In an app that I am working for, I need array variable that can be used in all UIViews. Currently when the array is changed in a view it is stored in the database, and when I need the updated version of the array in the previous view, I use the viewWillAppear method and retrieve the updated array from the database. While going to another view by a segue, I use the passing data by prepareForSegue, but if I use the back button, or just change the screens via tab bar, I use the viewWillAppear and a query.
Is there any way that when the array is created in a view, the data in it will be accessible in all views?
As I've stated in my comment, singletons are generally frowned upon for a myriad of reasons. However, there is much debate on this topic:
What is so bad about singletons?
Having said that, the best way I know to make a variable globally available for the session is by creating a singleton.
struct myArray {
static var data: [Int] = []
}
You could set this singleton up to fetch the records using CoreData and store the current working version in a static variable for quick access.
note: I am really very curious to see some other approaches.
Singleton
Singleton is basically a global variable that you can use them in any views, but some developers experience some bugs and difficulties, use it at your own risk, I recommend this method when you're definite that you will use that data a lot (STILL RISKY), but this method is like goddess of data handling :).
Create a NSObject subclass and call it DataManager.swift (I call it data manager cause it handle data.) as following:
import UIKit
class DataManager: NSObject {
//Store Data Globally
static var someData: NSArray! //This Boolean, you can choose whatever you want.
}
the static is what keep your data live.
Now you can store and receive someData from anywhere like you handle any data type like this.
//Store
DataManager.someData = []
//Receive
print(DataManager.someData)
Related
I have two view controllers that I am working on which both inherits from a Base view controller
class A_ViewController: BaseViewController
class B_ViewController: BaseViewController
Both of those VC interacts heavily with my firebase database. So I want a variable to keep track of all the downloaded items so those two VC can access it without the need to re-download the file again.
I tried to put a variable name in BaseViewController for the two A,B class to access
var allPostsDownloaded: [Post]!
So before A_VC downloads any data, it checks for this allPostsDownloaded variable and loads from it if the data exists. If it doesnt exist, I append to this variable. So when switching to B_VC, the same can be done and no data is re-downloaded when not required.
Reason I am not using segue or protocal to pass data around is that the two VC interacts quite heavly with my database. So it was alot cleaner to try and have a mutural data varaible to keep track of where things are.
However, the problem is that i
var allPostsDownloaded: [Post]!
gets called whenever I switch between A and B VC (Which are tabs). This will cause the variable to be empty and de-initialised.
I guess I could use a global variable instead but that is not good practice? Could anyone please explain why it gets re-called when new tab shows up? And the best solution for this.
as #avi mentioned create new singleton class, then you can pass and read easily. Below is an example
class PersistentData
{
static let sharedInstance = PersistentData()
// your global persistent variable
var allPostsDownloaded = [Post]()
}
So in your controllers you can simple read and set as below
// read
print(PersistentData.sharedInstance.allPostsDownloaded)
// set new data. this just example, hence depends on your case
PersistentData.sharedInstance.allPostsDownloaded.append(newPost)
PersistentData.sharedInstance.allPostsDownloaded = posts
also keep in mind that if you want to read new value when switching between tabs, you can get the updated in viewDidAppear or viewWillAppear
You can create a Singleton class with a instance variable and can save all downloaded data, and can access singleton class variable from any where of your project's classes.
New to iOS development and trying to develop my app with correct practices. My main issue is trying to figure out the best way to store a bunch of simple data globally.
The app connects to a computer adhoc wifi and gathers information about the current state. (ex// gpu temp, computer name, display resolution, date/time, etc). The various view controllers I have then allow the user to adjust some of these settings so I want to be able to keep track of these changes across all view controllers.
I have been wrestling between using a plist, core data, or singleton class to store all the information. Since the user will be connected to one computer for the life of the app I am leaning towards singleton since it seems easiest to get and set data. However, I have read that using a singleton is really not the best practice for various reasons. I have read up on plists and core data a little and seems like a decent amount of work just to get and set values. Should I spend the time using those or would some other method be a better way to accomplish all of this.
Update:
After both answers from Aaoli and Swift Rabbit. I wanted to clarify for my use case that the data I am storing did not need to be "saved" when the app was closed. So if you are coming to this question and need to store your data even if the app closes look at #SwiftRabbit's answer.
