Add ScrollPageToElement to action_runner.

telemetry/telemetry/internal/actions/action_runner_unittest.py

Index: telemetry/telemetry/internal/actions/action_runner_unittest.py
diff --git a/telemetry/telemetry/internal/actions/action_runner_unittest.py b/telemetry/telemetry/internal/actions/action_runner_unittest.py
index 8e89bce7663a0cd33bf1edd15d7470368460e49b..b8d562cc8415b8f84de1e1a255cd0e15920d1274 100644
--- a/telemetry/telemetry/internal/actions/action_runner_unittest.py
+++ b/telemetry/telemetry/internal/actions/action_runner_unittest.py
@@ -285,6 +285,32 @@ class ActionRunnerTest(tab_test_case.TabTestCase):
     self.assertTrue(action_runner.EvaluateJavaScript(
         '(document.scrollingElement || document.body).scrollLeft') > 75)
 
+    off_screen_element = 'document.querySelectorAll("#off-screen")[0]'

[nednguyen, 2016/10/14 00:19:01]

Can you move this to a separate test case, s.t like ScrollPageToElement ?

[charliea (OOO until 10-5), 2016/10/14 03:34:28]

+1

[rnephew (Reviews Here), 2016/10/14 17:57:18]

Done.

+    determine_if_on_screen = '''

[charliea (OOO until 10-5), 2016/10/14 03:34:28]

I think this variable is a little misleadingly named. I'd expect a
"determine_if_on_screen" variable to be a function that returns True or False
depending on whether the element is on the screen. This one instead returns 0,
1, or 2. 

Maybe something like "viewport_comparator" would be better? Or, because it's not
a python function but rather a js one, viewport_comparator_js?

[rnephew (Reviews Here), 2016/10/14 17:57:18]

Done.

+      function getOnScreen(elem) {

[charliea (OOO until 10-5), 2016/10/14 03:34:28]

nit: similar comment to before, where I think it makes sense to make this an
anonymous function to avoid polluting the global namespace:

(elem) => {
  var rect = elem.getBoundingClientRect();
  ...
}(%s)

[rnephew (Reviews Here), 2016/10/14 17:57:17]

Done.

+        var rect = elem.getBoundingClientRect();
+
+        if (rect.bottom < 0) {
+          // The bottom of the element is above the viewport.
+          return 2

[charliea (OOO until 10-5), 2016/10/14 03:34:28]

nit: all JS lines should end in semicolons

[charliea (OOO until 10-5), 2016/10/14 03:34:28]

I might go with -1 (above the viewport), 0 (in the viewport), and 1 (below the
viewport) to mirror the typical interface for compare functions (see "compare
function" here:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Obje...)

[rnephew (Reviews Here), 2016/10/14 17:57:18]

Done.

[rnephew (Reviews Here), 2016/10/14 17:57:18]

Done.

+        }
+        if (rect.top - window.innerHeight > 0) {
+          // rect.top provides the pixel offset of the element from the
+          // top of the page. Because that exceeds the viewport's height,
+          // we know that the element is below the viewport.
+          return 1

[charliea (OOO until 10-5), 2016/10/14 03:34:28]

nit: need semicolon

[rnephew (Reviews Here), 2016/10/14 17:57:18]

Done.

+        }
+        return 0

[charliea (OOO until 10-5), 2016/10/14 03:34:28]

nit: need semicolon

[rnephew (Reviews Here), 2016/10/14 17:57:17]

Done.

+      }
+      getOnScreen(%s)
+    ''' % off_screen_element
+
+    self.assertEqual(
+        action_runner.EvaluateJavaScript(determine_if_on_screen), 1)
+    action_runner.ScrollPageToElement(selector='#off-screen')
+    self.assertEqual(
+        action_runner.EvaluateJavaScript(determine_if_on_screen), 0)
+
   @decorators.Disabled('android',   # crbug.com/437065.
                        'chromeos')  # crbug.com/483212.
   def testSwipe(self):