Push from command line

2019-08-12 11:20:21 +02:00
parent 3a919dcb23
commit f1345eac14
512 changed files with 103288 additions and 1930 deletions
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+// Created by bryankeller on 7/24/17.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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 UIKit
+
+/// Encapsulates the vertical fitting priority for an element laid out by `MagazineLayout`.
+///
+/// Used by `UICollectionViewCell` and `UICollectionReusableView` subclasses to determine which
+/// vertical fitting priority to pass into
+/// `systemLayoutSizeFitting(_:withHorizontalFittingPriority:verticalFittingPriority)` via
+/// `preferredLayoutAttributesFitting(_:)`.
+public final class MagazineLayoutCollectionViewLayoutAttributes: UICollectionViewLayoutAttributes {
+
+  /// `MagazineLayout` supports self-sizing and static-sizing in the vertical direction. The value
+  /// of this property will change the layout priority used for sizing.
+  public var shouldVerticallySelfSize = true
+
+  override public func copy(with zone: NSZone? = nil) -> Any {
+    let copy = super.copy(with: zone) as! MagazineLayoutCollectionViewLayoutAttributes
+    copy.shouldVerticallySelfSize = shouldVerticallySelfSize
+    return copy
+  }
+
+  override public func isEqual(_ object: Any?) -> Bool {
+    return super.isEqual(object) &&
+      shouldVerticallySelfSize == (object as? MagazineLayoutCollectionViewLayoutAttributes)?.shouldVerticallySelfSize
+  }
+
+}
@@ -0,0 +1,27 @@
+// Created by bryankeller on 9/24/18.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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 UIKit
+
+/// `MagazineLayout`'s invalidation context type.
+///
+/// Used to indicate that collection view properties and/or delegate layout metrics changed.
+public final class MagazineLayoutInvalidationContext: UICollectionViewLayoutInvalidationContext {
+
+  /// Indicates whether to recompute the positions and sizes of elements based on the current
+  /// collection view and delegate layout metrics.
+  public var invalidateLayoutMetrics = true
+
+}
@@ -0,0 +1,40 @@
+// Created by bryankeller on 10/18/18.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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 UIKit
+
+extension MagazineLayout {
+
+  /// Constants for layout sizing and spacing defaults.
+  public enum Default {
+
+    public static let ItemSizeMode = MagazineLayoutItemSizeMode(
+      widthMode: .fullWidth(respectsHorizontalInsets: true),
+      heightMode: MagazineLayoutItemHeightMode.static(height: ItemHeight))
+    public static let HeaderVisibilityMode = MagazineLayoutHeaderVisibilityMode.hidden
+    public static let FooterVisibilityMode = MagazineLayoutFooterVisibilityMode.hidden
+    public static let BackgroundVisibilityMode = MagazineLayoutBackgroundVisibilityMode.hidden
+
+    public static let ItemHeight: CGFloat = 150
+    public static let HeaderHeight: CGFloat = 44
+    public static let FooterHeight: CGFloat = 44
+    public static let VerticalSpacing: CGFloat = 0
+    public static let HorizontalSpacing: CGFloat = 0
+    public static let SectionInsets: UIEdgeInsets = .zero
+    public static let ItemInsets: UIEdgeInsets = .zero
+
+  }
+
+}
@@ -0,0 +1,27 @@
+// Created by bryankeller on 10/18/18.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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.
+
+extension MagazineLayout {
+
+  /// Constants for supported supplementary view element kinds.
+  public enum SupplementaryViewKind {
+
+    public static let sectionHeader = "MagazineLayoutSupplementaryViewKindSectionHeader"
+    public static let sectionFooter = "MagazineLayoutSupplementaryViewKindSectionFooter"
+    public static let sectionBackground = "MagazineLayoutSupplementaryViewKindSectionBackground"
+
+  }
+
+}
@@ -0,0 +1,27 @@
+// Created by bryankeller on 10/15/18.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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.
+
+/// Represents the visibility mode for a background.
+public enum MagazineLayoutBackgroundVisibilityMode {
+
+  /// This visiblity mode will cause the background to be displayed behind the items and headers in
+  /// its respective section.
+  case visible
+
+  /// This visibility mode will cause the background to not be visibile behind the items and headers
+  /// in its respective section.
+  case hidden
+
+}
@@ -0,0 +1,78 @@
+// Created by Roman Laitarenko on 2/4/19.
