I am making a weather application. I basically created a class where I will only get data from API and return them as needed. I have variables like cityName, currentWeather etc.
Problem is sometimes API doesn't provide them all so I need to check if they are nil or not before return them. What comes to my mind is set variables like this first:
private var cityNamePrivate: String!
and then
var cityNamePublic: String {
if cityNamePrivate == nil {
//
}
else { return cityNamePrivate }
But as you can as imagine, its very bad because I have a lots of variables. Is there any better logic for this? Should I just return them and check later in wherever I send them?
The problem here is that you will have many variables to deal with. It's not just returning them from the API, it's also dealing with them in your app and perhaps in some processing code.
One solution is to have a big class or struct with many properties. This will work very well, is straightforward to implement but will require lots of repetitive code. Moreover, it will require to change your API and all your code, whenever some new properties are made available by the remote web service.
Another approach is to have replace the big inflexible class or struct, with a dynamic container, e.g. an array of items [ Item ] or a dictionary that associates names to items [ String : Item ]. If the data is just strings, it's straightforward. If it's several types, you may have to have to implement a small type system for the elements. Example:
var x : [ String: String] = [:]
x["city"]="Strasbourg"
x["temperature"]="34°C"
struct Item {
var name : String
var value : String
}
var y : [ Item ] = [Item(name:"city",value:"Strasbourg"),
Item(name:"temperature", value:"34°C")]
The other advantage of this approach is that you stay loyal to the semantics: an information that is not available in the API (e.g. "unknown") is not the same as a default value. I.e. if the weather API does not return a temperature, you will not display 0 in your app. because 0 is not the same as "unknown". While strings are more robust in this matter, the absence of a name is not the same as an empty name.
This last remark suggests that in your current scheme, of having a big data transfer object, you should consider to keep the properties as optional, and move the responsibility for the processing of unknown data to your app.
I would like to hide some property setters and initializers on my Swift model objects. These are reference data that the server provides, and under no circumstances should they be created or modified by the application. This is simple enough in Swift.
However, there is application in my project (a separate target) that needs to break this rule. It is a tool I use to populate the data in bulk, so of course needs to be able to initialize new model objects and set their properties.
What are my options for accomplishing this? I would rather not use a completely new project since it will mean a lot of code duplication. Is there some language-level way to keep this mutability hidden from one application but available to another?
If you declare a property with the let keyword. It can then only be set in the init of the type.
You can also declare a private setter to make the property readonly from the caller of the type but read/write inside the type
struct Foo {
private(set) var bar: Bool = true
func toggle() {
bar.toggle()
}
}
var foo = Foo()
let barState = foo.bar // This works
foo.toggle() // This works too
foo.bar.toggle() // This will make a compile time error
In swift, is there anyway I can create a shared class? That is, say there is a class called Value:
class Value{
var a = 0
}
I want to use this class to be shared among different object instances.
For example,
class Main{
func main(){
print(Value.a)
Value.a++
}
}
class OtherClass{
func otherMain(){
print(Value.a)
Value.a++
}
}
let main = Main()
let other = OtherClass()
//I want this call to print 0
main.main()
//I want this call to print 1
other.otherMain()
I tried static on var a, but then it won't let me do arithmetics such as addition... But it is working if I change the Value into static instead of class. I thought only difference between struct and class was either variable type or reference type...
I'll appreciate to any help. Thanks
The best way to go about what you want to do, unfortunately, is not to do it at all. It seems that the architecture you're designing is quite fragile and two independent classes shouldn't depend on a shared object state and if they do, they should be related via class inheritance or protocols.
A better solution would be:
class MyClass {
static var a = 0
}
class SubclassA: MyClass {
func someFunc() {
print(self.dynamicType.a)
self.dynamicType.a += 1
}
}
class SubclassB: MyClass {
func otherFunc() {
print(self.dynamicType.a)
}
}
There's a few reasons you should do it this way, over your previous solution:
Your shared state can stay private to those two classes. No one outside of those classes can or should modify that value.
Singleton patterns while sometimes useful, have a bunch of problems. This article explains it better than I can.
