I have a NSManagedObject class with two relationships: courseAand courseB.
These relationships should be represented in a dynamic variable. How is it possible to change this variable from outside the class?
#objc(Universtity)
public class Universtity: NSManagedObject {
dynamic var name: String {
get {
let name = self.courseA?.name
return name!
}
}
}
For example from within a ViewController like University.name = University.courseB.name ?
I was thinking about a Notifikation, but this seems maybe a little more complicated as it could be.
And if there is no other way, how should I implement the observer inside the University class?
Thank you for every idea.
Looking at your code, you have declared a "computed" or "ready-only" variable. This is a variable whose value comes from another variable or combination of variables.
I can't see your data model, so it's not clear if you have defined a name parameter in the Core Data model. Regardless, if you have the logic is somewhat confused, because the getter you have defined means any value it may hold would be ignored anyway. You would need to define a setter to set self.courseA.name if you want to ensure the value can be written to. You don't need to worry about key-value coding notifications, because they will be triggered by the Core Data Managed Object.
public class Universtity: NSManagedObject {
dynamic var name: String {
get {
let name = self.courseA?.name
return name!
}
set(newValue) {
courseA!.name = newValue
}
}
}
Also the pattern you have used to force unwrap a non-optional value in your getter isn't optimal. I haven't edited this because that is another discussion, but I would suggest asking yourself the question am I sure why I am doing this? for every "?" and "!" you use.
Related
As a preface, this might be an incredibly simple and/or ignorant question.
In ReactiveCocoa 2.x, we were able to use RACObserve and RAC to observe properties of an object. From the documentation I can find in Reactive 3 and 4, we now use PropertyType to observe changes to an object property. I have so far been unsuccessful in being able to observe any property change when using MutableProperty or DynamicProperty.
class TempObject {
var property: String
}
let tempObject = TempObject()
let propertyObserver: MutableProperty<String> = MutableProperty(tempObject.property)
From what I understand, I should be able to use propertyObserver to view changes to tempObject.property. I tried adding a map function to the signal producer from propertyObserver to see if it was firing, but don't see anything when updating tempObject.property. Again, could be a trivial thing that I am missing, thanks so much.
Edit
NachoSoto nailed it - I needed to make my property KVO compliant. I also ended doing this:
let tempObjectSignal: MutableProperty<TempObject> = MutableProperty(tempObject)
let propertyObserver: MutableProperty<String> <~ tempObjectSignal.producer.map({ $0.property })
And whenever tempObject.property is updated I make sure to call
tempObjectSignal.value = tempObject
This fires off all the necessary signals. I don't know if this breaks any best practices, though. Let me know what you think!
MutableProperty(value) creates a mutable property but only with value that as the initial value.
What you want to use is DynamicProperty, which will use the Objective-C runtime and KVO to detect changes to an object's property:
let property = DynamicProperty(tempObject, "property")
For that reason, however, you need to make sure that the property you want to observe is part of the Objective-C runtime, by making the class a subclass of NSObject, and by either using the dynamic keyword:
class TempObject: NSObject {
dynamic var property: String
}
Or using #objc to ensure that it gets exported to the runtime:
class TempObject: NSObject {
#objc var property: String
}
From a few initial tutorials, I see that properties belong to a Class and are essentially 'global variables' as used in the C++ world (coded in this years ago). I also see variables as more of a 'local' entities only used / storing information within a method.
Then I came across this Quora thread: https://www.quora.com/Apple-Swift-programming-language/What-is-the-difference-between-a-property-and-a-variable
Now I see properties being able to execute code associated with their invocation. This is very cool, but also opened up a whole bunch of other questions for me.
Are there other simple and clear ways to remember the distinction between a property and a variable?
Properties belong to an object, whereas variables do not. A variable can be declared without having to be associated with a particular class, or other object. A property must be associated with a particular object (i.e.: a class, enum, or struct)
Local variables are just things that you work with. You have full control over these, and if you change a variable in a function, nothing outside of your function is ever gonna know. If I write a framework and you use it, and I decide to change something about a function's local variables, your app that uses my framework will keep working just as if nothing changed.
Classes, on the other hand, describe a contract. When you use a class, you have access to everything they publicly advertise. This means that if I write a framework and you use it, if I ever change or remove a public member on a class, your code will break if you were previously using that member.
