Decodable

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[Probably deprecated] Swift 2/3 JSON unmarshalling done (more) right

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# Decodable
Simple and strict, yet powerful object mapping made possible by Swift 2's error handling. Greatly inspired by [Argo](http://github.com/thoughtbot/Argo), but without a bizillion functional operators.

[![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
[![Cocoapods version](https://cocoapod-badges.herokuapp.com/v/Decodable/badge.png)](https://cocoapods.org/pods/Decodable)
[![Platforms](https://cocoapod-badges.herokuapp.com/p/Decodable/badge.png)](https://cocoadocs.org/docsets/NSStringMask)
[![Travis](https://img.shields.io/travis/Anviking/Decodable/master.svg)](https://travis-ci.org/Anviking/Decodable/branches)

```swift

struct Repository {
    let name: String
    let description: String
    let stargazersCount: Int
    let language: String?
    let sometimesMissingKey: String?
    
    let owner: User // Struct conforming to Decodable
    let defaultBranch: Branch // Struct NOT conforming to Decodable
    
    var fullName: String { return "\(owner.login)/\(name)" }
}

extension Repository: Decodable {
    static func decode(j: Any) throws -> Repository {
        return try Repository(
                    name: j => "nested" => "name", 
                    description: j => "description", 
                    stargazersCount: j => "stargazers_count", 
                    language: j => "language", 
                    sometimesMissingKey: j =>? "sometimesMissingKey",
                    owner: j => "owner", 
                    defaultBranch: Branch(name: j => "default_branch")
                )
    }
}

do {
    let json = try NSJSONSerialization.JSONObjectWithData(data, options: [])
    let repo = try [Repository].decode(json)
} catch {
    print(error)
}
```

## How does it work?

### A protocol
```swift
public protocol Decodable {
    static func decode(json: Any) throws -> Self
}
```
### A parse-function
```swift
public func parse<T>(json: Any, path: [String], decode: (Any throws -> T)) throws -> T
```

### And shameless operator-overloading
The too-many generated overloads, all calling the `parse`-function, can be found in [Overloads.swift](https://github.com/Anviking/Decodable/blob/master/Sources/Overloads.swift). Return types include `T?`, `[T?]`, `[T?]?`, `Any` and `[String: T]?`. When conditional protocol conformance is supported in Swift this won't be necessary, and automagic decoding of infinitly nested generic types (like `[[[[[[[[[A???]]: B]]]?]]?]]`) would work.

An overload may look like this:
```swift
public func => <T: Decodable>(json: Any, keyPath: KeyPath) throws -> T
```

## KeyPaths
Keypaths can be created from string and array literals as well as with explicit initializers. They can also be joined using the operators `=>` and `=>?`. `=>?` is another operator that indicates that `nil` should be returned if the key to the right is missing.

- When composing `=>` and `=>?` operators in the same keypath, the strictness of `=>` is still honoured.
- Optional key paths (`=>?`) require an optional return type

```swift
let a: KeyPath = "a"
let b: KeyPath = ["a", "b"]
let c: KeyPath = "a" => "b" => "c"
let string: String? = json =>? "key1" => "key2" => "key3"`
                                ^^^^ allowed to be missing
```
## Errors
Errors will be caught and rethrown in the decoding process to backpropagate metadata, like the JSON object that failed decoding, the key path to it, and the root JSON object.

