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A Simple Class For Basic Persistent Storage
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README.md

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RVS_PersistentPrefs

A general-purpose Swift class for making storing persistent preferences incredibly easy and transparent.

Here is the technical documentation for this class.

Here is the GitHub repo for this class.

OVERVIEW

RVS_PersistentPrefs is an "abstract" base class, designed to be subclassed (Yes, I know. There's no such thing as an "abstract" class in Swift, but the class is designed to crash, if you instantiate it standalone).

Instances based on the class will have a simple, flexible Dictionary<String, Any> property. This will contain a set of values, stored on behalf of the derived subclass.

The base class also saves and retrieves the Dictionary, transparently, from UserDefaults.

Each instance will also have a stored property, called key, which is a String, used to "key" the stored data.

This means that multiple instances of subclasses, based on RVS_PersistentPrefs, can track multiple sets of persistent data.

RVS_PersistentPrefs is designed for ease of use and reliability. It is NOT a class that is meant to store mission-critical, or large volumes, of data. It is merely a convenient way to maintain small amounts of persistent data, such as app preferences.

WHAT PROBLEM DOES THIS SOLVE?

Storing persistent data (data that survives an app being started and stopped, or even, in some cases, transfers between apps) has always been a somewhat fraught process in app development.

Luckily, Apple has provided an excellent mechanism for this, called UserDefaults, which is part of the Foundation framework; meaning that it is supported by ALL Apple operating systems.

Saving and retrieving from UserDefaults is quite simple. You save and retrieve values in the same manner as you would a String-keyed Dictionary.

There is one major limitation, though: ALL stored data needs to be XML plist-compatible. This is usually not much of an issue, as there are plenty of types that work fine. The actual storage happens inside an XML Plist file, so there needs to be support for stored types.

This class will "vet" the preferences before attempting to store them (otherwise, the system simply crashes), and will let you know if there's a problem. It also completely abstracts the actual interface with UserDefaults, so all you need to worry about is interacting with a Dictionary<String, Any>.

Subclasses will also establish "allowed" keys, and can do things like translate the stored values into Key-Value Observer properties, so you can set up a completely "codeless" connection between user interface and stored preferences.

This class allows you to have an "implicit" global state that is accessed by simply instantiating a subclass, and you can associate "top-level keys" to instances, to maintain multiple sets of persistent preferences.

REQUIREMENTS

RVS_PersistentPrefs is an Apple Foundation-based resource. It will work equally well on all Apple development platforms (iOS, iPadOS, macOS, tvOS, watchOS). It will not work on non-Apple platforms, and is not designed to support anything other than native Swift development.

This requires Swift 4.0 or above, and has been tested with Swift 5.1 (prerelease, at the time of writing).

INSTALLATION

You can fetch the latest version of RVS_PersistentPrefs from its GitHub repo.

The class consists of one single Swift source file. All the other stuff in the project is for project support and testing.

Simply copy this file into your project, and add it to your current Swift native target.

IMPORTANT IMPLEMENTATION NOTES

Thread Safety

There is none. Deal with it and move on. I'll be looking to fix that (if I can) in the future, but it isn't a critical enough requirement at the moment to justify preventing release of the utility.

Because of the nature of the utility (a "quick and dirty" persistent save for small amounts of -usually- user-interface-linked data), thread safety is not a critical need. I am making a point of mentioning it, though, so you don't spend too much time searching under the cushions, if you come across inconsistent dealloc crashes. There is a commented-out test in the RVS_Persistent_Prefs_Thread_Tests.swift file. If you uncomment it, and run it repeatedly, you will eventually run into the issue. You can also jack up the number of tests to increase the likelihood of running into the issue.

It doesn't need to be on the main thread, but it shouldn't be called from different threads.

Must Be Subclassed

The implementation needs to be a concrete subclass of RVS_PersistentPrefs. At bare minimum, you need to override the keys: [String] calculated property, to return an Array of String, containing the internal keys. You might also override the key stored property, but it's probably easier to just set the base class one in an init.

