HorizonCalendar
A declarative and performant calendar UI component that supports use cases ranging from simple date pickers all the way up to fully-featured calendar apps.
Introduction
HorizonCalendar is a declarative and performant calendar UI component for iOS. It provides many customization points to support a diverse range of designs and use cases, and is used used to implement every calendar and date picker in the Airbnb iOS app.
Features:
- SwiftUI and UIKit support
- Vertical and horizontal month layouts
- Paging for horizontal month layout
- Right-to-left layout support
- Declarative API that encourages unidirectional data flow for updating the content of the calendar
- Supports displaying large (virtually-infinite) date ranges
- Animated content updates
- Customizable default views for days, month headers, and days of the week, and a month grid background
- Specify custom views for individual days, month headers, and days of the week
- Specify custom views to highlight date ranges
- Specify custom views to overlay parts of the calendar, enabling features like tooltips
- Specify custom views for month background decorations (colors, grids, etc.)
- Specify custom views for day background decorations (colors, patterns, etc.)
- A day selection handler to monitor when a day is tapped
- A multi-day selection handler to monitor when multiple days are selected via a drag gesture
- Customizable layout metrics
- Pin the days-of-the-week row to the top
- Show partial boundary months (exactly 2020-03-14 to 2020-04-20, for example)
- Scroll to arbitrary dates and months, with or without animation
- Robust accessibility support
- Inset the content without affecting the scrollable region using layout margins
- Separator below the days-of-the-week row
- Supports all calendars from
Foundation.Calendar(Gregorian, Japanese, Hebrew, etc.)
| Search | Stays Availability Calendar | Wish List | Experience Reservation | Experience Host Calendar Management |
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Table of Contents
Example App
An example app is available to showcase and enable you to test some of HorizonCalendar's features. It can be found in ./Example/HorizonCalendarExample.xcworkspace.
Note: Make sure to use the .xcworkspace file, and not the .xcodeproj file, as the latter does not have access to HorizonCalendar.framework.
Demos
The example app has several demo view controllers to try, with both vertical and horizontal layout variations:
Single Day Selection
| Vertical | Horizontal |
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Day Range Selection
| Vertical | Horizontal |
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Selected Day Tooltip
| Vertical | Horizontal |
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Scroll to Day with Animation
| Vertical | Horizontal |
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Integration Tutorial
Requirements
- Deployment target iOS 11.0+
- Swift 5+
- Xcode 10.2+
Installation
Swift Package Manager
To install HorizonCalendar using Swift Package Manager, add
.package(name: "HorizonCalendar", url: "https://github.com/airbnb/HorizonCalendar.git", from: "1.0.0")," to your Package.swift, then follow the integration tutorial here.
Carthage
To install HorizonCalendar using Carthage, add
github "airbnb/HorizonCalendar" to your Cartfile, then follow the integration tutorial here.
CocoaPods
To install HorizonCalendar using CocoaPods, add
pod 'HorizonCalendar' to your Podfile, then follow the integration tutorial here.
Creating a calendar
Once you've installed HorizonCalendar into your project, getting a basic calendar working is just a few steps.
