Refactor User-Agent generation, add documentation and test.

README.md

@@ -388,6 +388,14 @@ Alamofire.request(.GET, "https://httpbin.org/get", headers: headers)
          }
 ```
 
+The default Alamofire `Manager` provides a common set of headers for every request. These include:
+
+* `Accept-Encoding`, which defaults to `gzip;q=1.0, compress;q=0.5`, per [RFC7230](https://tools.ietf.org/html/rfc7230#section-4.2.3).
+* `Accept-Language`, which defaults to up to the top 6 preferred languages on the system, formatted like `en;q=1.0`, per [RFC7231](https://tools.ietf.org/html/rfc7231#section-5.3.5).
+* `User-Agent`, which contains versioning information about the current app. For example: `iOS Example/1.0 (com.alamofire.iOS-Example; build:1; iOS 9.3.0) Alamofire/3.4.2`, per [RFC7231](https://tools.ietf.org/html/rfc7231#section-5.5.3).
+
+Customizing these headers, since they need to be added to every request, should be done by creating a customized `Manager` instance and modifying the `defaultHTTPHeaders` dictionary, as shown in the [Modifying Session Configuration](#modifying-session-configuration) section.
+
 ### Caching
 
 Caching is handled on the system framework level by [`NSURLCache`](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSURLCache_Class/Reference/Reference.html#//apple_ref/occ/cl/NSURLCache).

Source/SessionManager.swift

@@ -64,6 +64,7 @@ open class SessionManager {
         }.joined(separator: ", ")
 
         // User-Agent Header; see https://tools.ietf.org/html/rfc7231#section-5.5.3
+        // Example: `iOS Example/1.0 (com.alamofire.iOS-Example; build:1; iOS 9.3.0) Alamofire/3.4.2`
         let userAgent: String = {
             if let info = Bundle.main.infoDictionary {
                 let executable = info[kCFBundleExecutableKey as String] as? String ?? "Unknown"
@@ -94,7 +95,16 @@ open class SessionManager {
                     return "\(osName) \(versionString)"
                 }()
 
-                return "\(executable)/\(bundle) (\(appVersion)/\(appBuild)); \(osNameVersion))"
+                let alamofireVersion: String = {
+                    guard
+                        let afInfo = Bundle(for: SessionManager.self).infoDictionary,
+                        let build = afInfo["CFBundleShortVersionString"]
+                    else { return "Unknown" }
+
+                    return "Alamofire/\(build)"
+                }()
+
+                return "\(executable)/\(appVersion) (\(bundle); build:\(appBuild); \(osNameVersion)) \(alamofireVersion)"
             }
 
             return "Alamofire"

Tests/SessionManagerTests.swift

@@ -177,6 +177,49 @@ class SessionManagerTestCase: BaseTestCase {
         XCTAssertNil(manager, "manager should be nil")
     }
 
+    // MARK: Tests - Default HTTP Headers
+
+    func testDefaultUserAgentHeader() {
+        // Given, When
+        let userAgent = SessionManager.defaultHTTPHeaders["User-Agent"]
+
+        // Then
+        let osNameVersion: String = {
+            let version = ProcessInfo.processInfo.operatingSystemVersion
+            let versionString = "\(version.majorVersion).\(version.minorVersion).\(version.patchVersion)"
+
+            let osName: String = {
+                #if os(iOS)
+                    return "iOS"
+                #elseif os(watchOS)
+                    return "watchOS"
+                #elseif os(tvOS)
+                    return "tvOS"
+                #elseif os(OSX)
+                    return "OS X"
+                #elseif os(Linux)
+                    return "Linux"
+                #else
+                    return "Unknown"
+                #endif
+            }()
+
+            return "\(osName) \(versionString)"
+        }()
+
+        let alamofireVersion: String = {
+            guard
+                let afInfo = Bundle(for: SessionManager.self).infoDictionary,
+                let build = afInfo["CFBundleShortVersionString"]
+            else { return "Unknown" }
+
+            return "Alamofire/\(build)"
+        }()
+
+        let expectedUserAgent = "Unknown/Unknown (Unknown; build:Unknown; \(osNameVersion)) \(alamofireVersion)"
+        XCTAssertEqual(userAgent, expectedUserAgent)
+    }
+
     // MARK: Tests - Start Requests Immediately
 
     func testSetStartRequestsImmediatelyToFalseAndResumeRequest() {