Implemented the DependencyService service

Sources/Dependencies/Services/DependencyService.swift

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+//
+//  DependencyService.swift
+//  Dependencies
+//
+//  Created by Javier Cicchelli on 11/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+/// This service provide write/read access to the injected dependencies.
+public struct DependencyService {
+    
+    // MARK: Properties
+    
+    private static var current = DependencyService()
+    
+    // MARK: Subscripts
+
+    public static subscript<DK: DependencyKey>(key: DK.Type) -> DK.Value {
+        get { key.currentValue }
+        set { key.currentValue = newValue }
+    }
+    
+    public static subscript<D>(_ keyPath: WritableKeyPath<DependencyService, D>) -> D {
+        get { current[keyPath: keyPath] }
+        set { current[keyPath: keyPath] = newValue }
+    }
+
+}

Tests/Dependencies/Helpers/TestServices.swift

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+//
+//  TestServices.swift
+//  DependenciesTests
+//
+//  Created by Javier Cicchelli on 11/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+import Dependencies
+
+// MARK: - Protocols
+
+protocol TestService {}
+
+// MARK: - Services
+
+struct SomeService: TestService, Equatable {}
+struct SomeOtherService: TestService, Equatable {}
+
+// MARK: - DependencyKey
+
+struct TestServiceKey: DependencyKey {
+    static var currentValue: TestService = SomeService()
+}
+
+// MARK: - DependencyService+Keys
+
+extension DependencyService {
+    var testService: TestService {
+        get { Self[TestServiceKey.self] }
+        set { Self[TestServiceKey.self] = newValue }
+    }
+}
+

Tests/Dependencies/Services/DependencyServiceTests.swift

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+//
+//  DependencyServiceTests.swift
+//  DependenciesTests
+//
+//  Created by Javier Cicchelli on 11/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+import Dependencies
+import XCTest
+
+final class DependencyServiceTests: XCTestCase {
+
+    // MARK: Setup
+    
+    override func setUp() {
+        DependencyService[\.testService] = SomeService()
+    }
+    
+    // MARK: Tests
+
+    func test_readDependencyKey() async throws {
+        // GIVEN
+        // WHEN
+        let service = DependencyService[\.testService]
+        
+        // THEN
+        XCTAssertNotNil(service)
+        XCTAssert(service is SomeService)
+    }
+    
+    func test_writeDependencyKey() async throws {
+        // GIVEN
+        DependencyService[\.testService] = SomeOtherService()
+        
+        // WHEN
+        let service = DependencyService[\.testService]
+        
+        // THEN
+        XCTAssertNotNil(service)
+        XCTAssert(service is SomeOtherService)
+    }
+    
+    func test_writeDependencyKeyTwice() async throws {
+        // GIVEN
+        DependencyService[\.testService] = SomeOtherService()
+        DependencyService[\.testService] = SomeService()
+        
+        // WHEN
+        let service = DependencyService[\.testService]
+        
+        // THEN
+        XCTAssertNotNil(service)
+        XCTAssert(service is SomeService)
+    }
+
+}