Basic Setup
Importing HorizonCalendar
At the top of the file where you'd like to use HorizonCalendar, import HorizonCalendar:
import HorizonCalendar Instantiating the view
SwiftUI
`CalendarViewRepresentable` is the SwiftUI view type that represents the calendar. Like other SwiftUI views, all customization is done through initializer parameters and modifiers. To create a basic calendar, you initialize a `CalendarViewRepresentable` with some initial data: ```swift let calendar = Calendar.current let startDate = calendar.date(from: DateComponents(year: 2020, month: 01, day: 01))! let endDate = calendar.date(from: DateComponents(year: 2021, month: 12, day: 31))! CalendarViewRepresentable( calendar: calendar, visibleDateRange: startDate...endDate, monthsLayout: .vertical(options: VerticalMonthsLayoutOptions()), dataDependency: nil) ```UIKit
`CalendarView` is the `UIView` subclass that renders the calendar. All visual aspects of `CalendarView` are controlled through a single type - `CalendarViewContent`. To create a basic `CalendarView`, you initialize one with an initial `CalendarViewContent`: ```swift let calendarView = CalendarView(initialContent: makeContent()) ``` ```swift private func makeContent() -> CalendarViewContent { let calendar = Calendar.current let startDate = calendar.date(from: DateComponents(year: 2020, month: 01, day: 01))! let endDate = calendar.date(from: DateComponents(year: 2021, month: 12, day: 31))! return CalendarViewContent( calendar: calendar, visibleDateRange: startDate...endDate, monthsLayout: .vertical(options: VerticalMonthsLayoutOptions())) } ``` At a minimum, `CalendarViewContent` must be initialized with a `Calendar`, a visible date range, and a months layout (either vertical or horizontal). The visible date range will be interpreted as a range of days using the `Calendar` instance passed in for the `calendar` parameter. For this example, we're using a Gregorian calendar, a date range of 2020-01-01 to 2021-12-31, and a vertical months layout. Make sure to add `calendarView` as a subview, then give it a valid frame either using Auto Layout or by manually setting its `frame` property. If you're using Auto Layout, note that `CalendarView` does not have an intrinsic content size. ```swift view.addSubview(calendarView) calendarView.translatesAutoresizingMaskIntoConstraints = false NSLayoutConstraint.activate([ calendarView.leadingAnchor.constraint(equalTo: view.layoutMarginsGuide.leadingAnchor), calendarView.trailingAnchor.constraint(equalTo: view.layoutMarginsGuide.trailingAnchor), calendarView.topAnchor.constraint(equalTo: view.layoutMarginsGuide.topAnchor), calendarView.bottomAnchor.constraint(equalTo: view.layoutMarginsGuide.bottomAnchor), ]) ```At a minimum, you need to provide a Calendar, a visible date range, and a months layout (either vertical or horizontal). The visible date range will be interpreted as a range of days using the Calendar instance passed in for the calendar parameter.
For this example, we're using a Gregorian calendar, a date range of 2020-01-01 to 2021-12-31, and a vertical months layout.
Adding the view
Next, we'll add the calendar to the view hierarchy.
SwiftUI
Add your calendar to the view hierarchy like any other SwiftUI view. Since the calendar doesn't have an intrinsic content size, you'll need to use the `frame` modifier to tell SwiftUI that it should consume all vertical and horizontal space. Optionally, use the `layoutMargins` modifier to apply internal padding, and the normal SwiftUI `padding` modifier to apply some external padding from the parent's edges. ```swift var body: some View { CalendarViewRepresentable(...) .layoutMargins(.init(top: 8, leading: 8, bottom: 8, trailing: 8)) .padding(.horizontal, 16) .frame(maxWidth: .infinity, maxHeight: .infinity) } ```UIKit
Add your calendar as a subview, then give it a valid frame either using Auto Layout or by manually setting its `frame` property. If you're using Auto Layout, note that `CalendarView` does not have an intrinsic content size. ```swift view.addSubview(calendarView) calendarView.translatesAutoresizingMaskIntoConstraints = false NSLayoutConstraint.activate([ calendarView.leadingAnchor.constraint(equalTo: view.layoutMarginsGuide.leadingAnchor), calendarView.trailingAnchor.constraint(equalTo: view.layoutMarginsGuide.trailingAnchor), calendarView.topAnchor.constraint(equalTo: view.layoutMarginsGuide.topAnchor), calendarView.bottomAnchor.constraint(equalTo: view.layoutMarginsGuide.bottomAnchor), ]) ```At this point, building and running your app should result in something that looks like this:
Customization
Providing a custom view for each day
HorizonCalendar comes with default views for month headers, day of week items, and day items. You can also provide custom views for each of these item types, enabling you to display whatever custom content makes sense for your app.