For this reason, in many languages, it's bad practice to mark instance variables as public. Instance variables having no logic attached, if I want at some point to trigger something when a field is changed or if I want to remove the field entirely (and instead report a value in a sub-object or something), then I'm stuck with changing the public contract (turning the field in a pair of get/set methods, for instance), and possibly breaking your code.
Swift makes properties an indirection for this reason. Swift properties can be treated as dumb values for the most part, but if you ever need to change from a stored value to a computed value or something, you can do it without changing your class's interface. That way, you don't break existing code that relies on the property.
Swift variable, constant, Property
[Swift types]
variable - named storage of address. Every variable has a type which defines a memory size, attributes and behaviours
Swift variable and constants
constant is a variable but can not be modified after definition.
//definition
var <name> = <initial_value>
//type annotation
var <name>: <Swift_type> [= <initial_value>] // [] is optional
//var - variable
var myVariable1 = 11
var myVariable2: Int
myVariable2 = 12
//let - constant
let myConstant1 = 21
let myConstant2: Int
myConstant2 = 22
Global and local variable
Global variable is a variable which is defined out of function, class.
Local variable is: variable inside a type context(class, struct, enum)[About], inside a function, function parameter
Property
property - associate value with a type context. It is a variable + bounded getter/setter. It has field syntax but uses methods(getter/setter) under the hood.
Stored properties and computed properties
They can belong to instance(instance property) or type(type property):
Stored property (class, structure)
Computed property (class, structure, enum)
Stored property - is a local variable -> variable inside a type context. Swift stored property does not support instance variable like Objective-C.
variable stored properties - var
constant stored properties - let
It supports property observers (willSet, didSet)
Computed property - provide getter and optional setter to calculate a value every time
public class Person {
var firstName = "John"
var lastName = "Wick"
var fullNameComputedProperty: String {
get {
return "\(firstName) \(lastName)"
}
//optional
set {
let arr = newValue.split(separator: " ")
firstName = String(arr[0])
lastName = String(arr[1])
}
}
var addressStoredProperty: String {
//Property Observers
willSet {
print("old address:\(addressStoredProperty)")
print("new address:\(newValue)")
//addressStoredProperty is not updated yet
}
didSet {
print("old address:\(oldValue)")
print("new address:\(addressStoredProperty)")
}
}
}
Lazy Stored property
Property is calculate during first access to it(on demand)
only var lazy because let must have a value during initialization
Init/customize stored property by closure
Official doc
You are able to init/setup/customise a stored property with a help of closure
() at the end executes the closure immediately and assign a value to stored property(calculate and return a value).
in initializing case it is not possible to access to any instance variable or function because it has not initialized yet
in initializing case it will be executed only once for every object or if you use static - once for the class[Example]
Examples
func testStoredPropertyWithClosure() {
class ClassA { }
class ClassB {
static let staticStoredProperty: ClassA = {
//is called only when you access to it like ClassB.staticStoredProperty
print("init staticStoredProperty")
return ClassA()
}()
var storedProperty: ClassA = {
print("init storedProperty")
//self.foo() //Error: Class declaration cannot close over value 'self' defined in outer scope
return ClassA()
}()
func foo () {
storedProperty = {
print("customize storedProperty")
return ClassA()
}()
}
}
let b = ClassB()
b.foo()
ClassB.staticStoredProperty
}
closure stored property vs Computed property
closure stored property is called once and can be changed after initialization(if it is var)
Computed property is calculated every time when it is called
[Java variable, property...]
I'm having trouble grasping the proper way of instantiating variables that always need to be set before an object is fully functional but may need to be instantiated after the constructor. Based on Swift's other conventions and restrictions it seems like there is a design pattern I'm unaware of.
Here is my use case:
I have a class that inherits from UIViewController and will programmatically create views based on user actions
I need to attach these views to this class, but to do so I need to retrieve their content based on configuration data supplied by another controller
I don't care if this configuration data is passed to the constructor (in which case it would always be required) or supplied by a secondary call to this object before it is used
My problem seems to be that both of the approaches in bullet 3 seem flawed.
