components/FlexibleHeader/tests/unit/ScrollViewContentOffsetSideEffectTests.swift

// Copyright 2016-present the Material Components for iOS authors. 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.MaterialFlexibleHeader

// Tests verifying contentOffset side effect behavior when changing contentInset.
class ScrollViewContentOffsetSideEffectTests: XCTestCase {

  let bounds = CGRect(x: 0, y: 0, width: 320, height: 20)
  let inset = CGFloat(6)

  let initialOffset = CGPoint(x: 0, y: 0)

  // Calculates the estimated contentOffset given the proposed new contentInsets and current value
  // of contentOffset + contentSize.
  class func estimatedContentOffsetForScrollView(
    _ scrollView: UIScrollView,
    withNewInsets newInsets: UIEdgeInsets
  ) -> CGPoint {
    let minimumPossibleYOffset = -newInsets.top
    let maximumPossibleYOffset =
      (scrollView.contentSize.height + newInsets.bottom) - scrollView.bounds.height
    let estimatedYOffset =
      max(minimumPossibleYOffset, min(maximumPossibleYOffset, scrollView.contentOffset.y))

    let minimumPossibleXOffset = -newInsets.left
    let maximumPossibleXOffset =
      (scrollView.contentSize.width + newInsets.right) - scrollView.bounds.width
    let estimatedXOffset =
      max(minimumPossibleXOffset, min(maximumPossibleXOffset, scrollView.contentOffset.x))

    return CGPoint(x: estimatedXOffset, y: estimatedYOffset)
  }

  func testEstimationAddingTopInsets() {
    let insets = UIEdgeInsets(top: inset, left: 0, bottom: 0, right: 0)

    enumerateHeightRangeWithInsets(UIEdgeInsets(), after: insets) { scrollView, contentOffset in
      XCTAssertEqual(scrollView.contentOffset, contentOffset)
    }
  }

  func testEstimationRemovingTopInsets() {
    let insets = UIEdgeInsets(top: inset, left: 0, bottom: 0, right: 0)

    enumerateHeightRangeWithInsets(insets, after: UIEdgeInsets()) { scrollView, contentOffset in
      XCTAssertEqual(scrollView.contentOffset, contentOffset)
    }
  }

  func testEstimationAddingBottomInsets() {
    let insets = UIEdgeInsets(top: 0, left: 0, bottom: inset, right: 0)

    enumerateHeightRangeWithInsets(UIEdgeInsets(), after: insets) { scrollView, contentOffset in
      XCTAssertEqual(scrollView.contentOffset, contentOffset)
    }
  }

  func testEstimationRemovingBottomInsets() {
    let insets = UIEdgeInsets(top: 0, left: 0, bottom: inset, right: 0)

    enumerateHeightRangeWithInsets(insets, after: UIEdgeInsets()) { scrollView, contentOffset in
      XCTAssertEqual(scrollView.contentOffset, contentOffset)
    }
  }

  func testEstimationAddingBothInsets() {
    let insets = UIEdgeInsets(top: inset, left: 0, bottom: inset, right: 0)

    enumerateHeightRangeWithInsets(UIEdgeInsets(), after: insets) { scrollView, contentOffset in
      XCTAssertEqual(scrollView.contentOffset, contentOffset)
    }
  }

  func testEstimationRemovingBothInsets() {
    let insets = UIEdgeInsets(top: inset, left: 0, bottom: inset, right: 0)

    enumerateHeightRangeWithInsets(insets, after: UIEdgeInsets()) { scrollView, contentOffset in
      XCTAssertEqual(scrollView.contentOffset, contentOffset)
    }
  }

  func enumerateHeightRangeWithInsets(
    _ before: UIEdgeInsets,
    after: UIEdgeInsets,
    work: (UIScrollView, CGPoint) -> Void
  ) {
    for contentHeight in -1...Int(bounds.height * 3) {
      let scrollView = UITableView(frame: bounds)

      scrollView.contentInset = before

      scrollView.contentOffset = initialOffset
      scrollView.contentSize = CGSize(width: 0, height: CGFloat(contentHeight))

      let estimatedContentOffset = type(of: self).estimatedContentOffsetForScrollView(
        scrollView,
        withNewInsets: after
      )

      scrollView.contentInset = after

      work(scrollView, estimatedContentOffset)
    }
  }
}