[feat] Add KVO support to query classes (#339)

Added at the `AbstractQuery` level, so it’s available for both `Query` and `PlacesQuery`.

Author: Clément Le Provost

Source/AbstractQuery.swift

@@ -172,6 +172,10 @@ import Foundation
 ///
 /// + Warning: This class is not meant to be used directly. Please see `Query` or `PlacesQuery` instead.
 ///
+/// ## KVO
+///
+/// Every parameter is observable via KVO under its own name.
+///
 @objc
 open class AbstractQuery : NSObject, NSCopying {
 
@@ -196,11 +200,18 @@ open class AbstractQuery : NSObject, NSCopying {
     /// - parameter value:  The parameter's value, or nill to remove it.
     ///
     @objc public func setParameter(withName name: String, to value: String?) {
+        let oldValue = parameters[name]
+        if value != oldValue {
+            self.willChangeValue(forKey: name)
+        }
         if value == nil {
             parameters.removeValue(forKey: name)
         } else {
             parameters[name] = value!
         }
+        if value != oldValue {
+            self.didChangeValue(forKey: name)
+        }
     }
 
     /// Convenience shortcut to `parameter(withName:)` and `setParameter(withName:to:)`.

Tests/QueryTests.swift

@@ -98,6 +98,60 @@ class QueryTests: XCTestCase {
         query["b"] = nil
         XCTAssertNil(query["b"])
     }
+    
+    // MARK: KVO
+    
+    func testKVO() {
+        let query = Query()
+        query.addObserver(self, forKeyPath: "hitsPerPage", options: [.new, .old], context: nil)
+        query.addObserver(self, forKeyPath: "attributesToRetrieve", options: [.new, .old], context: nil)
+        defer {
+            query.removeObserver(self, forKeyPath: "hitsPerPage")
+            query.removeObserver(self, forKeyPath: "attributesToRetrieve")
+        }
+        query.hitsPerPage = 666
+        query.hitsPerPage = 666 // setting the same value again should not trigger any call
+        query.hitsPerPage = nil
+        query.attributesToRetrieve = ["abc", "xyz"]
+        query.attributesToRetrieve = ["abc", "xyz"] // setting the same value again should not trigger any call
+        query.attributesToRetrieve = nil
+        XCTAssertEqual(4, iteration)
+    }
+
+    /// Tracks the number of calls to `observeValue(forKeyPath:of:change:context:)`.
+    var iteration: Int = 0
+    
+    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
+        iteration += 1
+        switch iteration {
+        case 1:
+            XCTAssertEqual(keyPath, "hitsPerPage")
+            let old = change![.oldKey] as? Int
+            let new = change![.newKey] as? Int
+            XCTAssert(nil == old)
+            XCTAssertEqual(666, new)
+        case 2:
+            XCTAssertEqual(keyPath, "hitsPerPage")
+            let old = change![.oldKey] as? Int
+            let new = change![.newKey] as? Int
+            XCTAssertEqual(666, old)
+            XCTAssert(nil == new)
+        case 3:
+            XCTAssertEqual(keyPath, "attributesToRetrieve")
+            let old = change![.oldKey] as? [String]
+            let new = change![.newKey] as? [String]
+            XCTAssert(nil == old)
+            XCTAssertEqual(["abc", "xyz"], new!)
+        case 4:
+            XCTAssertEqual(keyPath, "attributesToRetrieve")
+            let old = change![.oldKey] as? [String]
+            let new = change![.newKey] as? [String]
+            XCTAssertEqual(["abc", "xyz"], old!)
+            XCTAssert(nil == new)
+        default:
+            XCTFail("Unexpected call to `observeValue(forKeyPath:of:change:context:)`")
+        }
+    }
 
     // MARK: High-level