Data Stored Is Typeless

Remember that the internal storage of the data is a Dictionary<String, Any>. That means that the storage almost acts like a loosely-typed language. You can change the data type of a stored Dictionary value, simply by changing the type you give to it.

If you are a PHP programmer, that's great. Not so great, if you are a Swift programmer.

One of the jobs of a subclass is to hide this typelessness behind accessors that cast the stored data into consistent types.

All Stored Data Must Be XML-Plist-Compatible

Since the ultimate storage "bucket" for our persistent data is in a plist, all data types, anywhere in the hierarchy of stored data, needs to be in a form that can be serialized into an XML form. Most ObjC (NS and CF) classes can be stored in a plist, and you can usually store opaque types by rendering them into a Data value, and returning that via an NSCoding implementation.

The RVS_PersistentPrefs class will "vet" your data before attempting to save it. If it detects any plist-incompatible data, it will not save the data, and will set the lastError property to valuesNotPlistCompatible, which will have associated data. That data will be an Array of String, containing the top-level keys of the offending elements (remember that you can store a hierarchy, but the error will only report the top level of the hierarchy).

You Must Use the Keys Provided by the keys: [String] Calculated Property

You cannot submit data using a key that is not listed in the keys: [String] calculated property. If you attempt to do so the lastError property will be set to incorrectKeys, which will have associated data. That data will be an Array of String, containing the incorrect top-level keys.

Does Not Throw

The RVS_PersistentPrefs class does not throw. However, internally, it does. It also provides an error enum, containing the errors that it will put into the lastError property, if there was an error.

You should check lastError for problems. It will be nil, if there are none.

KVO

Although not required, it's a good idea to make the subclass Key-Value Observant. You do this by writing accessor calculated properties, and declaring them @objc dynamic. RVS_PersistentPrefs derives from NSObject, so there should be no issues.

You can also directly observe the RVS_PersistentPrefs.values property. It will change when ANY pref is changed (so might not be suitable for "pick and choose" observation).

In some of the included test harness apps, we will use KVO.

USAGE

Start by Including the Main Source File in Your Project

In order to use RVS_PersistentPrefs, you should include the RVS_PersistentPrefs.swift general-purpose source file into your target, and then create a subclass of the RVS_PersistentPrefs class, specific to your implementation.

You MUST Subclass the RVS_PersistentPrefs Class

It is not a protocol. It is a class. It is also not designed to be instantiated standalone. If you do that, it will deliberately crash upon first use.

At minimum, you need to override the keys calculated property, to assign keys to the various stored properties. The following example is from the test harness shared class:

•
•
•

/* ################################################################## */
/**
 This is an Array of String, containing the keys used to store and retrieve the values from persistent storage.
 */
 private static let _myKeys: [String] = ["Integer Value", "String Value", "Array Value", "Dictionary Value", "Date Value"]
 
•
•
•
 
/* ################################################################## */
/**
 This is an Array of String, containing the keys used to store and retrieve the values from persistent storage. READ-ONLY
*/
override public var keys: [String] {
    return type(of: self)._myKeys
}

•
•
•

You SHOULD Provide Type-Enforcing Accessors

The main storage is typeless. It is a simple Dictionary<String, Any>, with no enforcement of type for the data.

You should provide accessors to the stored data that enforces type. Again the following examples are from the test harness shared class:

•
•
•

/* ################################################################## */
/**
 The Integer Value. READ-WRITE
 */
@objc dynamic public var int: Int {
    get {
        if let ret = values[keys[_ValueIndexes.int.rawValue]] as? Int {
            return ret
        } else {
            #if DEBUG
                print("No legal variant of Integer Value")
            #endif
            return 0
        }
    }

    set {
        return values[keys[_ValueIndexes.int.rawValue]] = newValue
    }
}

•
•
•

/* ################################################################## */
/**
 The String Value. READ-WRITE
 */
@objc dynamic public var string: String {
    get {
        let value = values[keys[_ValueIndexes.string.rawValue]] as? String ?? ""
        return value
    }
        
    set {
        return values[keys[_ValueIndexes.string.rawValue]] = newValue
    }
}

•
•
•

Note also, that the two accessors above are declared @objc dynamic. That makes them eligible for Key-Value Observation. In Mac OS, this makes it quite simple to have a "codeless" connection between user interface elements and the persistent prefs (indeed, in the macOS Test Harness project, we demonstrate this).