+
+// 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 CoreGraphics
+
+// MARK: - MagazineLayoutFooterVisibilityMode
+
+/// Represents the visibility mode for a footer.
+public enum MagazineLayoutFooterVisibilityMode {
+
+  /// This visibility mode will cause the footer to be displayed using the specified height mode in
+  /// its respective section. If `pinToVisibleBounds` is true, the footer will pin to the visible
+  /// bounds of the collection view while its containing section is visible.
+  case visible(heightMode: MagazineLayoutFooterHeightMode, pinToVisibleBounds: Bool)
+
+  /// This visibility mode will cause the footer to not be visibile in its respective section.
+  case hidden
+
+  /// This visibility mode will cause the footer to be displayed using the specified height mode in
+  /// its respective section.
+  public static func visible(
+    heightMode: MagazineLayoutFooterHeightMode)
+    -> MagazineLayoutFooterVisibilityMode
+  {
+    return .visible(heightMode: heightMode, pinToVisibleBounds: false)
+  }
+
+}
+
+// MARK: - MagazineLayoutFooterHeightMode
+
+/// Represents the vertical sizing mode for a footer.
+public enum MagazineLayoutFooterHeightMode {
+
+  /// This height mode will force the footer to be displayed with a height equal to `height`.
+  ///
+  /// To properly support multiline labels, dynamic type, and other technologies that could affect
+  /// the height of your footers dynamically, consider using the `dynamic` height mode.
+  case `static`(height: CGFloat)
+
+  /// This height mode will cause the footer to self-size in the vertical direction.
+  ///
+  /// In practice, self-sizing in the vertical direction means that the footer will get its height
+  /// from the Auto Layout engine. Use this height mode for footers whose height is not known
+  /// upfront. For example, if you support multiline labels or dynamic type, your height is likely
+  /// not known until the Auto Layout engine resolves the layout at runtime.
+  case dynamic
+
+}
+
+// MARK: Equatable
+
+extension MagazineLayoutFooterHeightMode: Equatable {
+
+  public static func == (
+    lhs: MagazineLayoutFooterHeightMode,
+    rhs: MagazineLayoutFooterHeightMode)
+    -> Bool
+  {
+    switch (lhs, rhs) {
+    case (.static(let l), .static(let r)): return l == r
+    case (.dynamic, .dynamic): return true
+    default: return false
+    }
+  }
+
+}
@@ -0,0 +1,79 @@
+// Created by bryankeller on 10/15/18.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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 CoreGraphics
+
+// MARK: - MagazineLayoutHeaderVisibilityMode
+
+/// Represents the visibility mode for a header.
+public enum MagazineLayoutHeaderVisibilityMode {
+
+  /// This visibility mode will cause the header to be displayed using the specified height mode in
+  /// its respective section. If `pinToVisibleBounds` is true, the header will pin to the visible
+  /// bounds of the collection view while its containing section is visible.
+  case visible(heightMode: MagazineLayoutHeaderHeightMode, pinToVisibleBounds: Bool)
+
+  /// This visibility mode will cause the header to not be visibile in its respective section.
+  case hidden
+
+  /// This visibility mode will cause the header to be displayed using the specified height mode in
+  /// its respective section.
+  public static func visible(
+    heightMode: MagazineLayoutHeaderHeightMode)
+    -> MagazineLayoutHeaderVisibilityMode
+  {
+    return .visible(heightMode: heightMode, pinToVisibleBounds: false)
+  }
+
+}
+
+// MARK: - MagazineLayoutHeaderHeightMode
+
+/// Represents the vertical sizing mode for a header.
+public enum MagazineLayoutHeaderHeightMode {
+
+  /// This height mode will force the header to be displayed with a height equal to `height`.
+  ///
+  /// To properly support multiline labels, dynamic type, and other technologies that could affect
+  /// the height of your headers dynamically, consider using the `dynamic` height mode.
+  case `static`(height: CGFloat)
+
+  /// This height mode will cause the header to self-size in the vertical direction.
+  ///
+  /// In practice, self-sizing in the vertical direction means that the header will get its height
+  /// from the Auto Layout engine. Use this height mode for headers whose height is not known
+  /// upfront. For example, if you support multiline labels or dynamic type, your height is likely
+  /// not known until the Auto Layout engine resolves the layout at runtime.
