SessionsInitiator triggers new session for every foregrounding after 30 minutes of background (#10360)

* foreground after 30 minutes of background triggers new session

* PR review

* PR comments

* style

* copyright

* fix watchOS

* remove MockDate

* attempt to resolve conflict

* watchOS comment and rename currentTimeProvider

* change OSX to macOS

Author: leotianlizhan

FirebaseSessions/Sources/SessionInitiator.swift

@@ -13,6 +13,12 @@
 // limitations under the License.
 
 import Foundation
+#if os(macOS)
+  import Cocoa
+  import AppKit
+#elseif os(watchOS)
+  import WatchKit
+#endif
 
 ///
 /// The SessionInitiator is responsible for:
@@ -22,8 +28,73 @@ import Foundation
 ///      and comes to the foreground.
 ///
 class SessionInitiator {
+  let sessionTimeout: TimeInterval = 30 * 60 // 30 minutes
+  let currentTime: () -> Date
+  var backgroundTime = Date.distantFuture
+  var initiateSessionStart: () -> Void = {}
+
+  init(currentTimeProvider: @escaping () -> Date = Date.init) {
+    currentTime = currentTimeProvider
+  }
+
   func beginListening(initiateSessionStart: @escaping () -> Void) {
-    // Only cold start is implemented right now
-    initiateSessionStart()
+    self.initiateSessionStart = initiateSessionStart
+    self.initiateSessionStart()
+
+    let notificationCenter = NotificationCenter.default
+    #if os(iOS) || os(tvOS)
+      notificationCenter.addObserver(
+        self,
+        selector: #selector(appBackgrounded),
+        name: UIApplication.didEnterBackgroundNotification,
+        object: nil
+      )
+      notificationCenter.addObserver(
+        self,
+        selector: #selector(appForegrounded),
+        name: UIApplication.didBecomeActiveNotification,
+        object: nil
+      )
+    #elseif os(macOS)
+      notificationCenter.addObserver(
+        self,
+        selector: #selector(appBackgrounded),
+        name: NSApplication.didResignActiveNotification,
+        object: nil
+      )
+      notificationCenter.addObserver(
+        self,
+        selector: #selector(appForegrounded),
+        name: NSApplication.didBecomeActiveNotification,
+        object: nil
+      )
+    #elseif os(watchOS)
+      // Versions below WatchOS 7 do not support lifecycle events
+      if #available(watchOSApplicationExtension 7.0, *) {
+        notificationCenter.addObserver(
+          self,
+          selector: #selector(appBackgrounded),
+          name: WKExtension.applicationDidEnterBackgroundNotification,
+          object: nil
+        )
+        notificationCenter.addObserver(
+          self,
+          selector: #selector(appForegrounded),
+          name: WKExtension.applicationDidBecomeActiveNotification,
+          object: nil
+        )
+      }
+    #endif
+  }
+
+  @objc func appBackgrounded() {
+    backgroundTime = currentTime()
+  }
+
+  @objc func appForegrounded() {
+    let interval = currentTime().timeIntervalSince(backgroundTime)
+    if interval > sessionTimeout {
+      initiateSessionStart()
+    }
   }
 }

FirebaseSessions/Tests/Unit/InitiatorTests.swift

@@ -16,6 +16,9 @@ import XCTest
 @testable import FirebaseSessions
 
 class InitiatorTests: XCTestCase {
+  // 2021-11-01 @ 00:00:00 (EST)
+  let date = Date(timeIntervalSince1970: 1_635_739_200)
+
   func test_beginListening_initiatesColdStart() throws {
     let initiator = SessionInitiator()
     var initiateCalled = false
@@ -24,4 +27,33 @@ class InitiatorTests: XCTestCase {
     }
     assert(initiateCalled)
   }
+
+  func test_appForegrounded_initiatesNewSession() throws {
+    // Given
+    var pausedClock = date
+    let initiator = SessionInitiator(currentTimeProvider: { pausedClock })
+    var sessionCount = 0
+    initiator.beginListening {
+      sessionCount += 1
+    }
+    assert(sessionCount == 1)
+
+    // When
+    // Background, advance time by 30 minutes + 1 second, then foreground
+    initiator.appBackgrounded()
+    pausedClock.addTimeInterval(30 * 60 + 1)
+    initiator.appForegrounded()
+    // Then
+    // Session count increases because time spent in background > 30 minutes
+    assert(sessionCount == 2)
+
+    // When
+    // Background, advance time by exactly 30 minutes, then foreground
+    initiator.appBackgrounded()
+    pausedClock.addTimeInterval(30 * 60)
+    initiator.appForegrounded()
+    // Then
+    // Session count doesn't increase because time spent in background <= 30 minutes
+    assert(sessionCount == 2)
+  }
 }