We are a looking for a value type design pattern in swift that will allow us to create a shopping cart to hold Products. We are currently using a class but that is a reference type and when we try to add two different version of the same product (i.e. with a different colors or sizes), the first item we added gets changed to the second item we added because it points to the same object in memory.
The design pattern needs to be “global” so we can access it from any page in the app. Right now this is our Cart class that stores all the items in the cart. What do we need to do to make this a value type or how does it need to be reengineered to use a struct without a class?
class Cart : NSObject {
var allProductsInCart = [MainProduct]()
override init() {
super.init()
}
class var sharedCart: Cart {
struct Static {
static let instance = Cart()
}
return Static.instance
}
}
The problem we are getting is that we need the products in the cart to be of custom class “MainProduct.” Right now as you can see, they are stored as “MainProduct.” Do we need to switch the products to a struct or other design pattern as well? How would we do that?
Yes, given the desired behavior between a value type vs. reference type you should use a Struct.
A commonly used "pattern" for doing this is called "Redux".
The idea is that you have one, immutable version of the "state" of your app.
It can be accessed from anywhere and the only way to update it is through "actions". These will reconstruct the entire state with the required updates.
ViewControllers and views, etc... subscribe to updates of various parts of the state.
So you could have an AppState that contains a ShoppingCartState. When a product is added to it your CartViewController will be informed of this update and can update its view etc...
There are many different frameworks that are built to use Redux so I won't recommend one as you should find the one that is right for you. But I believe this pattern best suits the usage you are after.
Related
This is a bit confusing so I apologise. But:
I am trying to make an app where the user has the ability to add items to a list of individual items that will be displayed back to them(Something along the lines of a todo list app) as a table view.
However, I have hit a roadblock I need to store several different bits of data for each item(Some Strings, Some ints and a date) in the list.
I think that a class(or struct) would be the best way to do this where an instance of the class holds the information need for each item and then the name of that instance is stored in a list so it can be accessed via the indexPath in the table view.
However, I don't know how I am going to make a new instance of the class for every item because the app could have hundreds of individual items.
I'm sorry, this is so confusing and any help would be appreciated! Feel free to ask for more info
Edit: what I am looking for and I'm sure there's a stupidly easy way of doing it but I'm try to work out how to create an instance of a class when the name of the class is stored in a variable. Ecencialy I want the instance of the class to store the item. To be created when the user inputs the item to be added to the table.
Eg. They enter an item. item1 and the other data that goes along with then I want to be able to store that in instance of the item class but I don't know how to make the name of that instance because the name I want which is item 1 is stored in a variable.
Sorry that's so confusing that's the reason I need help
So first: You can't store the Name of a Class in a Variable and use this variable to get a new instance of a class.
What you need is an Array containing all the different Items. This array is unlimited so you can store as many items in it as you like and you don't need to set a name for each of these Instances.
First create an empty array containing all Items as a property:
var items : [Item] = []
Second call the function populateItemArray() in your viewDidLoad():
private func populateItemArray() {
// Populate the items Array (Are you using CoreData, Realm, ...?)
}
Third, use the Item Array to populate the TableView.
REMEMBER: If your only using one section in your tableView, indexPath.row is always equal to the corresponding item in the array.
E.G. items[indexPath.row]
Hope this helps you!
UPDATE:
Look at this example struct Item. As you can see you can also store a date in it:
struct Item {
// First create all the properties for your struct.
var data1: String
var data2: Int
var data3: String
// You can create properties of any type you want, also of type date.
var dateOfCreation : Date
// Then implement all the methods of your custom Struct Item.
func setDate(with date : Date) {
self.dateOfCreation = date
}
func returnDate() -> Date {
return self.dateOfCreation
}
}
We're building an iOS app using Realm as our model / database but we want to design the client so it can easily accommodate changes in the REST-ful API that may occur in the future. Lets say we're developing an app for sports competition organizations that accommodates different events. Each event has different types of event types based on what sports are being played. Right now the API only returns Soccer, Baseball, and Football but in the future it might expand to include Basketball too. Later it might eliminate Baseball. I've designed the Realm objects so that Events are decoupled from Event Types using a one-to many relationship like this:
class EventTypeGroup: Object {
dynamic var name = ""
let eventTypes = List<EventType>()
}
class EventType: Object {
dynamic var name = ""
dynamic var descriptionText = ""
}
An EventTypeGroup is the class describing the event types (in this case which sports) will be played at the event. I used this design because dictionaries aren't supported in Realm where we could store an event type with an associated set of properties.
