I am working on a scrollable chart that displays days on the horizontal axis. As the user scrolls, I always want them to be able to snap to a specific day. I implemented the following steps described in this WWDC23 session to achieve this.
I have set the chartScrollTargetBehavior to .valueAligned(matching: DateComponents(hour: 0))
I have set the x value unit on the BarMark to Calendar.Component.day
I ended up with the chart code that looks like this:
Chart(dates, id: \.self) { date in
BarMark(
x: .value("Date", date, unit: Calendar.Component.day),
y: .value("Number", 1)
)
.annotation {
Text(date.formatted(.dateTime.day()))
.font(.caption2)
}
}
.chartXAxis {
AxisMarks(format: .dateTime.day())
}
.chartScrollableAxes(.horizontal)
.chartScrollTargetBehavior(.valueAligned(matching: DateComponents(hour: 0)))
.chartXVisibleDomain(length: fifteenDays)
.chartScrollPosition(x: $selection)
However, this fails to work reliably. There is often a situation where the chart scroll position lands on, for instance, Oct 20, 11:56 PM, but the chart snaps to Oct 21.
I attempted to solve this problem by introducing an intermediate binding between a state value and a chart selection. This binding aims to normalize the selection always to be the first moment of any given date. But this hasn't been successful.
private var selectionBinding: Binding<Date> {
Binding {
Calendar.current.startOfDay(for: selection)
} set: { newValue in
self.selection = Calendar.current.startOfDay(for: newValue)
}
}
It's also worth mentioning that this issue also exists in Apple's sample project on Swift Charts.
How would you approach solving this? How can I find a way to make the chart scroll position blind to time values and only recognize whole days?
Here's the minimal reproducible example project for your reference.