Add mocked client + subclass of XCTestCase

Sources/XCTVapor/TestDoubles/Spies/SpyClient.swift

@@ -0,0 +1,42 @@
+import Foundation
+
+public final class SpyClient: Client {
+    private(set) var requestsUsed: [ClientRequest] = []
+
+    private var stubResponse: ClientResponse = .init(status: .ok)
+
+    public var eventLoop: EventLoop
+
+    public init(
+        eventLoop: EventLoop
+    ) {
+        self.eventLoop = eventLoop
+    }
+
+    // Conforming to the protocol: Client
+    public func delegating(to eventLoop: EventLoop) -> Client {
+        self.eventLoop = eventLoop
+
+        return self
+    }
+
+    // Conforming to the protocol: Client
+    public func send(_ request: ClientRequest) -> EventLoopFuture<ClientResponse> {
+        self.requestsUsed.append(request)
+
+        return self.eventLoop.future(self.stubResponse)
+    }
+
+    /// To be able to stub the response of a request made with the client.
+    /// - Parameters:
+    ///   - httpStatus: Provide HTTP status, you want the response to have
+    ///   - responseData: Provide response data, you want the response to have
+    public func stubResponse(httpStatus: HTTPStatus, responseData: (any Content)? = nil) throws {
+        self.stubResponse = .init(status: httpStatus)
+
+        if let responseData = responseData {
+            try self.stubResponse.content.encode(responseData, as: .json)
+        }
+    }
+}
+

Tests/VaporTests/ApplicationTests.swift

@@ -104,16 +104,31 @@ final class ApplicationTests: XCTestCase {
     func testBoilerplate() throws {
         let app = Application(.testing)
         defer { app.shutdown() }
-
-        app.get("hello") { req in
-            "Hello, world!"
+
+        let client = SpyClient(
+            eventLoop: app.eventLoopGroup.next()
+        )
+
+        app.clients.use { _ in
+            return client
+        }
+        struct Greeting: Content {
+            let message: String
         }
+
+        let greeting: Greeting = .init(message: "Hello, world!")
+        try client.stubResponse(httpStatus: .ok, responseData: greeting)
 
         app.environment.arguments = ["serve"]
         try app.start()
+
+        app.get("hello") { req in
+            "Hello, world!"
+        }
 
-        let res = try app.client.get("http://localhost:8080/hello").wait()
-        XCTAssertEqual(res.body?.string, "Hello, world!")
+        let response = try app.client.get("http://localhost:8080/hello").wait()
+        let message = try response.content.decode(Greeting.self).message
+        XCTAssertEqual(message, "Hello, world!")
     }
 
     func testAutomaticPortPickingWorks() {