[FlexibleHeader] Add scroll view did scroll tests.

Reviewers: #mdc_ios_owners, ajsecord

Reviewed By: #mdc_ios_owners, ajsecord

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D454

components/FlexibleHeader/tests/unit/FlexibleHeaderTrackingScrollViewDidScroll.swift

@@ -0,0 +1,74 @@
+/*
+Copyright 2016-present Google Inc. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import XCTest
+import MaterialComponents
+
+// Tests confirming contract with a tracking scroll view that has scrolled
+class FlexibleHeaderTrackingScrollViewDidScroll: XCTestCase {
+
+  // Implicitly unwrapped to indicate the contract of creating these values in setUp and make our
+  // accessors cleaner in tests.
+  var view: MDCFlexibleHeaderView!
+  var beforeFrame: CGRect!
+  var scrollView: UIScrollView!
+
+  override func setUp() {
+    view = MDCFlexibleHeaderView()
+
+    let originalFrame = CGRect(x: 0, y: 0, width: 100, height: 50)
+    view.frame = originalFrame
+    view.sizeToFit()
+
+    beforeFrame = view.frame
+
+    scrollView = UIScrollView()
+    scrollView.contentSize = CGSize(width: originalFrame.size.width, height: 1000)
+    scrollView.frame = CGRect(x: 0, y: 0, width: originalFrame.size.width, height: 250)
+
+    view.trackingScrollView = scrollView
+  }
+
+  // MARK: Initial changes are ignored.
+
+  func testFirstChangeDoesNothingNegative() {
+    scrollView.setContentOffset(CGPoint(x: 0, y: -view.maximumHeight), animated: false)
+
+    view.trackingScrollViewDidScroll()
+
+    XCTAssertEqual(beforeFrame, view.frame)
+  }
+
+  func testFirstChangeDoesNothingPositive() {
+    scrollView.setContentOffset(CGPoint(x: 0, y: 200), animated: false)
+
+    view.trackingScrollViewDidScroll()
+
+    XCTAssertEqual(beforeFrame, view.frame)
+  }
+
+  // MARK: Expansion on scroll to top.
+
+  func testFullyExpandedOnScrollToTop() {
+    view.trackingScrollViewDidScroll()
+
+    view.maximumHeight = 200
+    scrollView.setContentOffset(CGPoint(x: 0, y: -view.maximumHeight), animated: false)
+    view.trackingScrollViewDidScroll()
+
+    XCTAssertEqual(view.frame.size.height, view.maximumHeight)
+  }
+}