Is there a way to use tabs to evenly space out description strings in Swift?

Question

Overriding the description variable of a custom class:

override var description : String {
        let mirrored_object = Mirror(reflecting: self)
        let str:NSMutableString = NSMutableString()
        for (index, attr) in mirrored_object.children.enumerated() {
            if let property_name = attr.label as String! {
                //str.append(" Attr \(index): \(property_name) = \(attr.value)\n")
                str.append("\t\(property_name) = \t\t\(attr.value)\n")
            }
        }
        return str as String
    }

Produces output that looks like:

userID =        Optional(0)
username =          Optional("testuser")
email =         Optional("test@gmail.com")

Is there a way to set tabs in the output so that the attribute values line up nicely like this?

userID =        Optional(0)
username =      Optional("testuser")
email =         Optional("test@gmail.com")

Also, is there any way to get rid of or shorten the "Optional" part and just show the value?

For getting rid of the optional part, use null coalescing operator to avoid needing to unwrap it. So change \(attr.value) to \(attr.value ?? "") — totiDev, Apr 07 '17 at 07:24

score 1 · Accepted Answer · edited May 23 '17 at 12:17

I wouldn't use tabs but use padding(...):

var description : String {
    let mirrored_object = Mirror(reflecting: self)
    let childrenWithLabel = mirrored_object.children.filter { $0.label != nil }
    let maxLen = childrenWithLabel.map { Int($0.label!.characters.count) }.max() ?? 0
    let lines = childrenWithLabel.map { $0.label!.padding(toLength: maxLen, withPad: " ", startingAt: 0) + " = \($0.value)" }
    return lines.joined(separator: "\n")
}

For a struct like

struct Foo: CustomStringConvertible
{
    let userID = 42
    let username = "Foo"
    let verylongpropertyname: String? = "Bar"
}

this produces

userID               = 42
username             = Foo
verylongpropertyname = Optional("Bar")

As for the "Optional" part, it's not as easy as totiG suggests because the value you get from the mirror is of type Any. See this question.

Update

I overlooked that you wanted to have a slightly different format.

var description : String {
    let mirrored_object = Mirror(reflecting: self)
    let childrenWithLabel = mirrored_object.children.filter { $0.label != nil }
    let separator = " = "
    let firstColumnWidth = (childrenWithLabel.map { Int($0.label!.characters.count) }.max() ?? 0) + separator.characters.count
    let lines = childrenWithLabel.map {
        ($0.label! + separator).padding(toLength: firstColumnWidth, withPad: " ", startingAt: 0) + "\($0.value)"
    }
}

produces

userID =               42
username =             Foo
verylongpropertyname = Optional("Bar")

Update 2

To get rid of the "Optional" thing, please see my answer here.

If you use (from said answer) unwrap() or unwrapUsingProtocol() in description like this:

var description : String {
    let mirrored_object = Mirror(reflecting: self)
    let childrenWithLabel = mirrored_object.children.filter { $0.label != nil }
    let separator = " = "
    let firstColumnWidth = (childrenWithLabel.map { Int($0.label!.characters.count) }.max() ?? 0) + separator.characters.count
    let lines = childrenWithLabel.map {
        ($0.label! + separator).padding(toLength: firstColumnWidth, withPad: " ", startingAt: 0) + "\(unwrap($0.value))"
    }
    return lines.joined(separator: "\n")
}

this will produce

userID =               42
username =             Foo
verylongpropertyname = Bar

The unwrap solution in that link didn't work for me, but this answers the main question of formatting my description. — GoldenJoe, Apr 10 '17 at 04:47
@GoldenJoe I added an answer myself to the question I linked to and a second update here, does this work for you now? — thm, Apr 10 '17 at 12:35
Awesome. Thanks so much! I would add that you must subclass NSObject and override description for this to work. Since I have your ear I'll point you to my next question: http://stackoverflow.com/questions/43336724/how-can-i-nest-debug-output-in-swift — GoldenJoe, Apr 11 '17 at 04:22
@GoldenJoe Either that or you make whatever you want to print `CustomStringConvertible`, like `struct Foo` above. — thm, Apr 11 '17 at 10:57

score 1 · Answer 2 · answered Apr 07 '17 at 09:40

Try use this method. It first calculates the max length of the property, and then uses that to pad the property names:

let maxPropertyLength: Int = mirrored_object.children.map { Int($0.label?.characters.count ?? 0) }.max() ?? 0
for attr in mirrored_object.children {
    if let propertyName = attr.label {
        str.append("\(propertyName.padding(toLength: maxPropertyLength + 2, withPad: " ", startingAt: 0)) = \(attr.value)\n")
    }
}
return str as String

Is there a way to use tabs to evenly space out description strings in Swift?

2 Answers2

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