You have to think of your application and your code architecture as a state machine, moving from one state to another. The goal is to write code that's reusable, modular, generic, and has as little state as possible. The more stateless your code, the better. That's the whole idea behind platforms like ReactJS and ReactiveCocoa; they help simplify that application state.
What you might be looking for is data persistence, something like Realm or CoreData, where you persist data to disk and then you can read it back at a later time as needed. Imagine for example that instead of an Int your shared state was an array of UIImages. With the singleton pattern, you're keeping those images in memory at all times. What if the list grows to be 200 or 300 in length? Then you have memory issues.
So, I'm in Xcode and programming a small program from a friend.
I want to initialize several instances of an object, put them in an array and then iterate through it (via a function that returns a string). Each object adds some text to that string when it's iterated, depending on the variables of the object.
Let's say the class is Tree. The variables in the class are name (string), height(int) and a hasLeaves(bool)(weather it currently has leaves on it or not). I could easily print:
"This is a name that is height meters tall and has leaves.
The problem is that I want the string to be a bit different, depending on which kind of tree it is. Like this:
The oak(name) is big and lifeful, it's height meters tall and has leaves.
Apple trees(name) carries some apples, it's height meters tall and has leaves.
If you ever visit Sweden you should check out the firs(name), they are height tall and haven't got leaves.
I don't want you to write the code for me, but give me a clue. I don't know what to look for. I was thinking about creating a subclass for each Tree, but every subclass would only appear once in the program and I don't know if it's necessary or not.
As you recognize, I'm having a hard time even formulating this question, but if you understand my intentions I'm glad for any clue I can get.
Edit: So this is my attempt to show it in code:
Class:
class tree {
var treeHeight: Int?
var treeWidth: Int?
var hasLeaves: Bool
var treeString: String
init (height: Int?, width: Int?, leaves: Bool, theString: String) {
self.treeHeight = height
self.treeWidth = width
self.hasLeaves = leaves
self.treeString = theString
}
}
Main function:
var oak: tree = tree(height: 1, width: 2, leaves: true, theString:"Oh, the mighty oak")
var appleTree: tree = tree(height: 1, width: 2, leaves: false, theString: "Yummy Apples")
var treeArray: Array = [oak, appleTree]
var resultString = "This is the stories of the trees: "
for tree in treeArray {
if tree.hasLeaves == true {
resultString = resultString + tree.theString
}
}
So, I want the string added to the "resultString" to be different, depending on what kind of tree it is, but I don't want to set that string in the initialization of the object, but rather from what "kind" of tree it is. Does that make it easier to understand?
I want the string (treeString) to be static depending on what "kind" of tree it is. So if it is an oak, the string is always "Oh, the might oak".
It sounds like you want a tree class with some properties like leaves, etc. Maybe you also want to subclass leaves with additional properties like color, etc. I recommend the WWDC 2014 video:
http://youtu.be/W1s9ZjDkSN0
Somewhere around 30 minutes they have a demo of a Car class with RaceCar at subclass.
Regarding creating the objects, you can build each object individually and then collect them in an array as one option. For example, maybe in a form on your app the user inputs data for a class or subclass and then you create an object, store to a master array which you then archive to a file.
So, if anyone stumbles in to this question, this is what I learned:
I was looking for subclasses and protocols. There are methods to determining whether an object is of a certain subclass, and in my case, I could have a protocol called "Tree" with certain methods and/or methods, and then make subclasses to this protocol, called "oak" and "fir".
I am starting studying OOP and I want to learn what constitutes a class. I am a little confused at how loosely some core elements are being used and thus adding to my confusion.
I have looked at the C++ class, the java class and I want to know enough to write my own pseudo class to help me understand.
For instance in this article I read this (.. class attribute (or class property, field, or data member)
I have seen rather well cut out questions that show that there is a difference between class property and class field for instance What is the difference between a Field and a Property in C#?
Depending on what language I am studying, is the definition of
Property
Fields
Class variables
Attributes
different from language to language?
"Fields", "class variables", and "attributes" are more-or-less the same - a low-level storage slot attached to an object. Each language's documentation might use a different term consistently, but most actual programmers use them interchangeably. (However, this also means some of the terms can be ambiguous, like "class variable" - which can be interpreted as "a variable of an instance of a given class", or "a variable of the class object itself" in a language where class objects are something you can manipulate directly.)
