How do you instantiate a type dynamically based upon a lookup value in a dictionary in Swift?
Hopefully this is useful to others. It took some research to figure this out. The goal is to avoid the anti-pattern of giant if or switch statements to create each object type from a value.
class NamedItem : CustomStringConvertible {
let name : String
required init() {
self.name = "Base"
}
init(name : String) {
self.name = name
}
var description : String { // implement Printable
return name
}
}
class File : NamedItem {
required init() {
super.init(name: "File")
}
}
class Folder : NamedItem {
required init() {
super.init(name: "Folder")
}
}
// using self to instantiate.
let y = Folder.self
"\(y.init())"
let z = File.self
"\(z.init())"
// now put it in a dictionary.
enum NamedItemType {
case folder
case file
}
var typeMap : [NamedItemType : NamedItem.Type] = [.folder : Folder.self,
.file : File.self]
let p = typeMap[.folder]
"\(p!.init())"
let q = typeMap[.file]
"\(q!.init())"
Interesting aspects:
use of "required" for initializers
use of .Type to get the type for the dictionary value.
use of .self to get the "class" that can be instantiated
use of () to instantiate the dynamic object.
use of Printable protocol to get implicit string values.
how to init using a non parameterized init and get the values from subclass initialization.
Updated to Swift 3.0 syntax
Related
I have made a file called Constants.swift. Within this, I have made a class like so...
public class WebServices {
static let getMyPlants : String = "plant/getPlants"
static let getMyOrganizations: String = "organization/getOrganizations"
}
Now whenever, I use an api anywhere in my project, I do Webservices.getMyPlants.
Now I also have a base-url for each of the API's. That is mentioned below public class WebServices.... like so..
struct envDev {
var BASEURL : String = "http://api-proj-dev.ii.the-co.com/api/"
}
Now, the base-url for Webservices.getMyOrganizations is different. I want to use a condition within struct envDev that if I have selected Webservices.getMyOrganizations, then I can give a different BASEURL. Something like...
//The below code isn't right. I just wrote it to represent the kind of solution I wish to have.
struct envDev {
var BASEURL : String = "http://api-proj-dev.ii.the-co.com/api/"
if Webservices.getMyOrganizations {
var BASEURL : String = "http://my second base-url.."
}
}
EDIT 1 Giving below the signature of APIHelper
class APIHelper: NSObject {
var API: NSString
var json: NSString
var receivedJSON: NSString?
var arrResult: NSMutableArray = []
let esmatBaseUrl = AppDelegate().currentUser //This is given in AppDelegate as `var currentUser = envDev()`
()
EDIT 2 Inclusion of baseUrl computed property in APIHelper & the error.
class APIHelper: NSObject {
var API: NSString
var json: NSString
var receivedJSON: NSString?
var arrResult: NSMutableArray = []
let esmatBaseUrl = AppDelegate().currentUser //This is given in AppDelegate as `var currentUser = envDev()`
()
var baseUrl: String {
esmatBaseUrl.baseUrl(forApi: API as String) // Here I'm getting the error as `Value of type 'envDev' has no member 'baseUrl'`
}
envDev has no way of knowing what happens in APIHelper, so you need a way to pass in the API from APIHelper to envDev. This means that BASEURL should not be a property, but a method:
func baseUrl(forApi api: String) -> String {
switch api {
case WebServices.getMyPlants: return "some url"
case WebServices.getMyOrganizations: return "some other url"
default: fatalError()
}
}
Then in APIHelper, you can add a baseUrl computed property that calls the above method:
var baseUrl: String {
esmatBaseUrl.baseUrl(forApi: API as String)
}
This would mean that you need to change all occurrences of esmatBaseUrl.BASEURL in your existing code to just baseUrl.
Also, I would suggest not using NSString, NSArray, etc in Swift. You should their Swift counterparts: String and [T].
I understood your query. You want to create an ENUM for your server-environment's, instead of hard-coding baseUrl's you probably want to use ENUMS to select different environments, right.
So accordingly, I've created an ENUM for you to add different server-environments so it will be feasible for you to use it frequently every-where.
private enum ServerEnv: String {
case stage, prod, test, my_plants, my_organization
var domainValue: String {
switch self {
case .test, .my_plants: return "http://api-testing-proj-dev.ii.the-co.com/api/"
case .stage: return "http://api-staging-proj-dev.ii.the-co.com/api/"
case .prod: return "http://api-production-proj-dev.ii.the-co.com/api/"
case .my_organization: return "http://api-my_organization-proj-dev.ii.the-co.com/api/"
}
}
}
Example :
let baseUrl = ServerEnv.my_organization.domainValue
Output => baseURL = "http://api-my_organization-proj-dev.ii.the-co.com/api/"
let baseUrl = ServerEnv.my_plants.domainValue
Output => baseURL = "http://api-testing-proj-dev.ii.the-co.com/api/"
I hope, I've solved your query here.