+  case dynamic
+
+}
+
+// MARK: Equatable
+
+extension MagazineLayoutHeaderHeightMode: Equatable {
+
+  public static func == (
+    lhs: MagazineLayoutHeaderHeightMode,
+    rhs: MagazineLayoutHeaderHeightMode)
+    -> Bool
+  {
+    switch (lhs, rhs) {
+    case (.static(let l), .static(let r)): return l == r
+    case (.dynamic, .dynamic): return true
+    default: return false
+    }
+  }
+
+}
@@ -0,0 +1,164 @@
+// Created by bryankeller on 10/15/18.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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 CoreGraphics
+
+// MARK: - MagazineLayoutItemSizeMode
+
+/// Represents the horizontal and vertical sizing mode for an item.
+public struct MagazineLayoutItemSizeMode {
+
+  // MARK: Lifecycle
+
+  public init(widthMode: MagazineLayoutItemWidthMode, heightMode: MagazineLayoutItemHeightMode) {
+    self.widthMode = widthMode
+    self.heightMode = heightMode
+  }
+
+  // MARK: Public
+
+  /// The width mode for the item.
+  public let widthMode: MagazineLayoutItemWidthMode
+
+  /// The height mode for the item.
+  public let heightMode: MagazineLayoutItemHeightMode
+
+}
+
+// MARK: - MagazineLayoutItemWidthMode
+
+/// Represents the horizontal sizing mode for an item.
+///
+/// Consecutive items with the same width mode will display on the same row until there is no more
+/// room.
+public enum MagazineLayoutItemWidthMode {
+
+  /// Full width items will fill the available width in a section.
+  ///
+  /// Use this width mode to create lists of items.
+  /// `respectsHorizontalInsets` specifies whether the item should be edge-to-edge in a section, or
+  /// if it should be inset by the item insets specified for a section. `respectsHorizontalInsets`
+  /// does not take into account section insets or the collection view's content inset.
+  case fullWidth(respectsHorizontalInsets: Bool)
+
+  /// Fractional width items will take up `1/divisor` of the available width for a given row of
+  /// items.
+  ///
+  /// Use this width mode to create grids of items. Consider using `halfWidth`, `thirdWidth`,
+  /// `fourthWidth`, or `fifthWidth`, which are equivalent to using `fractionalWidth` with a
+  /// `divisor` of `2`, `3`, `4`, or `5`, respectively.
+  ///
+  /// Fractional width items respect `contentInset.left` and `contentInset.right`, and are affected
+  /// by the horizontal spacing specified for the section in which they're contained. On iOS 11 and
+  /// higher, they will also take the safe area insets into account if the collection view's
+  /// `contentInsetAdjustmentBehavior` property is set to a value that respects the safe area.
+  ///
+  /// - Warning: `divisor` must be greater than `0`. Specifying `0` as the `divisor` is a programmer
+  /// error and **will result in a runtime crash**.
+  case fractionalWidth(divisor: UInt)
+
+  /// Half width items will take up `1/2` of the available width for a given row of items.
+  public static var halfWidth: MagazineLayoutItemWidthMode {
+    return .fractionalWidth(divisor: 2)
+  }
+
+  /// Third width items will take up `1/3` of the available width for a given row of items.
+  public static var thirdWidth: MagazineLayoutItemWidthMode {
+    return .fractionalWidth(divisor: 3)
+  }
+
+  /// Fourth width items will take up `1/4` of the available width for a given row of items.
+  public static var fourthWidth: MagazineLayoutItemWidthMode {
+    return .fractionalWidth(divisor: 4)
+  }
+
+  /// Fifth width items will take up `1/5` of the available width for a given row of items.
+  public static var fifthWidth: MagazineLayoutItemWidthMode {
+    return .fractionalWidth(divisor: 5)
+  }
+
+}
+
+// MARK: Equatable
+
+extension MagazineLayoutItemWidthMode: Equatable {
+
+  public static func == (
+    lhs: MagazineLayoutItemWidthMode,
+    rhs: MagazineLayoutItemWidthMode)
+    -> Bool
+  {
+    switch (lhs, rhs) {
+    case (.fullWidth(let l), .fullWidth(let r)): return l == r
+    case (.fractionalWidth(let l), .fractionalWidth(let r)): return l == r
+    default: return false
+    }
+  }
+
+}
+
+// MARK: - MagazineLayoutItemHeightMode
+
+/// Represents the vertical sizing mode for an item.