"Properties" are, in most languages I use, something else entirely - they're a way to attach custom behaviour to reading / writing a field. (Or to replace it.)
So in Java, the canonical example would be:
class Circle {
// The radius field
private double radius;
public Circle(double radius) {
this.radius = radius;
}
// The radius property
public double getRadius() {
return radius;
}
public void setRadius(double radius) {
// We're doing something else besides setting the field value in the
// property setter
System.out.println("Setting radius to " + radius);
this.radius = radius;
}
// The circumference property, which is read-only
public double getCircumference() {
// We're not even reading a field here.
return 2 * Math.PI * radius;
}
}
(Note that in Java, a property foo is a pair of accessor methods called getFoo() and setFoo() - or just the getter if the property is read-only.)
Another way of looking at this is that "properties" are an abstraction - a promise by an object to allow callers to get or set a piece of data. While "fields" etc. are one possible implementation of this abstraction. The values for getRadius() or getCircumference() in the above example could be stored directly, or they could be calculated, it doesn't matter to the caller; the setters might or might not have side effects; it doesn't matter to the caller.
I agree with you, there's a lot of unnecessary confusion due to the loose definitions and inconsistent use of many OO terms. The terms you're asking about are used somewhat interchangeably, but one could say some are more general than others (descending order): Property -> Attributes -> Class Variables -> Fields.
The following passages, extracted from "Object-Oriented Analysis and Design" by Grady Booch help clarify the subject. Firstly, it's important to understand the concept of state:
The state of an object encompasses all of the (usually static) properties of the object plus the current (usually dynamic) values of each of these properties. By properties, we mean the totality of the object's attributes and relationships with other objects.
OOP is quite generic regarding certain nomenclature, as it varies wildly from language to language:
The terms field (Object Pascal), instance variable (Smalltalk), member object (C++), and slot (CLOS) are interchangeable, meaning a repository for part of the state of an object. Collectively, they constitute the object's structure.
But the notation introduced by the author is precise:
An attribute denotes a part of an aggregate object, and so is used during analysis as well as design to express a singular property of the class. Using the language-independent syntax, an attribute may have a name, a class, or both, and optionally a default expression: A:C=E.
Class variable: Part of the state of a class. Collectively, the class variables of a class constitute its structure. A class variable is shared by all instances of the same class. In C++, a class variable is declared as a static member.
In summary:
Property is a broad concept used to denote a particular characteristic of a class, encompassing both its attributes and its relationships to other classes.
Attribute denotes a part of an aggregate object, and so is used during analysis as well as design to express a singular property of the class.
Class variable is an attribute defined in a class of which a single copy exists, regardless of how many instances of the class exist. So all instances of that class share its value as well as its declaration.
Field is a language-specific term for instance variable, that is, an attribute whose value is specific to each object.
I've been doing oop for more than 20 years, and I find that people often use different words for the same things. My understanding is that fields, class variables and attributes all mean the same thing. However, property is best described by the stackoverflow link that you included in your question.
Generally fields, methods, static methods, properties, attributes and class (or static variables) do not change on a language basis... Although the syntax will probably change on a per language basis, they will be function in the way you would expect across languages (expect terms like fields/data members to be used interchangably across languages)
In C#....
A field is a variable that exists for a given instance of a class.
eg.
public class BaseClass
{
// This is a field that might be different in each instance of a class
private int _field;
// This is a property that accesses a field
protected int GetField
{
get
{
return _field;
}
}
}
Fields have a "visibility" this determines what other classes can see the field, so in the above example a private field can only be used by the class that contains it, but the property accessor provides readonly access to the field by subclasses.
A property lets you get (sometimes called an accessor) or set (sometimes called a mutator) the value of field... Properties let you do a couple of things, prevent writing a field for example from outside the class, change the visibility of the field (eg private/protected/public). A mutator allows you to provide some custom logic before setting the value of a field
So properties are more like methods to get/set the value of a field but provide more functionality
eg.
public class BaseClass
{
// This is a field that might be different in each instance of a class
private int _field;
// This is a property that accesses a field, but since it's visibility
// is protected only subclasses will know about this property
// (and through it the field) - The field and property in this case
// will be hidden from other classes.
protected int GetField
{
// This is an accessor
get
{
return _field;
}
// This is a mutator
set
{
// This can perform some more logic
if (_field != value)
{
Console.WriteLine("The value of _field changed");
_field = value;
OnChanged; // Call some imaginary OnChange method
} else {
Console.WriteLine("The value of _field was not changed");
}
}
}
}
A class or static variable is a variable which is the same for all instances of a class..