Let's start by customizing the view used for each day:
SwiftUI
Since all visual aspects of `CalendarViewRepresentable` are configured through modifiers, we'll use the `days` modifier to provide a custom view with a rounded border for each day in the calendar: ```swift CalendarViewRepresentable(...) .days { day in Text("\(day.day)") .font(.system(size: 18)) .foregroundColor(Color(UIColor.label)) .frame(maxWidth: .infinity, maxHeight: .infinity) .overlay { RoundedRectangle(cornerRadius: 12) .stroke(Color(UIColor.systemBlue), lineWidth: 1) } } ``` To use a UIKit view, wrap it using `UIViewRepresentable` and return it from the same function. > **Note** > > View-provider closures are invoked lazily as parts of the calendar come into view. If you read any view state in any of your view-provider closures, make sure you capture it explicitly using a capture list. If you don't, SwiftUI will fail to identify that state as a dependency of your view unless it was read during the initial body evaluation of your view. This will lead to missed updates when your state changes.UIKit
Since all visual aspects of `CalendarView` are configured through `CalendarViewContent`, we'll expand on our `makeContent` function. Let's start by providing a custom view for each day in the calendar: ```swift private func makeContent() -> CalendarViewContent { return CalendarViewContent(...) .dayItemProvider { day in // Return a `CalendarItemModel` representing the view for each day } } ``` The `dayItemProvider(_:)` function on `CalendarViewContent` returns a new `CalendarViewContent` instance with the custom day item model provider configured. This function takes a single parameter - a provider closure that returns a `CalendarItemModel` for a given `DayComponents`. `CalendarItemModel` is a type that abstracts away the creation and configuration of a view displayed in the calendar. It's generic over a `ViewRepresentable` type, which can be any type conforming to `CalendarItemViewRepresentable`. You can think of `CalendarItemViewRepresentable` as a blueprint for creating and updating instances of a particular type of view to be displayed in the calendar. For example, if we want to use a `UILabel` for our custom day view with a rounded border, we'll need to create a type that knows how to create and update that label. Here's a simple example: ```swift import HorizonCalendar struct DayLabel: CalendarItemViewRepresentable { /// Properties that are set once when we initialize the view. struct InvariantViewProperties: Hashable { let font: UIFont let textColor: UIColor let borderColor: UIColor } /// Properties that will vary depending on the particular date being displayed. struct Content: Equatable { let day: DayComponents } static func makeView( withInvariantViewProperties invariantViewProperties: InvariantViewProperties) -> UILabel { let label = UILabel() label.isUserInteractionEnabled = true label.layer.borderWidth = 1 label.layer.borderColor = invariantViewProperties.borderColor.cgColor label.font = invariantViewProperties.font label.textColor = invariantViewProperties.textColor label.textAlignment = .center label.clipsToBounds = true label.layer.cornerRadius = 12 return label } static func setContent(_ content: Content, on view: UILabel) { view.text = "\(content.day.day)" } } ``` `CalendarItemViewRepresentable` requires us to implement a `static` `makeView` function, which should create and return a view given a set of invariant view properties. We want our label to have a configurable font and text color, so we've made those configurable via the `InvariantViewProperties` type. In our `makeView` function, we use those invariant view properties to create and configure an instance of our label. `CalendarItemViewRepresentable` also requires us to implement a `static` `setContent` function, which should update all data-dependent properties (like the day text) on the provided view. Now that we have a type conforming to `CalendarItemViewRepresentable`, we can use it to create a `CalendarItemModel` to return from the day item model provider: ```swift return CalendarViewContent(...) .dayItemProvider { day in DayLabel.calendarItemModel( invariantViewProperties: .init( font: .systemFont(ofSize: 18), textColor: .label, borderColor: .systemBlue), content: .init(day: day)) } ``` Using a SwiftUI view is even easier - simply initialize your SwiftUI view and call `.calendarItemModel` on it. There's no need to create a custom type conforming to `CalendarItemViewRepresentable` like we had to do with the UIKit example above, or have separate concepts for invariant and variant (content) view properties. ```swift return CalendarViewContent(...) .dayItemProvider { day in Text("\(day.day)") .font(.system(size: 18)) .foregroundColor(Color(UIColor.label)) .frame(maxWidth: .infinity, maxHeight: .infinity) .overlay { RoundedRectangle(cornerRadius: 12) .stroke(Color(UIColor.systemBlue), lineWidth: 1) } .calendarItemModel } ```Similar item-provider functions are available to customize the views used for month headers, day-of-the-week items, and more.
If you build and run your app, it should now look like this:
Adjusting layout metrics
We can improve the layout of our current calendar by adding some additional spacing between individual days and months:
SwiftUI
```swift CalendarViewRepresentable(...) .days { ... } .interMonthSpacing(24) .verticalDayMargin(8) .horizontalDayMargin(8) ```UIKit
```swift return CalendarViewContent(...) .dayItemProvider { ... } .interMonthSpacing(24) .verticalDayMargin(8) .horizontalDayMargin(8) ``` Just like when we configured a custom day view via the day provider, changes to layout metrics are also done through `CalendarViewContent`. `interMonthSpacing(_:)`, `verticalDayMargin(_:)`, and `horizontalDayMargin(_:)` each return a mutated `CalendarViewContent` with the corresponding layout metric value updated, enabling you to chain function calls together to produce a final content instance.After building and running your app, you should see a much less cramped layout:
Adding a day range indicator
Day range indicators are useful for calendars that need to highlight not just individual days, but ranges of days. To do this, we can create a custom view that represents the entire highlighted region, and then provide that view to the calendar for day ranges that we care about.