+///
+/// `MagazineLayout` supports vertically self-sizing and statically sized items. Since height modes
+/// are specified for each item, you can mix vertically self-sizing and statically sized items in
+/// the same sections, and even in the same rows.
+public enum MagazineLayoutItemHeightMode {
+
+  /// This height mode mode will cause the item to be displayed with a height equal to `height`.
+  ///
+  /// To properly support multiline labels, dynamic type, and other technologies that could affect
+  /// the height of your items dynamically, consider using one of the dynamic height modes.
+  case `static`(height: CGFloat)
+
+  /// This height mode will cause the item to self-size in the vertical direction.
+  ///
+  /// In practice, self-sizing in the vertical direction means that the item will get its height
+  /// from the Auto Layout engine. Use this height mode for items whose height is not known upfront.
+  /// For example, if you support multiline labels or dynamic type, your height is likely not known
+  /// until the Auto Layout engine resolves the layout at runtime.
+  case dynamic
+
+  /// This height mode will cause the item to self-size in the vertical direction, then resize to
+  /// match the height of the tallest item in the same row of items.
+  ///
+  /// If the item _is_ the tallest item in the row (after being self-sized), then it will stay
+  /// at its self-sized height until it's no longer the tallest item in the row.
+  ///
+  /// Note that items with this height mode will resize to match the height of the tallest item in
+  /// the same row of items, even if the tallest item has a `static` height mode.
+  case dynamicAndStretchToTallestItemInRow
+
+}
+
+// MARK: Equatable
+
+extension MagazineLayoutItemHeightMode: Equatable {
+
+  public static func == (
+    lhs: MagazineLayoutItemHeightMode,
+    rhs: MagazineLayoutItemHeightMode)
+    -> Bool
+  {
+    switch (lhs, rhs) {
+    case (.static(let l), .static(let r)): return l == r
+    case (.dynamic, .dynamic): return true
+    case (.dynamicAndStretchToTallestItemInRow, .dynamicAndStretchToTallestItemInRow): return true
+    default: return false
+    }
+  }
+
+}
@@ -0,0 +1,138 @@
+// Created by bryankeller on 7/17/17.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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 UIKit
+
+public protocol UICollectionViewDelegateMagazineLayout: UICollectionViewDelegate {
+
+  ///   Asks the delegate for the size mode of the specified item.
+  ///
+  ///   - Parameters:
+  ///      - collectionView: The collection view using the layout.
+  ///      - collectionViewLayout: The layout requesting the information.
+  ///      - indexPath: The index path of the item.
+  ///
+  ///   - Returns: The size mode of the specified item.
+  func collectionView(
+    _ collectionView: UICollectionView,
+    layout collectionViewLayout: UICollectionViewLayout,
+    sizeModeForItemAt indexPath: IndexPath)
+    -> MagazineLayoutItemSizeMode
+
+  ///   Asks the delegate for the visibility mode of the header in the specified section.
+  ///
+  ///   - Parameters:
+  ///      - collectionView: The collection view using the layout.
+  ///      - collectionViewLayout: The layout requesting the information.
+  ///      - index: The index of the section containing the header.
+  ///
+  ///   - Returns: The visibility mode of the header in the specified section.
+  func collectionView(
+    _ collectionView: UICollectionView,
+    layout collectionViewLayout: UICollectionViewLayout,
+    visibilityModeForHeaderInSectionAtIndex index: Int)
+    -> MagazineLayoutHeaderVisibilityMode
+
+  ///   Asks the delegate for the visibility mode of the footer in the specified section.
+  ///
+  ///   - Parameters:
+  ///      - collectionView: The collection view using the layout.
+  ///      - collectionViewLayout: The layout requesting the information.
+  ///      - index: The index of the section containing the footer.
+  ///
+  ///   - Returns: The visibility mode of the footer in the specified section.
+  func collectionView(
+    _ collectionView: UICollectionView,
+    layout collectionViewLayout: UICollectionViewLayout,
+    visibilityModeForFooterInSectionAtIndex index: Int)
+    -> MagazineLayoutFooterVisibilityMode
+
+  ///   Asks the delegate for the visibility mode of the background in the specified section.
+  ///
+  ///   - Parameters:
+  ///      - collectionView: The collection view using the layout.
+  ///      - collectionViewLayout: The layout requesting the information.
+  ///      - index: The index of the section containing the background.
+  ///
+  ///   - Returns: The visibility mode of the background in the specified section.
