Improve object lookup

[zenangst]

This PR adds a new method on Inspectable objects.

It can be used to safely get values via key from any object that conforms to the Inspectable protocol.

Example

do {
  let firstName: String = try taylor.value("firstName")
  let lastName: String? = try taylor.value("lastName")
  let age: Int = try taylor.value("age")
} catch { print(error) }

Implementation

  public func value<T>(key: String) throws -> T {
    let value = Mirror(reflecting: self)
      .children
      .filter { $0.label == key }
      .map { $1 }.first

    guard let objectValue = value as? T else {
      throw MappableError.TypeError(message: "Tried to get value \(value!) for \(key) as \(T.self) when expecting \(types()[key]!)")
    }

    return objectValue
  }

The method throws to make sure that you are getting the correct value type back.

As an added bonus, the error message that the method throws should give you enough information to point you to the right direction of what when wrong when trying to get the value.

Failure example

let age: String = try taylor.value("age")

Would render this error message:

TypeError("Tried to get value 27 for age as String when expecting Int")

[vadymmarkov]

@zenangst Is it used?

[zenangst]

Oh, I think I used them when going with the other approach, the one where you had object.value("firstName", OptionalString.self). These should be removed now, good catch!

[zenangst]

Think I'll just merge this an start working on #23. This might change before it's ready for release anyways.