First, we need to create our custom day range indicator view. This view is responsible for drawing the entire highlighted region for a particular day range, which can potentially span multiple weeks, months, or even years. We'll use UIKit and Core Graphics to implement this, but it can easily be done in SwiftUI as well:
import UIKit
final class DayRangeIndicatorView: UIView {
private let indicatorColor: UIColor
init(indicatorColor: UIColor) {
self.indicatorColor = indicatorColor
super.init(frame: .zero)
backgroundColor = .clear
}
required init?(coder: NSCoder) { fatalError("init(coder:) has not been implemented") }
var framesOfDaysToHighlight = [CGRect]() {
didSet {
guard framesOfDaysToHighlight != oldValue else { return }
setNeedsDisplay()
}
}
override func draw(_ rect: CGRect) {
let context = UIGraphicsGetCurrentContext()
context?.setFillColor(indicatorColor.cgColor)
// Get frames of day rows in the range
var dayRowFrames = [CGRect]()
var currentDayRowMinY: CGFloat?
for dayFrame in framesOfDaysToHighlight {
if dayFrame.minY != currentDayRowMinY {
currentDayRowMinY = dayFrame.minY
dayRowFrames.append(dayFrame)
} else {
let lastIndex = dayRowFrames.count - 1
dayRowFrames[lastIndex] = dayRowFrames[lastIndex].union(dayFrame)
}
}
// Draw rounded rectangles for each day row
for dayRowFrame in dayRowFrames {
let roundedRectanglePath = UIBezierPath(roundedRect: dayRowFrame, cornerRadius: 12)
context?.addPath(roundedRectanglePath.cgPath)
context?.fillPath()
}
}
}Next, we need to create a ClosedRange<Date> that represents the day range for which we'd like to display our day range indicator view. The Dates in our range will be interpreted as DayComponentss using the Calendar instance that we used when initially setting up our calendar.
let lowerDate = calendar.date(from: DateComponents(year: 2020, month: 01, day: 20))!
let upperDate = calendar.date(from: DateComponents(year: 2020, month: 02, day: 07))!
let dateRangeToHighlight = lowerDate...upperDateSwiftUI
Next, we'll use the `dayRanges` modifier on our `CalendarViewRepresentable`: ```swift CalendarViewRepresentable(...) ... .dayRanges(for: [dateRangeToHighlight]) { dayRangeLayoutContext in DayRangeIndicatorViewRepresentable( framesOfDaysToHighlight: dayRangeLayoutContext.daysAndFrames.map { $0.frame }) } ``` For each day range derived from the `SetUIKit
Next, we need to invoke the `dayRangeItemProvider(for:_:)` on our `CalendarViewContent`: ```swift return CalendarViewContent(...) ... .dayRangeItemProvider(for: [dateRangeToHighlight]) { dayRangeLayoutContext in // Return a `CalendarItemModel` representing the view that highlights the entire day range } ``` For each day range derived from the `SetIf you build and run the app, you should see a day range indicator view that highlights 2020-01-20 to 2020-02-07:
Adding grid lines
HorizonCalendar provides an API to add a decorative background behind each month. By using the included MonthGridBackgroundView, we can easily add grid lines to each of the months in the calendar:
SwiftUI
```swift CalendarViewRepresentable(...) .monthBackgrounds { monthLayoutContext in MonthGridBackgroundViewRepresentable( framesOfDays: monthLayoutContext.daysAndFrames.map { $0.frame }) } ``` Since `MonthGridBackgroundView` is a `UIView`, we need to bridge it to SwiftUI using `UIViewRepresentable`: ```swift struct MonthGridBackgroundViewRepresentable: UIViewRepresentable { let framesOfDays: [CGRect] func makeUIView(context: Context) -> MonthGridBackgroundView { MonthGridBackgroundView( invariantViewProperties: .init(horizontalDayMargin: 8, verticalDayMargin: 8)) } func updateUIView(_ uiView: MonthGridBackgroundView, context: Context) { uiView.framesOfDays = framesOfDays } } ``` > **Note** > > When wrapping a `UIKit` view in a `UIViewRepresentable`, there is no equivalent concept of invariant view properties; all customizable properties must be updated in `updateUIView` to prevent view-reuse issues.UIKit
```swift return CalendarViewContent(...) .monthBackgroundItemProvider { monthLayoutContext in MonthGridBackgroundView.calendarItemModel( invariantViewProperties: .init(horizontalDayMargin: 8, verticalDayMargin: 8), content: .init(framesOfDays: monthLayoutContext.daysAndFrames.map { $0.frame })) } ```The month background provider works similarly to the overlay provider and day range provider; for each month in the calendar, the provider closure will be invoked with a layout context. This layout context contains information about the size and positions of elements in the month. Using this information, you can draw grid lines, borders, backgrounds, and more.
