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.
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.)
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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.
The example app has several demo view controllers to try, with both vertical and horizontal layout variations:
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- Deployment target iOS 11.0+
- Swift 5+
- Xcode 10.2+
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.
To install HorizonCalendar using Carthage, add
github "airbnb/HorizonCalendar" to your Cartfile, then follow the integration tutorial here.
To install HorizonCalendar using CocoaPods, add
pod 'HorizonCalendar' to your Podfile, then follow the integration tutorial here.
Once you've installed HorizonCalendar into your project, getting a basic calendar working is just a few steps.
At the top of the file where you'd like to use HorizonCalendar, import HorizonCalendar:
import HorizonCalendar 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:
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:
let calendarView = CalendarView(initialContent: makeContent())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.
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.
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.
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.
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:
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:
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:
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:
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:
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.
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:
We can improve the layout of our current calendar by adding some additional spacing between individual days and months:
SwiftUI
CalendarViewRepresentable(...)
.days { ... }
.interMonthSpacing(24)
.verticalDayMargin(8)
.horizontalDayMargin(8)UIKit
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:
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:
CalendarViewRepresentable(...)
...
.dayRanges(for: [dateRangeToHighlight]) { dayRangeLayoutContext in
DayRangeIndicatorViewRepresentable(
framesOfDaysToHighlight: dayRangeLayoutContext.daysAndFrames.map { $0.frame })
}For each day range derived from the Set<ClosedRange<Date>> passed into this modifier, our day range provider closure will be invoked with a context instance that contains all of the information needed for us to create a view to be used to highlight a particular day range. Since DayRangeIndicatorView is a UIView, we need to bridge it to SwiftUI using UIViewRepresentable:
struct DayRangeIndicatorViewRepresentable: UIViewRepresentable {
let framesOfDaysToHighlight: [CGRect]
func makeUIView(context: Context) -> DayRangeIndicatorView {
DayRangeIndicatorView(indicatorColor: UIColor.systemBlue.withAlphaComponent(0.15))
}
func updateUIView(_ uiView: DayRangeIndicatorView, context: Context) {
uiView.framesOfDaysToHighlight = framesOfDaysToHighlight
}
}Note
When wrapping a
UIKitview in aUIViewRepresentable, there is no equivalent concept of invariant view properties; all customizable properties must be updated inupdateUIViewto prevent view-reuse issues.
UIKit
Next, we need to invoke the dayRangeItemProvider(for:_:) on our CalendarViewContent:
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 Set<ClosedRange<Date>> passed into this function, our day range item model provider closure will be invoked with a context instance that contains all of the information needed for us to render a view to be used to highlight a particular day range. Here is an example implementation of such a view:
Next, we need a type that conforms to CalendarItemViewRepresentable that knows how to create and update instances of DayRangeIndicatorView. To make things easy, we can just make our view conform to this protocol:
import HorizonCalendar
extension DayRangeIndicatorView: CalendarItemViewRepresentable {
struct InvariantViewProperties: Hashable {
let indicatorColor: UIColor
}
struct Content: Equatable {
let framesOfDaysToHighlight: [CGRect]
}
static func makeView(
withInvariantViewProperties invariantViewProperties: InvariantViewProperties)
-> DayRangeIndicatorView
{
DayRangeIndicatorView(indicatorColor: invariantViewProperties.indicatorColor)
}
static func setContent(_ content: Content, on view: DayRangeIndicatorView) {
view.framesOfDaysToHighlight = content.framesOfDaysToHighlight
}
}Last, we need to return a CalendarItemModel representing our DayRangeIndicatorView from the day range item model provider closure:
return CalendarViewContent(...)
...
.dayRangeItemProvider(for: [dateRangeToHighlight]) { dayRangeLayoutContext in
DayRangeIndicatorView.calendarItemModel(
invariantViewProperties: .init(indicatorColor: UIColor.blue.withAlphaComponent(0.15)),
content: .init(framesOfDaysToHighlight: dayRangeLayoutContext.daysAndFrames.map { $0.frame }))
}If you build and run the app, you should see a day range indicator view that highlights 2020-01-20 to 2020-02-07:
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
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:
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
UIKitview in aUIViewRepresentable, there is no equivalent concept of invariant view properties; all customizable properties must be updated inupdateUIViewto prevent view-reuse issues.
UIKit
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.
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:
@State var selectedDate: Date?Then, update the selected date using the onDaySelection modifier:
CalendarViewRepresentable(...)
...
.onDaySelection { day in
selectedDate = calendar.date(from: day.components)
}Last, return a different view in your day provider closure:
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:
calendarView.daySelectionHandler = { [weak self] day in
self?.selectedDate = calendar.date(from: day.components)
}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:
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:
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:
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.
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.
Bryan Keller
Bryan Keller
Bryn Bodayle
If you or your company has found HorizonCalendar to be useful, let us know!
HorizonCalendar is released under the Apache License 2.0. See LICENSE for details.
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