Update README

Author: vadymmarkov

README.md

@@ -18,6 +18,9 @@
   * [Sync APIs](#sync-apis)
   * [Async APIS](#async-apis)
   * [Expiry date](#expiry-date)
+* [Observations](#observations)
+  * [Storage observations](#storage-observations)
+  * [Key observations](#key-observations)
 * [Handling JSON response](#handling-json-response)
 * [What about images?](#what-about-images)
 * [Installation](#installation)
@@ -70,8 +73,8 @@ let diskConfig = DiskConfig(name: "Floppy")
 let memoryConfig = MemoryConfig(expiry: .never, countLimit: 10, totalCostLimit: 10)
 
 let storage = try? Storage(
-  diskConfig: diskConfig, 
-  memoryConfig: memoryConfig, 
+  diskConfig: diskConfig,
+  memoryConfig: memoryConfig,
   transformer: TransformerFactory.forCodable(ofType: User.self) // Storage<User>
 )
 ```
@@ -158,7 +161,7 @@ let diskConfig = DiskConfig(
   // Maximum size of the disk cache storage (in bytes)
   maxSize: 10000,
   // Where to store the disk cache. If nil, it is placed in `cachesDirectory` directory.
-  directory: try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask, 
+  directory: try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask,
     appropriateFor: nil, create: true).appendingPathComponent("MyPreferences"),
   // Data protection is used to store files in an encrypted format on disk and to decrypt them on demand
   protectionType: .complete
@@ -237,7 +240,7 @@ try? storage.setObject(user, forKey: "character")
 
 ### Async APIs
 
-In `async` fashion, you deal with `Result` instead of `try catch` because the result is delivered at a later time, in order to not block the current calling queue. In the completion block, you either have `value` or `error`. 
+In `async` fashion, you deal with `Result` instead of `try catch` because the result is delivered at a later time, in order to not block the current calling queue. In the completion block, you either have `value` or `error`.
 
 You access Async APIs via `storage.async`, it is also thread safe, and you can use Sync and Async APIs in any order you want. All Async functions are constrained by `AsyncStorageAware` protocol.
 
@@ -290,7 +293,7 @@ storage.async.removeExpiredObjects() { result in
 By default, all saved objects have the same expiry as the expiry you specify in `DiskConfig` or `MemoryConfig`. You can overwrite this for a specific object by specifying `expiry` for `setObject`
 
 ```swift
-// Default cexpiry date from configuration will be applied to the item
+// Default expiry date from configuration will be applied to the item
 try? storage.setObject("This is a string", forKey: "string")
 
 // A given expiry date will be applied to the item
@@ -304,6 +307,61 @@ try? storage.setObject(
 storage.removeExpiredObjects()
 ```
 
+## Observations
+
+[Storage](#storage) allows you to observe changes in the cache layer, both on
+a store and a key levels. The API lets you pass any object as an observer,
+while also passing an observation closure. The observation closure will be
+removed automatically when the weekly captured observer has been deallocated.
+
+## Storage observations
+
+```swift
+// Add observer
+let token = storage.addStorageObserver(self) { observer, storage, change
+  switch change {
+  case .add(let key):
+    print("Added \(key)")
+  case .remove(let key):
+    print("Removed \(key)")
+  case .removeAll:
+    print("Removed all")
+  case .removeExpired:
+    print("Removed expired")
+  }
+}
+
+// Remove observer
+token.cancel()
+
+// Remove all observers
+storage.removeAllStorageObservers()
+```
+
+## Key observations
+
+```swift
+let key = "user1"
+
+let token = storage.addObserver(self, forKey: key) { observer, storage, change in
+  switch change {
+  case .edit(let before, let after):
+    print("Changed object for \(key) from \(String(describing: before)) to \(after)")
+  case .remove:
+    print("Removed \(key)")
+  }
+}
+
+// Remove observer by token
+token.cancel()
+
+// Remove observer for key
+storage.removeObserver(forKey: "user1")
+
+// Remove all observers
+storage.removeAllKeyObservers()
+```
+
 ## Handling JSON response
 
 Most of the time, our use case is to fetch some json from backend, display it while saving the json to storage for future uses. If you're using libraries like [Alamofire](https://github.com/Alamofire/Alamofire) or [Malibu](https://github.com/hyperoslo/Malibu), you mostly get json in the form of dictionary, string, or data.
@@ -360,7 +418,7 @@ You also need to add `SwiftHash.framework` in your [copy-frameworks](https://git
 ## Author
 
 - [Hyper](http://hyper.no) made this with ❤️
-- Inline MD5 implementation from [SwiftHash](https://github.com/onmyway133/SwiftHash) 
+- Inline MD5 implementation from [SwiftHash](https://github.com/onmyway133/SwiftHash)
 
 ## Contributing