So, for example, if you wanted a description for a class that description would be the same for all instance of the class and could be accessed by using the class
eg.
public class BaseClass
{
// A static (or class variable) can be accessed from anywhere by writing
// BaseClass.DESCRIPTION
public static string DESCRIPTION = "BaseClass";
}
public class TestClass
{
public void Test()
{
string BaseClassDescription = BaseClass.DESCRIPTION;
}
}
I'd be careful when using terminology relating to an attribute. In C# it is a class that can be applied to other classes or methods by "decorating" the class or method, in other context's it may simply refer to a field that a class contains.
// The functionality of this attribute will be documented somewhere
[Test]
public class TestClass
{
[TestMethod]
public void TestMethod()
{
}
}
Some languages do not have "Attributes" like C# does (see above)
Hopefully that all makes sense... Don't want to overload you!
Firstly, you need to select a language. For example, I would recommend you to select Ruby language and community. Until you select a language, you cannot escape confusion, as different communities use different terms for the same things.
For example, what is known as Module in Ruby, Java knows as abstract class. What is known as attributes in some languages, is known as instance variables in Ruby. I recommend Ruby especially for its logical and well-designed OOP system.
Write the following in a *.rb file, or on the command line in irb (interactive Ruby interpreter):
class Dog # <-- Here you define a class representing all dogs.
def breathe # <-- Here you teach your class a method: #breathe
puts "I'm breathing."
end
def speak # <-- Here you teach your class another method: #speak
puts "Bow wow!"
end
end
Now that you have a class, you can create an instance of it:
Seamus = Dog.new
You have just created an instance, a particular dog of class Dog, and stored it in the constant Seamus. Now you can play with it:
Seamus.breathe # <-- Invoking #breathe instance method of Seamus
#=> I'm breathing.
Seamus.speak # <-- Invoking #speak instance method of Seamus
#=> Bow wow!
As for your remaining terminology questions, "property" or "attribute" is understood as "variable" in Ruby, almost always an instance variable. And as for the term "data member", just forget about it. The term "field" is not really used in Ruby, and "class variable" in Ruby means something very rarely used, which you definitely don't need to know at this moment.
So, to keep the world nice and show you that OOP is really simple and painless in Ruby, let us create an attribute, or, in Ruby terminology, an instance variable of Dog class. As we know, every dog has some weight, and different dogs may have different weights. So, upon creation of a new dog, we will require the user to tell us dog's weight:
class Dog
def initialize( weight ) # <-- Defining initialization method with one argument 'weight'
#weight = weight # <-- Setting the dog's attribute (instance variable)
end
attr_reader :weight # <-- Making the dog's weight attribute visible to the world.
end
Drooly = Dog.new( 16 ) # <-- Weight now must provide weight upon initialization.
Drooly.weight # <-- Now we can ask Drooly about his weight.
#=> 16
Remember, with Ruby (or Python), things are simple.
I discovered in my question that Properties as defined in .Net are just a convenience syntax for code, and they are not tied to underlying variables at all (except for Auto-Implemented Properties, of course). So, saying "what is the difference between class property and class field" is like saying: what is the difference between a method and an attribute. No difference, one is code and the other is data. And, they need not have anything to do with each other.
It is really too bad that the same words, like "attribute" and "property", are re-used in different languages and ideologies to have starkly different meanings. Maybe someone needs to define an object-oriented language to talk about concepts in OOP? UML?
In The Class
public class ClassSample
{
private int ClassAttribute;
public int Property
{
get { return ClassAttribute; }
set { ClassAttribute = value; }
}
}
In the Program
class Program
{
static void Main(string[] args)
{
var objectSample = new ClassSample();
//Get Object Property
var GetProperty = objectSample.Property;
}
}