Add ScreenMock to helpers

Mocks DOM API calls for position and dimension of
elements and provides an API to allow access to
them.

tests/javascripts/support/helpers.js

@@ -1,3 +1,20 @@
+function getDescriptorForProperty (prop, obj) {
+  const descriptors = Object.getOwnPropertyDescriptors(obj);
+  const prototype = Object.getPrototypeOf(obj);
+
+  if ((descriptors !== {}) && (prop in descriptors)) {
+    return descriptors[prop];
+  }
+
+  // if not in this object's descriptors, check the prototype chain
+  if (prototype !== null) {
+    return getDescriptorForProperty(prop, prototype);
+  }
+
+  // no descriptor for this prop and no prototypes left in the chain
+  return null;
+};
+
 const triggerEvent = (el, evtType, options) => {
   const eventInit = {
     bubbles: true,
@@ -235,6 +252,7 @@ class WindowMock {
     document.documentElement.scrollTop = scrollPosition;
     window.scrollY = scrollPosition;
     window.pageYOffset = scrollPosition;
+
     triggerEvent(window, 'scroll');
 
   }
@@ -297,6 +315,162 @@ class SelectionMock extends DOMInterfaceMock {
   }
 
 }
+class ScreenRenderItem {
+  constructor (jest, node) {
+
+    this._jest = jest;
+    this._node = node;
+    this._storeProps();
+    this._mockAPICalls();
+
+  }
+
+  setData (itemData) {
+
+    // check all the item data is present
+    const itemProps = Object.keys(itemData);
+    const missingKeys = ScreenRenderItem.REQUIRED_PROPS.filter(prop => !itemProps.includes(prop));
+
+    this._data = {};
+
+    if (missingKeys.length) {
+      throw Error(`${itemData.name ? itemData.name : itemProps.join(', ')} is missing these properties: ${missingKeys.join(', ')}`);
+    }
+
+    // default left if not set
+    if (!('offsetLeft' in itemData)) { itemData.offsetLeft = 0; }
+
+    // copy onto internal store
+    Object.assign(this._data, itemData);
+
+  }
+
+  _getBoundingClientRect () {
+    const {offsetHeight, offsetWidth, offsetTop, offsetLeft} = this._data;
+    const x = offsetLeft - window.scrollX;
+    const y = offsetTop - window.scrollY;
+
+    return {
+      'x': x,
+      'y': y,
+      'top': (offsetHeight < 0) ? y + offsetHeight : y,
+      'left': (offsetWidth < 0) ? x + offsetWidth : x,
+      'bottom': (offsetTop + offsetHeight) - window.scrollY,
+      'right': (offsetLeft + offsetWidth) - window.scrollX,
+    };
+
+  }
+
+  reset () {
+
+    // reset DOMRect mock
+    this._node.getBoundingClientRect.mockClear();
+
+    ScreenRenderItem.OFFSET_PROPS.forEach(prop => {
+
+      if (prop in this._propStore) {
+        // replace property implementation
+        Object.defineProperty(this._node, prop, this._propStore[prop]);
+      }
+
+    });
+
+  }
+
+  _storeProps () {
+
+    this._propStore = {};
+
+    ScreenRenderItem.OFFSET_PROPS.forEach(prop => {
+      const descriptor = getDescriptorForProperty(prop, this._node);
+
+      if (descriptor !== null) {
+        this._propStore[prop] = descriptor;
+      }
+    });
+
+  }
+
+  // mock any calls to the node's DOM API for position/dimension
+  _mockAPICalls () {
+
+    // proxy boundingClientRect property calls to item data
+    this._jest.spyOn(this._node, 'getBoundingClientRect').mockImplementation(() => this._getBoundingClientRect());
+
+    // handle calls to offset properties
+    ScreenRenderItem.OFFSET_PROPS.forEach(prop => {
+
+      this._jest.spyOn(this._node, prop, 'get').mockImplementation(() => this._data[prop]);
+
+      // proxy DOM API sets for offsetValues (not possible to mock directly)
+      Object.defineProperty(this._node, prop, {
+        configurable: true,
+        set: jest.fn(value => {
+          this._data[prop] = value;
+          return true;
+        })
+      });
+
+    });
+
+  }
+
+}
+ScreenRenderItem.OFFSET_PROPS = ['offsetHeight', 'offsetWidth', 'offsetTop', 'offsetLeft'];
+ScreenRenderItem.REQUIRED_PROPS = ['name', 'offsetHeight', 'offsetHeight', 'offsetWidth', 'offsetTop'];
+
+class ScreenMock {
+  constructor (jest) {
+
+    this._jest = jest
+    this._items = {};
+
+  }
+
+  mockPositionAndDimension (itemName, node, itemData) {
+
+    if (itemName in this._items) { throw new Error(`${itemName} already has its position and dimension mocked`); }
+
+    const data = Object.assign({ 'name': itemName }, itemData);
+    const item = new ScreenRenderItem(this._jest, node);
+
+    item.setData(data);
+
+    this._items[itemName] = item;
+
+  }
+
+  setWindow (windowData) {
+
+    this.window = new WindowMock(this._jest);
+
+    // check all the window data is present
+    const missingKeys = Object.keys(windowData).filter(key => !ScreenMock.REQUIRED_WINDOW_PROPS.includes(key));
+
+    if (missingKeys.length) {
+      throw Error(`Window definition is missing these properties: ${missingKeys.join(', ')}`);
+    }
+
+    this.window.setHeightTo(windowData.height);
+    this.window.setWidthTo(windowData.width);
+    this.window.scrollTo(windowData.scrollTop);
+
+  }
+
+  scrollTo (scrollTop) {
+
+    this.window.scrollTo(scrollTop);
+
+  }
+
+  reset () {
+
+    Object.keys(this._items).forEach(itemName => this._items[itemName].reset());
+
+  }
+
+}
+ScreenMock.REQUIRED_WINDOW_PROPS = ['height', 'width', 'scrollTop'];
 
 // function to ask certain questions of a DOM Element
 const element = function (el) {
@@ -311,3 +485,4 @@ exports.RangeMock = RangeMock;
 exports.SelectionMock = SelectionMock;
 exports.element = element;
 exports.WindowMock = WindowMock;
+exports.ScreenMock = ScreenMock;