In the first case, there is only one legitimate constructor this class can be called with, yet I'm forced to override other constructors and initialize member variables with fake values even if the other constructors are never intended to be used (I'm also trying to keep these variables as let types based on Swift's best practices).
In the second case, I'm effectively splitting my constructor into two parts and introduce an additional point of failure in case the second part fails to be called prior to class being used. I also can't move this second part to a method that's guaranteed to be called prior to usage (such as viewDidLoad) because I still need to pass in additional arguments from the config. While I can make sure to call the initPartTwo manually, I'd prefer to have a mechanism that better groups it with the actual constructor. I can't be the first one to run into this and it seems like there is a pattern I'm not seeing to make this cleaner.
UPDATE:
I ended up going with a modified version of the pattern matt suggested:
struct Thing {
let item1: String
let item2: String
struct Config {
let item3: String
let item4: String
}
var config:Config! {
willSet {
if self.config != nil {
fatalError("tried to initialize config twice")
}
}
}
init() {
self.item1 = ...
self.item2 = ...
...
}
public func phaseTwoInit(item3: String, item4: String) {
self.item3 = item3
self.item4 = item4
...
}
}
var t = Thing()
...
t.phaseTwoInit(...)
...
// start using t
If an initial instance variable property value can't be supplied at object initialization time, the usual thing is to declare it as an Optional. That way it doesn't need to be initialized by the class's initializers (it has a value - it is nil automatically), plus your code subsequently can distinguished uninitialized (nil) from initialized (not nil).
If the Optional if an implicitly unwrapped Optional, this arrangement need have no particular effect on your code (i.e. it won't have to be peppered with unwrappings).
If your objection is that you are forced to open the door to multiple settings of this instance variable because now it must be declared with var, then close the door with a setter observer:
struct Thing {
var name:String! {
willSet {
if self.name != nil {
fatalError("tried to set name twice")
}
}
}
}
var t = Thing()
t.name = "Matt" // no problem
t.name = "Rumplestiltskin" // crash
Xcode6 ios swift
I have created my own class and trying to make an autogetter and autosetter, but i don't really know if it's allowed.
var Birthday:NSDate {
get {return birthday}
set(newValue){birthday = newValue}
}
var BirthYear:Int32 {
get {}
set {}
}
The last part of code triggers error, missing return, so my question is that - Is there any possibility to make getter and setter without making a second variable
Stored properties in swift are backed by hidden instance variables - the property itself is its own getter and setter, unless you implement it as a computed property, in that case you have to provide your own getter and/or setter. So when you write:
var birthday: NSDate
you use it as:
let value = classInstance.birthday
to read its value, and
classInstance.birthday = someDate
to assign a new value. You don't have to do anything special to make that work.
Suggested reading: Properties
Side note: by convention variables and property should use lower camel case notation, so they should start with lowercase, and if made up of multiple words, make the first letter of each word in uppercase. For instance:
var single: Int
var multipleWordsVariable: String
So in a regular Class, you can override the setter method for a class property:
-(void)setSortBy:(NSString *)sortBy {
// Do other stuff
_sortBy = sortBy;
}
using the _ prevents an infinite loop of the method calling its self.
I am trying to do something similar with a NSManagedObject class, but it does not give the option to use the underscore (_):
-(void)setHasNewData:(NSNumber *)hasNewData {
// update self.modifiyDate
_hasNewData = hasNewData;
}
Gives me an error and suggests I replace _hasNewData to hasNewData.
Is this how it should be done or will it give me an infinite loop?
I want it to update the NSManagedObject's property modifyDate anytime I set hasNewData.
Your first example for a "regular class" works if _sortBy is the instance variable
backing up the sortBy property (e.g. the default synthesized instance variable for
that property).
But Core Data properties are not backed up by instance variables.
When overriding Core Data accessors, you have to use the "primitive accessors",
and also trigger the Key-Value Observing notifications:
-(void)setHasNewData:(NSNumber *)hasNewData {
[self willChangeValueForKey:#"hasNewData"];
[self setPrimitiveValue:hasNewData forKey:#"hasNewData"];
[self didChangeValueForKey:#"hasNewData"];
// do other things, e.g.
self.modifyDate = ...;
}
More examples can be found in the "Core Data Programming Guide".