What does .decode return in swift 4? - swift

Question

When you call .decode() to decode a struct, what exactly does it return?
I have look it up on the Apple Documentation, but all it says is "native format into in-memory representations." But what does this mean? Can anyone help me?
I'm asking this because my app is crashing when I get a null value from the JSON data, from this line of code:
let plantData = try decoder.decode([Plants].self, from: data)
Here is my struct:
struct Plants: Codable {
let date: String
let monthlyAVG: String?
enum CodingKeys : String, CodingKey {
case date = "Date"
case monthlyAVG = "30_Day_MA_MMBTU"
}
}
And Here is my Parsing code:
func parseJson() {
let url = URL(string: ebr_String)
// Load the URL
URLSession.shared.dataTask(with:url!, completionHandler: {(data, response, error) in
// If there are any errors don't try to parse it, show the error
guard let data = data, error == nil else { print(error!); return }
let decoder = JSONDecoder()
do{
let plantData = try decoder.decode([Plants].self, from: data)
print(plantData)
And Here is just a snippet of the information I am getting back:
MorrowTabbedApp.Plants(date: "2018-02-22", monthlyAVG: Optional("1210.06")), MorrowTabbedApp.Plants(date: "2018-02-23", monthlyAVG: nil)]
Here is the snippet of JSON from the web:
[
{"Date":"2018-02-21","30_Day_MA_MMBTU":"1210.06"},
{"Date":"2018-02-22","30_Day_MA_MMBTU":"1210.06"},
{"Date":"2018-02-23","30_Day_MA_MMBTU":null}
]

Answer 1

The decode method of JSONDecoder is a "generic" method. It returns an instance of whatever type you specified in the first parameter of the method. In your case, it returns a [Plants], i.e. a Array<Plants>, i.e. a Swift array of Plants instances.
If it's crashing because of a null value in your JSON, then you have to identify what was null, whether it was appropriate to be null, and if so, make sure that any Plants properties associated with values that might be null should be optionals.
Given your updated answer with code snippets, I'd suggest:
// Personally, I'd call this `Plant` as it appears to represent a single instance
struct Plant: Codable {
let date: String
let monthlyAVG: String? // Or you can use `String!` if you don't want to manually unwrap this every time you use it
enum CodingKeys : String, CodingKey {
case date = "Date"
case monthlyAVG = "30_Day_MA_MMBTU"
}
}
And:
do {
let plantData = try JSONDecoder().decode([Plant].self, from: data)
.filter { $0.monthlyAVG != nil }
print(plantData)
} catch let parseError {
print(parseError)
}
Note the filter line which selects only those occurrences for which monthlyAVG is not nil.
A couple of other suggestions:
Personally, if you could, I'd rather see the web service designed to only return the values you want (those with an actual monthlyAVG) and then change the monthlyAVG property to not be an optional. But that's up to you.
If monthlyAVG is really a numeric average, I'd change the web service to not return it as a string at all, but as a number without quotes. And then change the property of Plant to be Double or whatever.
You could, if you wanted, change the date property to be a Date and then use dateDecodingStrategy to convert the string to a Date:
struct Plant: Codable {
let date: Date
let monthlyAVG: String?
enum CodingKeys : String, CodingKey {
case date = "Date"
case monthlyAVG = "30_Day_MA_MMBTU"
}
}
and
do {
let decoder = JSONDecoder()
let formatter = DateFormatter()
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.dateFormat = "yyyy-MM-dd"
decoder.dateDecodingStrategy = .formatted(formatter)
let plantData = try decoder.decode([Plant].self, from: data)
.filter { $0.monthlyAVG != nil }
print(plantData)
} catch let parseError {
print(parseError)
}
You might do this if, for example, you wanted the x-axis of your chart to actually represent time rather than an evenly spaced set of data points.