Happy Coding :-)
I know that in swift we can use Extensions to add new methods to existing classes.
But what about if i want to add a variable?
extension UIViewController {
var myVar = "xyz"
}
It gives like :
Extensions must not contain stored properties
We can't add the stored properties to extensions directly but we can have the computed variables.
extension UIViewController {
var myVar: String {
return "xyz"
}
}
Extensions in Swift can:
Add computed instance properties and computed type properties
...
For more please visit
https://docs.swift.org/swift-book/LanguageGuide/Extensions.html
You can only add computed properties to extensions as follows...
extension UIViewController {
var someProperty = "xyz" : String {
return "xyz"
}
}
If you wish to use it the way you are defining it, you might need to subclass your UIViewController
class YourCustomViewController: UIViewController {
var someProperty: String = "xyz"
}
you can only use computed variables:
for example we have the type Int in swift, and we want it extend in a way that swift generates a random number from 0 to our number :
extension Int
{
var arc4random : Int{
if self > 0
{return Int(arc4random_uniform(UInt32(UInt(self))))}
else if self < 0
{return -Int(arc4random_uniform(UInt32(UInt(abs(self)))))}
else
{return 0}
}
}
and usage :
myArray.count.arc4random
here my array.count is an Int , and arc4random is the computed variable we have defined in our extension, u cant store a value in it
You can try ( This is a readOnly computed property )
extension UIViewController {
var someProperty : String {
return "xyz"
}
}
I'm trying to implement a ORM layer on top of Couchbase Lite iOS 2.0, now it did removed the CBLModel apis which make it a little bit difficult to use.
I'm using a Reflection lib and its Mappable protocol(which is renamed to ORMMappable in the following code) to simplify the mapping apis.
Here's the error message:
let t = Self.cast_id_type(type: link, obj: value)
Cannot invoke 'cast_id_type' with an argument list of type '(type: ORMMappable.Type, obj: Any?)'
Expected an argument list of type '(type: D.Type, obj: Any?)'
And here's the problematic code
typealias MappableDictionary = [String : Any]
class IDString<T:ORMMappable> : NSString{
func load_object(){
}
}
struct MapMeta{
var ignores : [String] = []
var mapping : [String:ORMMappable.Type] = [:]
}
protocol ORMMappable {
var id : NSString {get set}
static var _meta : MapMeta{get}
init(dictionary: MappableDictionary) throws
}
extension ORMMappable {
init() throws{
try self.init(dictionary: [:] as MappableDictionary)
}
static func cast_id_type<D:ORMMappable>(type: D.Type,obj: Any?) -> IDString<D>?{
if let o = obj as? IDString<D>{
return o
}
return nil
}
init(dictionary: MappableDictionary) throws {
self = try construct { property in
let meta = Self._meta
if let value = dictionary[property.key] {
if let type = property.type as? ORMMappable.Type, let value = value as? MappableDictionary {
return try type.init(dictionary: value)
}
else if let link = meta.mapping[property.key]
{
let t = Self.cast_id_type(type: link, obj: value)
print(link)
//return t
return nil
}
else {
return value
}
} else {
return nil
//throw Error.missingRequiredValue(key: property.key)
}
}
}
}
A example of usage is
struct TestObject : ORMMappable{
static var _meta: MapMeta{
return MapMeta(ignores: [], mapping: ["link_id":TestObject2.self])
}
var id : NSString
var name : String?
var age : Int?
var link_id : IDString<TestObject2>?
}
IDString is holder for a link to other ORMMappable compatible class, mapping maps from String(property name) to a ORMMappable compatible class, and cast_id_type does check the mapping and trying to cast from the pointer of value to the StringID object. The error itself makes me quite confused here,
static func cast_id_type<D:ORMMappable>(type: D.Type,obj: Any?) -> IDString<D>?
D should be a ORMMappable compatible class, where I give is a value of a [String:ORMMappable.Type], but it rises ORMMappable.Type is not D.Type, how does this comes from?