From [DecodingError.swift](https://github.com/anviking/decodable/tree/master/Sources/DecodingError.swift):
```swift
public enum DecodingError: ErrorProtocol, Equatable {
    /// Thrown when optional casting from `Any` fails.
    ///
    /// This can happen both when trying to access a key on a object
    /// that isn't a `NSDictionary`, and failing to cast a `Castable`
    /// primitive.
    case typeMismatch(expected: Any.Type, actual: Any.Type, Metadata)
    
    /// Thrown when a given, required, key was not found in a dictionary.
    case missingKey(String, Metadata)
    
    /// Thrown from the `RawRepresentable` extension when
    /// `init(rawValue:)` returned `nil`.
    case rawRepresentableInitializationError(rawValue: Any, Metadata)
    
    /// When an error is thrown that isn't `DecodingError`, it 
    /// will be wrapped in `DecodingError.other` in order to also provide
    /// metadata about where the error was thrown.
    case other(ErrorProtocol, Metadata)
}
```

```swift
let dict: NSDictionary = ["object": ["repo": ["owner": ["id" : 1, "login": "anviking"]]]]

do {
    let username: String = try dict => "object" => "repo" => "owner" => "name"
} catch let error {
    print(error)
}
//
// MissingKeyError at object.repo.owner: name in {
//    id = 1;
//    login = anviking;
// }
```

## Handling Errors
Expressions like `j => "key"` will throw directly, and `catch`-statements can be used to create the most complex error handling behaviours. This also means that `try?` can be used to return nil if *anything* goes wrong instead of throwing.

For convenience there is an operator, `=>?`, that only returns nil on missing keys, for APIs that indicate `null` in that manner, and to aid working with different response formats.

| Overload | Null Behaviour | Missing Key Behavior  |Type Mismatch Behaviour | Errors in subobjects | 
| ------------- |:-------------:|:-----:|:-----:|:-----:|
|  `=> -> T`| throws | throws | throws | uncaught (throws) | 
|  `=> -> T?`| nil | throws | throws | uncaught (throws) | 
|  `=>? -> T?`| nil | nil | throws | uncaught (throws) | 
|  `try? => -> T `| nil | nil | nil | caught (nil) |

## Customization
`Int`, `Double`,`String`, `Bool`, `Date` (ISO8601), `NSArray`, and `NSDictionary` types that conform to `DynamicDecodable` with the following declaration:
```swift
public protocol DynamicDecodable {
    associatedtype DecodedType
    static var decoder: (Any) throws -> DecodedType {get set}
}
```
This allows Decodable to implement default decoding closures while allowing you to override them as needed.
```swift
// Lets extend Bool.decoder so that it accepts certain strings:
Bool.decoder = { json in
    switch json {
    case let str as String where str == "true":
        return true
    case let str as String where str == "false":
        return false
    default:
        return try cast(json)
    }
}
```

Note that when extending new types to conform to `Decodable` there is really no point in conforming to `DynamicDecodable` since you already control the implementation. Also note that the `decoder` properties are intended as "set once". If you need different behaviour on different occations, please create custom decode functions.

The default `Date.decoder` uses a ISO8601 date formatter. If you don't want to create your own decode closure there's a helper:
```swift
Date.decoder = Date.decoder(using: formatter)
```

## When `Decodable` isn't enough
Don't be afraid of not conforming to `Decodable`.
```swift
let array = try NSArray.decode(json => "list").map {
    try Contribution(json: $0, repository: repo)
}
```

## Tips
- You can use `Decodable` with classes. Just make sure to either call a `required` initializer on self (e.g `self.init`) and return `Self`, or make your class `final`. ( [This](http://stackoverflow.com/questions/26495586/best-practice-to-implement-a-failable-initializer-in-swift) might be a problem though)
- The `Decodable`-protocol and the `=>`-operator should in no way make you committed to use them everywhere.

## Compatibility

| Swift version | Compatible tag or branch |
| --- | --- |
| Swift 4.0 | `0.6.0` |
| Swift 3.0 | `v0.5` |
| Swift 2.3 | `v0.4.4`|
| Swift 2.2 | `v0.4.3`|

## Note on Swift 4.0 usage
Due to collisions with the standard library you will have to import ambiguous symbols specifically, in addition to `Decodable` as a whole.

This means you likely want the following
```swift
import Decodable
import protocol Decodable.Decodable
```
and you can import other symbols, e.g `KeyPath`, `DecodingError`, in a simlilar fashion (using `import struct` and `import enum`)

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