Responding to day selection
If you're building a date picker, you'll most likely need to respond to the user tapping on days in the calendar.
SwiftUI
In SwiftUI, responding to day selection is easy. First, define a state property for the current selected date: ```swift @State var selectedDate: Date? ``` Then, update the selected date using the `onDaySelection` modifier: ```swift CalendarViewRepresentable(...) ... .onDaySelection { day in selectedDate = calendar.date(from: day.components) } ``` Last, return a different view in your day provider closure: ```swift CalendarViewRepresentable(...) ... .days { [selectedDate] day in let date = calendar.date(from: day.components) let borderColor: UIColor = date == selectedDate ? .systemRed : .systemBlue Text("\(day.day)") .font(.system(size: 18)) .foregroundColor(Color(UIColor.label)) .frame(maxWidth: .infinity, maxHeight: .infinity) .overlay { RoundedRectangle(cornerRadius: 12) .stroke(Color(borderColor), lineWidth: 1) } } ``` > **Note** > > View-provider closures are invoked lazily as parts of the calendar come into view. If you read any view state in any of your view-provider closures, make sure you capture it explicitly using a capture list. If you don't, SwiftUI will fail to identify that state as a dependency of your view unless it was read during the initial body evaluation of your view. This will lead to missed updates when your state changes.UIKit
In UIKit, provide a day selection handler closure by setting `CalendarView`'s `daySelectionHandler`: ```swift calendarView.daySelectionHandler = { [weak self] day in self?.selectedDate = calendar.date(from: day.components) } ``` ```swift private var selectedDate: Date? ``` The day selection handler closure is invoked whenever a day in the calendar is selected. You're provided with a `DayComponents` instance for the day that was selected. If we want to highlight the selected day once its been tapped, we'll need to create a new `CalendarViewContent` with a day calendar item model that looks different for the selected day: ```swift let selectedDay = self.selectedDay return CalendarViewContent(...) ... .dayItemProvider { [selectedDate] day in let date = calendar.date(from: day.components) let borderColor: UIColor = date == selectedDate ? .systemRed : .systemBlue return DayLabel.calendarItemModel( invariantViewProperties: .init( font: .systemFont(ofSize: 18), textColor: .label, borderColor: borderColor), content: .init(day: day)) } ``` Last, we'll change our day selection handler so that it not only stores the selected date, but also sets an updated content instance on `calendarView`: ```swift calendarView.daySelectionHandler = { [weak self] day in guard let self else { return } selectedDate = calendar.date(from: day.components) let newContent = makeContent() calendarView.setContent(newContent) } ```After building and running the app, tapping days should cause them to turn blue:
Technical Details
If you'd like to learn about how HorizonCalendar was implemented, check out the Technical Details document. It provides an overview of HorizonCalendar's architecture, along with information about why it's not implemented using UICollectionView.
Contributions
HorizonCalendar welcomes fixes, improvements, and feature additions. If you'd like to contribute, open a pull request with a detailed description of your changes.
As a rule of thumb, if you're proposing an API-breaking change or a change to existing functionality, consider proposing it by opening an issue, rather than a pull request; we'll use the issue as a public forum for discussing whether the proposal makes sense or not. See CONTRIBUTING for more details.
Authors
Bryan Keller
Maintainers
Bryan Keller
Bryn Bodayle
If you or your company has found HorizonCalendar to be useful, let us know!
License
HorizonCalendar is released under the Apache License 2.0. See LICENSE for details.