+  func collectionView(
+    _ collectionView: UICollectionView,
+    layout collectionViewLayout: UICollectionViewLayout,
+    visibilityModeForBackgroundInSectionAtIndex index: Int)
+    -> MagazineLayoutBackgroundVisibilityMode
+
+  ///   Asks the delegate for the horizontal spacing for items in the specified section.
+  ///
+  ///   - Parameters:
+  ///      - collectionView: The collection view using the layout.
+  ///      - collectionViewLayout: The layout requesting the information.
+  ///      - index: The index of the section whose horizontal item spacing is needed.
+  ///
+  ///   - Returns: The horizontal spacing for items in the specified section.
+  func collectionView(
+    _ collectionView: UICollectionView,
+    layout collectionViewLayout: UICollectionViewLayout,
+    horizontalSpacingForItemsInSectionAtIndex index: Int)
+    -> CGFloat
+
+  ///   Asks the delegate for the vertical spacing for items in the specified section.
+  ///
+  ///   - Parameters:
+  ///      - collectionView: The collection view using the layout.
+  ///      - collectionViewLayout: The layout requesting the information.
+  ///      - index: The index of the section whose vertical item spacing is needed.
+  ///
+  ///   - Returns: The vertical spacing for items in the specified section.
+  func collectionView(
+    _ collectionView: UICollectionView,
+    layout collectionViewLayout: UICollectionViewLayout,
+    verticalSpacingForElementsInSectionAtIndex index: Int)
+    -> CGFloat
+
+  ///   Asks the delegate for the amount by which to inset elements in the specified section.
+  ///
+  ///   Section insets are relative to the content's bounds, which is impacted by the collection
+  ///   view's content inset.
+  ///
+  ///   - Parameters:
+  ///      - collectionView: The collection view using the layout.
+  ///      - collectionViewLayout: The layout requesting the information.
+  ///      - index: The index of the section whose element insets are needed.
+  ///
+  ///   - Returns: The amount by which to inset elements in the specified section.
+  func collectionView(
+    _ collectionView: UICollectionView,
+    layout collectionViewLayout: UICollectionViewLayout,
+    insetsForSectionAtIndex index: Int)
+    -> UIEdgeInsets
+
+  ///   Asks the delegate for the amount by which to inset items in the specified section.
+  ///
+  ///   Item insets are relative to the section's bounds, which is impacted by the section's insets
+  ///   and, transitively, the collection view's content inset.
+  ///
+  ///   - Parameters:
+  ///      - collectionView: The collection view using the layout.
+  ///      - collectionViewLayout: The layout requesting the information.
+  ///      - index: The index of the section whose item insets are needed.
+  ///
+  ///   - Returns: The amount by which to inset items in the specified section.
+  func collectionView(
+    _ collectionView: UICollectionView,
+    layout collectionViewLayout: UICollectionViewLayout,
+    insetsForItemsInSectionAtIndex index: Int)
+    -> UIEdgeInsets
+
+}
@@ -0,0 +1,61 @@
+// Created by bryankeller on 11/29/18.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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 UIKit
+
+/// A collection reusable view that coordinates with `MagazineLayoutCollectionViewLayoutAttributes`
+/// to determine how to size itself: with self-sizing, or without self-sizing. Use this class
+/// (or subclasses) for displaying header and background supplementary views with `MagazineLayout`.
+///
+/// Note that this class is very similar to `MagazineLayoutCollectionViewCell`.
+///
+/// `UIKit` invokes `preferredLayoutAttributesFitting(_:)` with an initial set of layout attributes,
+/// giving this reusable view subclass a chance to modify the `size` property of the attributes
+/// based on whether or not we want to self-size.
+///
+/// Subclassing and/or adding additional protocol conformances is encouraged, although modifying
+/// the behavior of `preferredLayoutAttributesFitting(_:)` is not recommended.
+///
+/// This class exists because `MagazineLayout` supports self-sizing supplementary views in just the
+/// vertical dimension - a use case that `UICollectionReusableView` does not support out-of-the-box.
+open class MagazineLayoutCollectionReusableView: UICollectionReusableView {
+
+  override open func preferredLayoutAttributesFitting(
+    _ layoutAttributes: UICollectionViewLayoutAttributes)
+    -> UICollectionViewLayoutAttributes
+  {
+    guard let attributes = layoutAttributes as? MagazineLayoutCollectionViewLayoutAttributes else {
+      assertionFailure("`layoutAttributes` must be an instance of `MagazineLayoutCollectionViewLayoutAttributes`")
+      return super.preferredLayoutAttributesFitting(layoutAttributes)
+    }
+
+    let size: CGSize
+    if attributes.shouldVerticallySelfSize {
+      // Self-sizing is required in the vertical dimension.