In order to make the model adaptable to future changes in the API in case sports for a particular organization are added or removed, I used an abstract factory pattern like below. This way an event cannot be created without using an enum in keeping with modern Swift design principles. The problem I'm having is, assuming we only check for changes in the API to the event types (sports) once upon the user opening the app, how do we change the model with the app already open? Will the database need to be migrated if these fields change?
protocol EventTypeGroupFactory {
func createEventTypeGroup(List<EventType>) -> EventTypeGroup
}
protocol EventTypeFactory {
func createEventTypes() -> List<EventType>
}
class SportEventGroupFactory: EventTypeGroupFactory {
func createEventTypeGroup(withEventTypes: List<EventType>) ->
EventTypeGroup {
//implement logic to create an EventTypeGroup for the SportEventGroup
}
}
class SportEventTypeFactory: EventTypeFactory {
EventTypeGroup {
func createEventType() -> EventType {
//implement logic to create an EventType for the SportEventType
}
}
class EventTypeGroup: Object {
let eventTypes = List<Int>
enum EventType {
}
}
class EventType: Object {
var type: Int?
name: String?
description: String?
}
class Event: Object {
static enum EventType
init(eventTypeWithRawValue:) {
}
}
Also, how will I refer to the different variations of the classes in the code I write now if I don't know how they'll be defined. I'm guessing the abstract factory pattern may not be the best way to deal with this but am not sure what other options I should consider or how to approach the issue of making types easily extensible in a model based on API changes.
You are overcomplicating it, I think. Just add a string property called "eventType" to your Event model.
For example, normally, if you didn't need to keep things dynamic, you might do something like this:
enum EventType {
case soccer
case baseball
case football
}
// Your Event model
struct Event {
var date: Date
var eventType: EventType // a static type :)
}
But in your case, instead you can do something like this:
// Your Event model without any enums
struct Event {
var date: Date
var eventType: String // a dynamic type :(
}
Property eventType can then be "soccer" or "baseball" or "football". (But the compiler cannot help you catch errors now.) As for your persistent storage, just have a field there of type eventType and store the string.
Dynamic types make me sad given how nicely static Swift is, but it gets you what you want. Just make sure to think about edge cases. To not end up with undefined behavior, think ahead about what your app is supposed to do if, for example, you end up with event types on disk that are no longer being supported by your REST API.
For example, say you have an /eventTypes endpoint, so that your app's users can add events and categorize them accordingly, and it's been returning "soccer", "baseball" and "football" and your users have been adding these types of events and you have been storing them on disk (in Realm or CoreData or whatever). But then one day someone on the backend (or through the backend) renames "football" to "american football", and let's hope no one renames "soccer" to "football" too. (And so now you can't tell if a thing was renamed or removed and another added.) Do you then take the union of the event types your /eventTypes endpoint returns and what you find on disk? Do you let users add old event types that still live on disk but are not longer supported by your REST API or only display them?
With active users, you will likely end up with these kinds of edge cases if your backend folks rename event types or remove event types (as opposed to simply adding them). Just discuss with your stakeholders what the behavior should be.
I am making an instagram clone but I am having abit of question about what would be the best way to pass data arround.
I have 3 post arrays and three VCs as shown below
myPosts = [Post]()
nearbyPosts = [Post]()
followingPosts = [Post]()
myPostVC
nearbyPostVC
followingPostVC
The interesting part is when I download a post that is close by and it happens to be one of my post that I am also following. So when I download that post, I will need to put it in all three arrays so they are the same reference. This is so that if the user "likes" that post, it will be reflected in all three array because they point to the same post. (If they dont have the same reference, and the user likes or perform any sort of action on a post in myPosts array, I will have to manually loop through each nearbyPosts and followingPosts to update their "like count" individually)
Initially, my code looks this
myPost -> Initiated inside myPostVC
nearbyPost -> Initiated inside nearByVC
followingPostVC -> Initiated inside followingPostVC
So in myPostVC, I would need to create an instance of nearbyVC and followingPostVC so that everytime I download a post inside myPostVC, I loop through nearbyVC.nearbyPosts and followingPostVC.followingPosts and update anyting that has the same poast as the one that was just downloded. And I do this for all three VC. Imagine if I have moreVC, this gets very messy as each VC is linked to other VC (In my case, the threeVC as three tabs)
This is when I changed to Singleton... I made myPosts, nearbyPosts and followingPosts to be all inside a singleton as shown below. This allows the three VC to be disconnected and they just need to access whichever array they wish to by calling something like PostService.ps.myPosts (Without the need to create a refrence to other VC)
My question is...
I know it "might" be good to use the first method because it is not singleton, but is that actually a method people use in this case? (It makes all VC linked together. Very messy)
Is singleton the right way to solve this problem or is there a much better way? (Note that similar to instagram, I dont really need to store these posts on the phone to look at in future, so no coredata?)
class PostService {
static let ps = PostService()
private var _myPosts = [Post]()
private var _nearbyPosts = [Post]()
private var _followingPosts = [Post]()
var myPosts: [Post] {
return _myPosts
}
var nearbyPosts: [Post] {
return _nearbyPosts
}
var followingPosts: [Post] {
return _followingPosts
}
So many ways to approach this.
