In this tutorial, we will learn how to use `NSCache` in Swift. `NSCache` is a class provided by Apple that allows you to store and retrieve temporary objects in memory. It is commonly used to cache data that is expensive to compute or retrieve, such as images or network responses. By using `NSCache`, you can improve the performance of your app by reducing the need to recreate or fetch data from expensive sources.
Here’s what we’ll cover in this tutorial:
1. What is NSCache?
2. Creating an NSCache instance
3. Adding and retrieving objects from NSCache
4. Setting limits and eviction policies
5. Handling memory warnings
6. Example usage scenarios
Let’s dive in!
1. What is NSCache?
`NSCache` is a subclass of `NSObject` that provides a way to store and retrieve temporary objects in memory. It is similar to `NSMutableDictionary`, but with additional features specifically designed for caching purposes. `NSCache` automatically manages the memory used by its objects, and can automatically evict objects when memory is low.
2. Creating an NSCache instance
To use `NSCache`, you need to create an instance of it. Here’s how you can do it:
let myCache = NSCache<AnyObject, AnyObject>()
In the above code, we create an instance of `NSCache` called `myCache`. The generic types `AnyObject` can be replaced with the specific types of the keys and values you want to store in the cache.
3. Adding and retrieving objects from NSCache
Once you have an instance of `NSCache`, you can add and retrieve objects from it. Here’s how you can do it:
let myObject = MyObject() myCache.setObject(myObject, forKey: "myKey")
In the above code, we create an instance of `MyObject` and add it to the cache using the `setObject(_:forKey:)` method. The key `”myKey”` is used to identify the object in the cache.
To retrieve an object from the cache, you can use the `object(forKey:)` method:
if let cachedObject = myCache.object(forKey: "myKey") {
// Use the cached object
} else {
// Object not found in cache
}
In the above code, we retrieve an object from the cache using the key `”myKey”`. If the object is found in the cache, it is assigned to the `cachedObject` variable and we can use it. If the object is not found in the cache, the `cachedObject` variable will be `nil`.
4. Setting limits and eviction policies
`NSCache` provides several properties and methods to control the behavior of the cache. Here are a few important ones:
– `countLimit`: The maximum number of objects the cache can hold. When this limit is reached, older objects may be evicted to make room for new ones.
– `totalCostLimit`: The maximum total cost of the objects in the cache. The cost can be any numerical value you assign to each object, representing its relative “weight” in the cache. When the total cost limit is reached, older objects may be evicted to make room for new ones.
– `evictsObjectsWithDiscardedContent`: A Boolean value that determines whether the cache should evict objects that have been discarded by the system, such as during a memory warning.
Here’s an example of how to set these properties:
myCache.countLimit = 100 myCache.totalCostLimit = 1024 * 1024 // 1MB myCache.evictsObjectsWithDiscardedContent = true
In the above code, we set the `countLimit` to 100, meaning the cache can hold up to 100 objects. We set the `totalCostLimit` to 1MB, meaning the total cost of the objects in the cache should not exceed 1MB. We also enable `evictsObjectsWithDiscardedContent` to automatically evict objects that have been discarded by the system.
5. Handling memory warnings
`NSCache` provides a way to handle memory warnings and automatically evict objects to free up memory. To handle memory warnings, you can implement the `didReceiveMemoryWarning()` method in your view controller or app delegate and call the `removeAllObjects()` method on your cache instance:
func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
myCache.removeAllObjects()
}
In the above code, we call `removeAllObjects()` on our cache instance when a memory warning is received. This clears the cache and frees up memory.
6. Example usage scenarios
Here are a few example scenarios where you can use `NSCache`:
– Caching downloaded images: When you download an image from the internet, you can cache it in `NSCache` so that you don’t have to download it again if it is needed later.
– Caching computed data: If you have expensive computations that generate data, you can cache the computed data in `NSCache` so that you can reuse it instead of recomputing it.
– Caching network responses: If your app makes network requests and receives responses, you can cache the responses in `NSCache` so that you can reuse them instead of making the same request again.
By using `NSCache` in these scenarios, you can improve the performance of your app by reducing the need to recreate or fetch data from expensive sources.
Conclusion
In this tutorial, we learned how to use `NSCache` in Swift. We covered creating an `NSCache` instance, adding and retrieving objects from the cache, setting limits and eviction policies, handling memory warnings, and provided example usage scenarios. By using `NSCache`, you can efficiently cache temporary objects in memory and improve the performance of your app.
I hope you found this tutorial helpful! If you have any further questions, feel free to ask.