queued requests. They will all be executed when the
+   * background page injects screen code into this page
+   */
+  var queuedRequests = [];
+
+  /** The next id to assign. Used for mapping id to callback. */
+  var nextId = 0;
+
+  /** This is set to true when the background page injects screenshot code into
+   * this page
+   */
+  var extensionInjected = false;
+
+  /** Generate a new id, which maps to the given callbacks.
+   *
+   * @param {function(string)} onSuccess
+   * @param {function(string)} onError
+   * @return {number} The id.
+   */
+  function addCallback(onSuccess, onError) {
+    var id = nextId++;
+    callbackMap[id] = [onSuccess, onError];
+    return id;
+  }
+
+  /** Call the callback mapped to |id|.
+   *
+   * @param {number} id
+   * @param {boolean} success true to call the success callback, false for the
+   *                          error callback.
+   * @param {...} A variable number of arguments to pass to the callback.
+   */
+  function callCallback(id, success) {
+    var callbacks = callbackMap[id];
+    if (!callbacks) {
+      console.log('Unknown id: ' + id);
+      return;
+    }
+
+    delete callbackMap[id];
+    var callback = success ? callbacks[0] : callbacks[1];
+    if (callback)
+      callback(Array.prototype.slice.call(arguments, 2));
+  }
+
+  /** Post a message to take a screenshot.
+   *
+   * This message will be enqueued if the extension has not yet injected the
+   * screenshot code.
+   *
+   * @param {number} id An id to associate with this request. When the
+   *                    screenshot is complete, the background page will return
+   *                    a result with this id.
+   */
+  function postScreenshotMessage(id) {
+    if (!extensionInjected) {
+      queuedRequests.push(id);
+      return;
+    }
+
+    window.postMessage({id: id, target: 'background'}, '*');
+  }
+
+  /** Post all queued screenshot requests.
+   *
+   * Should only be called after the screenshot code has been injected by the
+   * background page.
+   */
+  function postQueuedRequests() {
+    for (var i = 0; i < queuedRequests.length; ++i) {
+      var id = queuedRequests[i];
+      postScreenshotMessage(id);
+    }
+    queuedRequests = [];
+  }
+
+  /** Predicate whether the extension has injected code yet.
+   *
+   * @return {boolean}
+   */
+  function isInjected() {
+    // NOTE: This attribute name must match the one in injected.js.
+    return document.body &&
+        document.body.getAttribute('screenshot_extension_injected');
+  }
+
+  /** Start an interval that polls for when the extension has injected code
+   * into this page.
+   *
+   * The extension first adds a postMessage handler to listen for requests,
+   * then adds an attribute to the body element. If we see this attribute, we
+   * know the listener is ready.
+   */
+  function pollForInjection() {
+    var intervalId = window.setInterval(function() {
+      if (!isInjected())
+        return;
+
+      // Finally injected!
+      window.clearInterval(intervalId);
+      extensionInjected = true;
+      postQueuedRequests();
+    }, 100);  // Every 100ms.
+  }
+
+  // Add a postMessage listener for when the injected code returns a result
+  // from the background page.
+  window.addEventListener('message', function(event) {
+    // If the message comes from another window, or is outbound (i.e.
+    // event.data.target === 'background'), ignore it.
+    if (event.source !== window || event.data.target !== 'page')
+      return;
+
+    var success = event.data.error === undefined;
+    callCallback(event.data.id, success, event.data.data);
+  }, false);
+
+  if (isInjected())
+    extensionInjected = true;
+  else
+    pollForInjection();
+
+  // Public functions.
+
+  /** Capture the current visible area of the tab as a PNG.
+   *
+   * If the request succeeds, |onSuccess| will be called with one parameter:
+   * the image encoded as a data URL.
+   *
+   * If the request fails, |onError| will be called with one parameter: an
+   * informational error message.
+   *
+   * @param {function(string)} onSuccess The success callback.
+   * @param {function(string)} onError The error callback.
+   */
+  function captureTab(onSuccess, onError) {
+    var id = addCallback(onSuccess, onError);
+    postScreenshotMessage(id);
+  }
+
+  /** Capture the current visible area of a given element as a PNG.
+   *
+   * If the request succeeds, |onSuccess| will be called with one parameter:
+   * the image encoded as a data URL.
+   *
+   * If the request fails, |onError| will be called with one parameter: an
+   * informational error message.
+   *
+   * @param {Element} element The element to capture.
+   * @param {function(string)} onSuccess The success callback.
+   * @param {function(string)} onError The error callback.
+   */
+  function captureElement(element, onSuccess, onError) {
+    var elementRect = element.getBoundingClientRect();
+    var elX = elementRect.left;
+    var elY = elementRect.top;
+    var elW = elementRect.width;
+    var elH = elementRect.height;
+
+    function onScreenCaptured(dataURL) {
+      // Create a canvas of the correct size.
+      var canvasEl = document.createElement('canvas');
+      canvasEl.setAttribute('width', elW);
+      canvasEl.setAttribute('height', elH);
+      var ctx = canvasEl.getContext('2d');
+
+      var imgEl = new Image();
+      imgEl.onload = function() {
+        // Draw only the element region of the image.
+        ctx.drawImage(imgEl, elX, elY, elW, elH, 0, 0, elW, elH);
+
+        // Extract the canvas to a new data URL, and return it via the callback.
+        onSuccess(canvasEl.toDataURL());
+      };
+
+      // Load the full screenshot into imgEl.
+      imgEl.src = dataURL;
+    }
+
+    var id = addCallback(onScreenCaptured, onError);
+    postScreenshotMessage(id);
+  }
+
+  return {
+    captureTab: captureTab,
+    captureElement: captureElement
+  };
+})();