Latest 0.6.7
License MIT
Platforms ios 8.0, watchos 2.0, requires ARC



MagneticFields is a library for adding fields to your model objects. It’ll give you:

  • type-safe change observation
  • automatic timestamps
  • validations
  • load state


MagneticFields is available through CocoaPods. To install it, add the following line to your Podfile:

pod "MagneticFields"

To run the example project, clone the repo, and run pod install from the Example directory first.

Basic Usage

class Person {
  let age = Field<Int>()
person.age.value = 10

Field types

The basic field type is Field, a generic class whose type parameter is its value type. For multivalued fields, there is ArrayField, which wraps another field that describes the single-valued type.

let tag = Field<String>(name: "Tag")
let tags = ArrayField(Field<String>(), name: "Tag")

The inner field is responsible for validations, transformations, etc.. The ArrayField owns top-level attributes like name, key, etc. — but for convenience, it will copy them from the inner field at initialization.

The unary prefix operator * is provided to wrap a Field in an ArrayField. So you can also write the above declaration like this:

let tags = *Field<String>(name: "Tag")


Simple closure validations:

let age = Field<Int>().require { $0 > 0 }

Rules can be chained, too, implying an AND. Order is not important.

let age = Field<Int>().require { $0 > 0 }.require { $0 % 2 == 0 }

By default, nil values will be considered valid. To change that for a given rule, pass allowNil: false to require.

To validate a field value, either call field.valid (returning a Bool) or field.validate(), which returns a ValidationState enum:

public enum ValidationState:Equatable {
    case Unknown
    case Invalid([String])
    case Valid

The associated value of the .Invalid case is a list of error messages (e.g., ["must be greater than 0", "is required"]).


Fields will automatically have the following timestamps:

  • updatedAt: the last time any value was set
  • changedAt: the last time a new value was set (compared using ==)


This library includes the Observer and Observable protocols for generic, type-safe change observation. Fields implement both protocols.

An Observable can have any number of registered Observer objects. The --> operator is a shortcut for the addObserver method (<-- works the same, only with its arguments swapped). Observation events are triggered once when the observer is added, and after that whenever a field value is set.

Adding an observer

An observer can be added if it implements the Observer protocol, which has a valueChanged(observable, value: value) method.

field --> observer

Or, a closure can be provided. In place of an observer object, an owner is used only to identify each closure; each owner can only have one associated closure.

field --> owner { value in

We can still register a closure even if no observer is given. This is effectively registering the closure with a null observer. There can only be one of these at a time.

age --> { value in 
  print("Age was changed to (value)")

Binding a field to another field

Since Field itself implements both Observable and Observer, the --> operator can be used to create a link between two field values.

sourceField --> destinationField

This will set the value of destinationField to that of sourceField immediately, and again whenever sourceField‘s value changes.

The <--> operator is a shortcut for <-- followed by --> (and can only be used between two Fields).

field1 <--> field2

Since <-- is called first, both fields will initially have the value of field2.

Type safety

Fields and Observables have a strongly typed value property, which must match an Observer’s associated type (ValueType) or the closure’s parameter.

So this will fail to compile:

let name = Field<String>()
let age = Field<Int>()
name --> age


Unregistering observers is done with the removeObserver method, or the -/-> operator. All observers can be removed with removeAllObservers().

Load State

It can be useful to distinguish between a value that’s nil because hasn’t been loaded yet (e.g., from an API), and one that is known to be nil. For this, fields provide the state property, whose values are in the LoadState enum:

public enum LoadState {
    case NotSet
    case Set
    case Loading
    case Error

All fields are initially in the .NotSet state, but automatically become .Set when their value is set to anything.

The .Loading state can be useful when the process of loading takes time. You might decide to show a spinner in the UI while making an API request, for example.

Latest podspec

    "name": "MagneticFields",
    "version": "0.6.7",
    "summary": "Type-safe model fields with validation and observation",
    "homepage": "",
    "license": "MIT",
    "authors": {
        "Sam Williams": "[email protected]"
    "source": {
        "git": "",
        "tag": "0.6.7"
    "platforms": {
        "ios": "8.0",
        "watchos": "2.0"
    "requires_arc": true,
    "source_files": "Pod/Classes/**/*"

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