Other than these two things, there's very little that you need to do in order to use the class. You can provide a distinct String key, so you can store multiple sets of preferences. Remember that this is slightly different from the way that UserDefaults is traditionally used, where each data item is given a separate key. Using RVS_PersistentPrefs, each set of parameters has a "root key." If, for example, you are using the iOS Settings.bundle to display a preferences screen in the Settings app, you can't easily access the RVS_PersistentPrefs preferences directly. It's usually a good idea to manage a set of UserDefaults separately, in order to provide a suitable user experience. We demonstrate this in the iOS Test Harness app.

You CAN Add Your Own Initializer[s]

You may also want to set up a custom init(). In our case, we set one up to allow us to set a key:

•
•
•

/* ################################################################## */
/**
 The keyed initializer. It sends in our default values, if there were no previous ones. Bit primitive, but this is for test harnesses.
 */
public init(key inKey: String) {
    super.init(key: inKey)  // Start by initializing with the key. This will load any asved values.
    if values.isEmpty { // If we didn't already have something, we send in our defaults.
        values = type(of: self)._myValues
    }
}

Once all this is set up, usage is incredibly simple. You just read and write via the accessors. Storage to, and loading from, the UserDefaults is handled completely transparently in the background. It is quite robust, with storage happening immediately upon saving the data, or reading the accessor. With this in mind, you should realize that "saving is for keeps." There's no store in a cache, then flushing the cache. Full storage happens immediately.

Needless to say, this favors robustness over efficiency. It's not recommended to use individual keys for data that may be composed of many parts. It's usually better to have it as an Array or Dictionary that is stored or fetched at once, under one key.

TEST HARNESS PROJECTS

There are a number of included test harness applications. These cover iOS/iPadOS, macOS, watchOS and tvOS.

All Apps Are Localizable

The test harness apps are all complete, production-quality apps, designed to demonstrate release-quality implementation of the preferences class. They are localizable, and written as "ship-quality" apps.

Common Preferences File

All the test harnesses will share the same Preferences Subclass. This is a fairly simple variant that has the following data types:

  • An Integer (key: "Integer Value").

  • A String (key: "String Value").

  • An Arry of String (key: "Array Value").

  • A Dictionary of String-keyed Any (key: "Dictionary Value").

  • A Date Object (key: "Date Object").

It presents Key-Value Observable accessors for all of these values, which are directly used in the macOS test harness.

The iOS Test Harness

The iOS Test Harness App is a very simple one-screen app that presents direct interface to edit and view the values in the common prefs instance. Additionally, it gives a simple demo of using a Settings Bundle to show a "Preferences Pane" in the Settings App. We only access two values, in order to keep the demonstration as basic as possible, but it is possible to get fancier with this.

The iOS test harness also integrates a watchOS test harness that shares the preferences instance with the device app.

The watchOS Test Harness

The watchOS Test Harness is actually a part of the iOS Test Harness app. It shares a RVS_PersistentPrefs state with the iOS Test Harness app instance.

It is a tiny app that merely demonstrates transferring the prefs to the Watch, and displays only a couple of values (and updates them in response to them being changed on the phone). Its main reason for existence is to show that the class works as well in watchOS, as it does in iOS. The only thing that you can do with the Watch app to affect the data, is send a reset command to the phone. Otherwise, it is display-only.

The macOS Test Harness

The macOS test harness app uses KVO for some of its UI, so there are "codeless" connections between some user entry fields and the persistent prefs.

Upon startup, there is no window displayed. You need to go into the app menu, and select "Preferences...". That will bring up the window.

The tvOS Test Harness

The tvOS test harness displays a very similar layout to all the others, and allows demonstration of the class working in tvOS.

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