Added shared() method to assist in migration. (#240)

* Added shared() method for easier migration

* Added shared() documentation

* fixed test

Author: bsneed

Sources/Segment/Analytics.swift

@@ -26,6 +26,34 @@ public class Analytics {
 
     public var timeline: Timeline
 
+    static internal let deadInstance = "DEADINSTANCE"
+    static internal weak var firstInstance: Analytics? = nil
+    
+    /**
+     This method isn't a traditional singleton implementation.  It's provided here
+     to ease migration from analytics-ios to analytics-swift.  Rather than return a
+     singleton, it returns the first instance of Analytics created, OR an instance
+     who's writekey is "DEADINSTANCE".
+     
+     In the case of a dead instance, an assert will be thrown when in DEBUG builds to
+     assist developers in knowning that `shared()` is being called too soon.
+     */
+    static func shared() -> Analytics {
+        if let a = firstInstance {
+            if a.isDead == false {
+                return a
+            }
+        }
+        
+        #if DEBUG
+        if isUnitTesting == false {
+            assert(true == false, "An instance of Analytice does not exist!")
+        }
+        #endif
+        
+        return Analytics(configuration: Configuration(writeKey: deadInstance))
+    }
+    
     /// Initialize this instance of Analytics with a given configuration setup.
     /// - Parameters:
     ///    - configuration: The configuration to use
@@ -40,6 +68,8 @@ public class Analytics {
 
         storage.analytics = self
 
+        checkSharedInstance()
+        
         // Get everything running
         platformStartup()
     }
@@ -142,8 +172,8 @@ extension Analytics {
         }
     }
 
+    /// Returns a list of currently active flush policies.
     public var flushPolicies: [FlushPolicy] {
-        
         get {
             configuration.values.flushPolicies
         }
@@ -332,3 +362,25 @@ extension Analytics {
         track(name: "Deep Link Opened", properties: jsonProperties)
     }
 }
+
+// MARK: Private Stuff
+
+extension Analytics {
+    private func checkSharedInstance() {
+        // is firstInstance a dead one?  If so, override it.
+        if let firstInstance = Self.firstInstance {
+            if firstInstance.isDead {
+                Self.firstInstance = self
+            }
+        }
+        // is firstInstance nil?  If so, set it.
+        if Self.firstInstance == nil {
+            Self.firstInstance = self
+        }
+    }
+        
+    /// Determines if an instance is dead.
+    internal var isDead: Bool {
+        return configuration.values.writeKey == Self.deadInstance
+    }
+}

Tests/Segment-Tests/Analytics_Tests.swift

@@ -620,4 +620,25 @@ final class Analytics_Tests: XCTestCase {
         XCTAssertTrue(destHit)
 
     }
+    
+    func testSharedInstance() {
+        Analytics.firstInstance = nil
+        
+        let dead = Analytics.shared()
+        XCTAssertTrue(dead.isDead)
+        
+        let alive = Analytics(configuration: Configuration(writeKey: "1234"))
+        XCTAssertFalse(alive.isDead)
+        
+        let shared = Analytics.shared()
+        XCTAssertFalse(shared.isDead)
+        
+        XCTAssertTrue(alive === shared)
+        
+        let alive2 = Analytics(configuration: Configuration(writeKey: "ABCD"))
+        let shared2 = Analytics.shared()
+        XCTAssertFalse(alive2 === shared2)
+        XCTAssertTrue(shared2 === shared)
+        
+    }
 }