Create a new class (such as your PostService) to represent your data model, outside of any view controller. This data model class could be a singleton. I would put all of the posts into a single Set (not an array) named allPosts. Using a Set rather than an array will prevent any duplicates. Each Post object could have boolean vars indicating if it belongs to specific subsets or not. This could be stored or calculated depending on your data.
Then to access a specific subset, use the filter function on the set, such as:
var allPosts: Set<Post> = ...
var myPosts: [Post] {
return allPosts.filter { $0.isMine }
}
var nearbyPosts: [Post] {
return allPosts.filter { $0.isNearby }
}
The advantage here is no duplication of objects and a shared resource for each of your view controllers.
Note: Since this is a Set rather than an Array, it will not have any order to the items it contains. But you can always use the sort() function to do that.
The application in question is built as follows:
A user selects a job
the job can have many components
each component can have many lineitems.
I am not clear on how this should be structured - should this be class or structs? Seeing that only one job is being processed at a time, I am fairly confident that jobs should be a class. However, when there are multiples of a certain object type, I am not exactly clear on how to form them, like the components and lineitem objects.
The application consists of ViewControllers and TableViewControllers. All the data is fetched from a server in JSON and populated into the appropriate view as needed. Here are the object types as they are currently setup:
A job object:
// Job Object
//
public struct Job {
static var globalId : String?
static var name : String?
static var status : String?
static var client = Client()
static var components = Array<Component>()
// etc..
}
A Component like so:
// JobComponent Object
//
public struct Component {
var name:String? = ""
var fmRecordId : String?
var startTS:NSDate?
var endTS:NSDate?
var notes:String? = ""
var items = Array<Lineitem>()
// etc...
}
and finally, a lineitem:
// Lineitem Object
//
public struct Lineitem {
var fmRecordId = String()
var itemName = String()
var itemNumber = String()
// etc...
}
All of these object are built within a public class called "PL".
When a user selects lineitem and edits it's values, the values are not available outside the VC in which they are edited because the VC isn't referencing the lineitem that is was passed, it is simply copying it. The same happens with components.
A workaround I found was to use the the Job struct PL.Job.self and always modify the components and lineitems like so where i = a desired index in the array:
PL.Job.components[i] to access a component
PL.Job.components[i].items[i] to access a specific item within that component.
However, this doesn't scale very well.
The desired behavior is to be able to pass a reference to a particular instance of an object around rather than pass around the index path of those objects in the PL.Job object.
I am well aware there is something wrong with how this is currently structured, but could someone please point me in the right direction?
A couple of points:
You can only pass class instances by reference. If you want to be able to pass a reference to a particular LineItem or Component or Job, and you want to be able to make changes to that object that are effective everywhere, then you need to define them as classes and not structs. Instances of struct types are always passed by value and not be reference. And when a value type is passed, it is copied, meaning that you create an entirely new copy of the object, and mutating the copy has no effect on the original.
Your Job struct only has static properties - i.e., there will only ever be one globalId, name, status etc. throughout your entire application. If you want to have multiple instances of Job, then these should not be static properties. You say that only one Job will be processed at a time, so maybe that was intentional. Either way, it is still often preferable to create an instance of a Job class that has those properties. It certainly would give you more flexibility later if you decide to make it possible to hold references to multiple jobs in memory, or to allow the user to select between different jobs, or switch between jobs, etc. For example, you may want to allow a user to switch to the Job they were processing earlier without necessarily destroying the Job that they are working on now.
But I think the main point is that you will need to define your objects as classes if you want to be able to pass them by reference. If you modify an object that is passed by reference, all other references to the same object will show the same changes (because, after all, they are just references to the same object). That doesn't work with value types, like structs.
I develop web part with custom editor part and faced with this question.
Is it possible in web part set Personalizable attribute to generic List?
For example I want something like this:
[WebBrowsable(false)]
[Personalizable(PersonalizationScope.Shared)]
public List<AnnouncementItem> Announcements
{
get { return _announcements; }
set { _announcements = value; }
}
Is it possible, and what kind of types at all can be used as "Personalizable"?
Thanks.
Solution:
I use a custom EditorPart to select multiple lists using AssetUrlSelector, but I need a way to personalize this collection for end user.List<of custom objects> doesn't work, but I found that List<string> (and only string) work perfectly. So, I get required lists in EditorPart and pass their to the web part using List<string>.
Try using a custom EditorPart to add/remove items from the collection. I've never built a web part that personalized a collection so I don't know if it works but I'd definitely try the collection with an EditorPart. If it doesn't work, serialize XML into a string property.
Your question does not seem to match your code. Your code shows a collection of custom objects. I doubt an end user will be able to set such a property. To have a property that points to a generic list, you would probably be better off defining the property as a string that contains the URL to a list.