How does the closure work in the method mapValues? - ios

Question

I'm trying to understand this how the mapValues method works in the following code from Calendar Heatmap.
First, a function loads a dictionary:
private func readHeatmap() -> [String: Int]? {
guard let url = Bundle.main.url(forResource: "heatmap", withExtension: "plist") else { return nil }
return NSDictionary(contentsOf: url) as? [String: Int]
}
heatmap.plist is a key/value list like this:
<key>2019.5.3</key>
<integer>3</integer>
<key>2019.5.5</key>
<integer>4</integer>
<key>2019.5.7</key>
<integer>3</integer>
A property is initialized using the above function:
lazy var data: [String: UIColor] = {
guard let data = readHeatmap() else { return [:] }
return data.mapValues { (colorIndex) -> UIColor in
switch colorIndex {
case 0:
return UIColor(named: "color1")!
case 1:
return UIColor(named: "color2")!
case 2:
return UIColor(named: "color3")!
case 3:
return UIColor(named: "color4")!
default:
return UIColor(named: "color5")!
}
}
}()
Finally, the data property defined above is used in the following function:
func colorFor(dateComponents: DateComponents) -> UIColor {
guard let year = dateComponents.year,
let month = dateComponents.month,
let day = dateComponents.day else { return .clear}
let dateString = "\(year).\(month).\(day)"
return data[dateString] ?? UIColor(named: "color6")!
}
Apple's documentation states that mapValues returns a dictionary "containing the keys of this dictionary with the values transformed by the given closure."
My questions is, what exactly is the value colorIndex passed into the closure in data.mapValues { (colorIndex) -> UIColor in? Is it from heatmap.plist? I'm confused how a String is passed into date, date[dateString] from the colorFor(dateComponents: ) function, but colorIndex is Int.

Answer 1

Originally, data is like this:
"2019.5.3" : 3
"2019.5.5" : 4
"2019.5.7" : 3
Suppose you did data.mapValues(f), where f is a function, the resulting dictionary will look like this:
"2019.5.3" : f(3)
"2019.5.5" : f(4)
"2019.5.7" : f(3)
So now, the value type of the dictionary changes to the return type of f, while the key type remains unchanged.
what exactly is the value colorIndex passed into the closure?
It's every value in data. Every value will be passed into closure once.
To see this more clearly, I've written one possible way that mapValues could be implemented:
extension Dictionary {
func myMapValues<T>(_ transform: (Value) throws -> T) rethrows -> [Key: T] {
var retVal = [Key: T]()
for entry in self {
retVal[entry.key] = try transform(entry.value)
}
return retVal
}
}
Is it from heatmap.plist?
Indirectly, yes. The contents of the local variable data (the [String: Int]) was originally from heatmap.plist, but mapValues operates directly on the data already read from the file.
I'm confused how a String is passed into data, data[dateString] from the colorFor(dateComponents: ) function, but colorIndex is Int.
colorIndex is irrelevant here. colorIndex is simply the name of the function parameter of the function that you pass to mapValues. mapValues has been called at this point, and the dictionary's values have been transformed.
You can pass a String into data because the data dictionary has Strings as keys. Recall that mapValues doesn't change the key type. Note that this data is different from the local variable data. I'm talking about the lazy property data, of type [String: UIColor].