Also I'm looking forward if there any better ways to do ORM in swift, currently the code does working with dynamic object creation, but when comes with ORM relation, it really drove me nuts here, just looking for ways to manage it in a easier and more manageable ways, where currently it looks like there are not much functionalities to do meta programming, a lot of other ORM libs still using objc, which is much more easier(but boilerplate) on dynamic instance creation or class inspection.
Thanks very much for the help, any hints will be real appreciated :)
Removed all generics solved the problem, it won't inferred in runtime environment.
In my project I am having an issue with Swift, that it doesn't recognize variables containing protocol types. This means I can't use a variable that stores a type, to check if the type of an instance matches it.
I attached problematic part of the code with some brief context.
Is this some kind of bug or am I overseeing something really badly?
Using XCode 7.3, Swift 2.2
//Context of Issue BEGIN
class TaskValueObject {
//ManyData, VeryComplexity, SoBig, wow..
}
typealias TaskListSorterBlock = (TaskValueObject, TaskValueObject) -> Bool
protocol TaskListSorter : class {
init()
var localizedTitle : String { get }
var sorterBlock : TaskListSorterBlock { get }
}
class PriorityTaskListSorter : TaskListSorter {
// ... Implementation ...
}
// Many other TaskListSorter implementation classes ...
//Context of Issue END
class TaskListContainer {
weak var currentSorter : TaskListSorter?
var sorters : [TaskListSorter]
init() {
self.sorters = [ PriorityTaskListSorter(), /* ... Instances created for <TaskListSorter> implementing class ... */ ]
loadDefaultSorter()
}
static var defaultSorterType : TaskListSorter.Type = PriorityTaskListSorter.self
private func loadDefaultSorter() {
let defaultSorterType = TaskListContainer.defaultSorterType
for sorter in self.sorters {
if sorter is defaultSorterType {
// ^ ERROR HERE : defaultSorterType is not a 'type'
self.currentSorter = sorter
}
}
}
}
Update 1: I get the same error if I replace the problematic line with the following:
if let defaultSorter = sorter as? defaultSorterType {
Update 2: Replacing the problematic line with the one below, makes the code work. However I am using here the 'dynamicType' which is not offered by code completion (must be a reason for that...). Also the question remains, why the first 2 attempts didn't work?
if sorter.dynamicType == defaultSorterType {
Can someone please help me understand the correct use of getters and setters in swift. I get the impression its not the same as say Java.
Is this the correct usage in Swift to store and access a class variable?
class Person {
private var name: String
init(name: String) {
self.name = name
}
func setName(name: String) {
self.name = name
}
func getName() -> String {
return name
}
}
Swift provides a much more structured approach to getters and setters than Java.
You can, but you should not, write setters and getters as you did in your code.
Instead (if you are using stored properties) just declare the property with a visibility non private (e.g. internal in my example). This way callers outside of your class will be able to see the property and change it.
class Person {
var name: String {
willSet(newValue) {
print("\(self.name) is going to be renamed as \(newValue)")
}
didSet(oldValue) {
print("\(oldValue) has been renamed as \(self.name)")
}
}
init(name: String) {
self.name = name
}
}
Ok but in java getter and setters do allow me to add custom logic to be executed before or after the value is changed.
Right! In Swift, you can do this using the willSet and didSet observers.
willSet(newValue)
You write here the code you want to run before a new value is written in the property.
Here you can access the current value (that is going to be overwritten) with self.name while the new value is available with newValue.
didSet(oldValue)
You write here the code you want to run after a new value is written in the property.
Here you can access the old value (that has been overwritten) with oldValue while the new value is available in self.name.
Both willSet and didSet are optional [I am not talking about Optional Type! I mean you are not forced to write them :)].
If you don't need to run some code just before or after the property has been changed, just omit them.
Example
let aVerySmartPerson = Person(name: "Walter White")
aVerySmartPerson.name = "Heisenberg"
// > Walter White is going to be renamed as Heisenberg
// > Walter White has been renamed as Heisenberg
If you assign to self., you will just be calling this method again. Also, there is no "get" like the old Java bean property pattern. Finally, if you actually need to use methods for property computation or actions after setting, they can be built right into the variable definition.
class Person
{
private var name: String;
init( name: String )
{
self.name = name
}
}
should be sufficient for your simple case, although you can also
private var name: String {
didSet {
// updating something after setting
}
};
This is how setter and getter works as in Java:
class Person {
private var _name
private var _age
// .... other properties
var name: String {
get {
return _name
}
set {
_name = newValue
}
}
var age: String {
get {
return _age
}
set {
_age = newValue
}
}
}