+      size = super.systemLayoutSizeFitting(
+        layoutAttributes.size,
+        withHorizontalFittingPriority: .required,
+        verticalFittingPriority: .fittingSizeLevel)
+    } else {
+      // No self-sizing is required; respect whatever size the layout determined.
+      size = layoutAttributes.size
+    }
+
+    layoutAttributes.size = size
+
+    return layoutAttributes
+  }
+
+}
@@ -0,0 +1,88 @@
+// Created by bryankeller on 11/29/18.
+// Copyright © 2018 Airbnb, Inc.
+
+// 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 UIKit
+
+/// A cell that coordinates with `MagazineLayoutCollectionViewLayoutAttributes` to determine how to
+/// size itself: with self-sizing, or without self-sizing. Use this class (or subclasses) for
+/// displaying cells with `MagazineLayout`.
+///
+/// Note that this class is very similar to `MagazineLayoutCollectionReusableView`.
+///
+/// `UIKit` invokes `preferredLayoutAttributesFitting(_:)` with an initial set of layout attributes,
+/// giving this cell subclass a chance to modify the `size` property of the attributes based on
+/// whether or not we want to self-size.
+///
+/// Subclassing and/or adding additional protocol conformances is encouraged, although modifying
+/// the behavior of `preferredLayoutAttributesFitting(_:)` is not recommended.
+///
+/// This class exists because `MagazineLayout` supports self-sizing in just the vertical dimension -
+/// a use case that `UICollectionViewCell` does not support out-of-the-box.
+///
+/// As of iOS 12, `UICollectionReusableView` is tightly coupled with `UICollectionViewFlowLayout`'s
+/// private `_estimatesSizes` property, which is used to determine how to size cells displayed in a
+/// `UICollectionViewFlowLayout`. If `_estimatesSizes` is `true`, then `UICollectionReusableView`
+/// will self-size in both the horizontal and vertical dimensions. If it is `false`, no self-sizing
+/// will occur. In short, `UICollectionReusableView` is only optimized to work correctly with
+/// Apple's own layout.
+open class MagazineLayoutCollectionViewCell: UICollectionViewCell {
+
+  override open func preferredLayoutAttributesFitting(
+    _ layoutAttributes: UICollectionViewLayoutAttributes)
+    -> UICollectionViewLayoutAttributes
+  {
+    guard let attributes = layoutAttributes as? MagazineLayoutCollectionViewLayoutAttributes else {
+      assertionFailure("`layoutAttributes` must be an instance of `MagazineLayoutCollectionViewLayoutAttributes`")
+      return super.preferredLayoutAttributesFitting(layoutAttributes)
+    }
+
+    // In some cases, `contentView`'s required width and height constraints
+    // (created from its auto-resizing mask) will not have the correct constants before invoking
+    // `systemLayoutSizeFitting(...)`, causing the cell to size incorrectly. This seems to be a
+    // UIKit bug.
+    // https://openradar.appspot.com/radar?id=5025850143539200
+    // The issue seems most common when the collection view's bounds change (on rotation).
+    // We correct for this by updating `contentView.bounds`, which updates the constants used by the
+    // width and height constraints created by the `contentView`'s auto-resizing mask.
+
+    if contentView.bounds.width != layoutAttributes.size.width {
+      contentView.bounds.size.width = layoutAttributes.size.width
+    }
+
+    if
+      !attributes.shouldVerticallySelfSize &&
+      contentView.bounds.height != layoutAttributes.size.height
+    {
+      contentView.bounds.size.height = layoutAttributes.size.height
+    }
+
+    let size: CGSize
+    if attributes.shouldVerticallySelfSize {
+      // Self-sizing is required in the vertical dimension.
+      size = super.systemLayoutSizeFitting(
+        layoutAttributes.size,
+        withHorizontalFittingPriority: .required,
+        verticalFittingPriority: .fittingSizeLevel)
+    } else {
+      // No self-sizing is required; respect whatever size the layout determined.
+      size = layoutAttributes.size
+    }
+
+    layoutAttributes.size = size
+
+    return layoutAttributes
+  }
+
+}