To use Global variable in Swift you need to define struct outside the class and because you don't need the variable to be swimming in the whole app memory without binding:
struct GlobalVar {
static var myvar = 0
}
To use simply the global variable from anywhere use :
GlobalVar.myvar = 10
In my case, using "simple" data, which is not sensitive, I usually use the user defaults. They can be accessed globally through the project. For example, it can be set this way :
var prefs:NSUserDefaults = NSUserDefaults.standardUserDefaults()
prefs.setObject(myValue, forKey: "myKey") // myValue is of type typeOfValue
prefs.synchronize()
It can be accessed this way :
var prefs:NSUserDefaults = NSUserDefaults.standardUserDefaults()
prefs.valueForKey("myKey") as? typeOfValue
Using setObject can be used for any kind of value, that's why I cast it when retrieving it from the userDefaults. You could actually use other methods to insert it and retrieve it that would make your task easier. For example, you could tell you are inserting a bool with :
prefs.setBool(<#value: Bool#>, forKey: <#String#>)
And specify you are retrieving one with :
prefs.boolForKey(<#defaultName: String#>)
All methods can be found in the apple documentation here
My opinion: create a singleton, e.g. in this way:
class MyClass {
/// The singleton instance
private static var singletonInstance: MyClass!
class var sharedInstance: MyClass! {
get {
singletonInstance = singletonInstance ?? MyClass()
return singletonInstance
}
}
private init() { }
}
Or in this way:
class MySingletonClass {
static let sharedInstance = MySingletonClass()
private init() {
}
Many people consider Singleton a poor design pattern, but in my opinion it is not. It is good to have a pattern which ensures that a piece of information is only available once. What is not good is to have global access to this Singleton from anywhere in the application. Some people counter-act this with "Dependency Injection", which basically means that your classes does not "pull" the information from the Singleton but the information is "pushed" into the class from (some other instance) outside. This promotes loose coupling and better test-ability. But this is an advanced topic which needs much more information. (Google "typhoon framework" for iOS if you are interested in this)
I am developing an iOS application where I want to record the time when a user presses a particular button and keep it. Later I will use this time record in background. Is there a nice way of doing that without invoking NSUserDefaults or CoreData or whatever other database?
I am very new to iOS development. I think this is very likely to be a naive question. But I'm just curious. Please don't laugh at me haha.
Edit: This is indeed a very naive question haha.
A simple way to make sure your data is available everywhere in your app and only persists for each app session would be to use a singleton. Something like this.
// Create a class to store the data
class SessionData : NSObject {
// Create a shared instance of your class
static let sharedInstance = SessionData()
// Create a variable to store the date object
var timeStore:NSDate?
}
This will mean that anywhere in your app you can get and set this data as below.
// Get
if let time = SessionData.sharedInstance.timeStore {
println(time)
}
// Set
SessionData.sharedInstance.timeStore = NSDate()
It is worth mentioning that despite the above being a valid method, You may be able to avoid doing this by re-thinking your app structure and passing data between classes instead. You should have a look at Delegation.
Also as #CouchDeveloper mentions in their comment below, you may want to included a dispatch queue to prevent crashes or locks in the situation where two or more classes try to read and or write data at the same time.
I been reading about avoiding the mutable state, and how the singleton pattern is bad for having a global state.
I see few answers about Dependency injection http://www.objc.io/issue-13/singletons.html, but I can not find how to solve this basic approach:
How maintain the user data arround the app?, the solution is to pass the user information from the one view (where is requested by webservice) through the views by parameter to the seven push view (where is needed again) ?
there is a better way?, or the singleton pattern is sometimes necessary?
I use a singleton to represent my Engine class in Swift. It is initialized once on launch and contains static class variables stored in a struct. Works fine for me.
class Engine{
struct properties{
static var resourceManager:ResourceManager!;
...
}
init(){
properties.resourceManager = ResourceManager();
}
In another class I can then call
Engine.properties.resourceManager
I got a app that uses navigation controller and tableViews and I want to create a class to do some simple storage of information that stays persistent while navigating through the different views without saving to disk.
I can either create an singleton with only class methods, but in this case I´d need to create
the collection class holding the data as an instance variable (as #properties don´t work with class methods). I only ever see objects declared in properties in iOS, so is this frowned upon?
The class would look something like this
header:
+ (BOOL) addObject: (id) object;
+ (BOOL) removeObject: (id) object;
+ (NSInteger) count;
and privately I´ll have an NSArray for storage
NSArray *cache;
But is this a good way of achieving the task? or would it be possible to have a non-singelton class with instance methods and use that same instance of the class in the different table views? if so, how would I do that?
First, ALL readwrite properties auto-synthesize instance variables (unless you implement BOTH setter AND getter).
Second, if that information is global to the entire (or most of the) App, a singleton is just what you need. You don't need to keep it as a property (or an ivar). It's a singleton, it keeps its own pointer.
If you still want to go with a property, you will have to pass it some how to every VC in your App (prepareForSegue:sender: probably if you're using storyboards).
First figure out what global information you need. Then figure out what objects you already have that have a lifetime consistent with that global information, and which are logically associated with the info. Eg, if you need an array of info to "back up" a UITableView, store the pointer to that array in the table view data source instance.
It is rarely necessary to create a "singleton", and having lots of singletons is usually a sign of poor programming.