fix: Reading applicationState in the background

Sources/Sentry/SentryUIApplication.m

@@ -1,10 +1,40 @@
 #import "SentryUIApplication.h"
+#import "SentryDependencyContainer.h"
+#import "SentryNSNotificationCenterWrapper.h"
 
 #if SENTRY_HAS_UIKIT
 
 #    import <UIKit/UIKit.h>
 
-@implementation SentryUIApplication
+@implementation SentryUIApplication {
+    UIApplicationState appState;
+}
+
+- (instancetype)init
+{
+    if (self = [super init]) {
+
+        [SentryDependencyContainer.sharedInstance.notificationCenterWrapper
+            addObserver:self
+               selector:@selector(didEnterBackground)
+                   name:UIApplicationDidEnterBackgroundNotification];
+
+        [SentryDependencyContainer.sharedInstance.notificationCenterWrapper
+            addObserver:self
+               selector:@selector(didBecomeActive)
+                   name:UIApplicationDidBecomeActiveNotification];
+        // We store the application state when the app is initialized
+        // and we keep track of its changes by the notifications
+        // this way we avoid calling sharedApplication in a background thread
+        appState = self.sharedApplication.applicationState;
+    }
+    return self;
+}
+
+- (void)dealloc
+{
+    [SentryDependencyContainer.sharedInstance.notificationCenterWrapper removeObserver:self];
+}
 
 - (UIApplication *)sharedApplication
 {
@@ -58,7 +88,17 @@ - (UIApplication *)sharedApplication
 
 - (UIApplicationState)applicationState
 {
-    return self.sharedApplication.applicationState;
+    return appState;
+}
+
+- (void)didEnterBackground
+{
+    appState = UIApplicationStateBackground;
+}
+
+- (void)didBecomeActive
+{
+    appState = UIApplicationStateActive;
 }
 
 @end

Tests/SentryTests/Helper/TestNSNotificationCenterWrapper.swift

@@ -2,6 +2,9 @@ import Foundation
 import SentryTestUtils
 
 @objcMembers public class TestNSNotificationCenterWrapper: SentryNSNotificationCenterWrapper {
+
+    var ignoreRemoveObserver = false
+
     var addObserverInvocations = Invocations<(observer: Any, selector: Selector, name: NSNotification.Name)>()
     public var addObserverInvocationsCount: Int {
         return addObserverInvocations.count
@@ -24,6 +27,8 @@ import SentryTestUtils
         return removeObserverInvocations.count
     }
     public override func removeObserver(_ observer: Any) {
-        removeObserverInvocations.record(observer)
+        if ignoreRemoveObserver == false {
+            removeObserverInvocations.record(observer)
+        }
     }
 }

Tests/SentryTests/SentryUIApplicationTests.swift

@@ -45,6 +45,32 @@ class SentryUIApplicationTests: XCTestCase {
 
         XCTAssertEqual(sut.windows?.count, 0)
     }
+
+    @available(iOS 13.0, tvOS 13.0, *)
+    func test_ApplicationState() {
+        let notificationCenter = TestNSNotificationCenterWrapper()
+        notificationCenter.ignoreRemoveObserver = true
+        SentryDependencyContainer.sharedInstance().notificationCenterWrapper = notificationCenter
+
+        let sut = MockSentryUIApplicationTests()
+        XCTAssertEqual(sut.applicationState, .active)
+
+        notificationCenter.addObserverInvocations.invocations.forEach { (observer: Any, selector: Selector, name: NSNotification.Name) in
+            if name == UIApplication.didEnterBackgroundNotification {
+                sut.perform(selector, with: observer)
+            }
+        }
+
+        XCTAssertEqual(sut.applicationState, .background)
+
+        notificationCenter.addObserverInvocations.invocations.forEach { (observer: Any, selector: Selector, name: NSNotification.Name) in
+            if name == UIApplication.didBecomeActiveNotification {
+                sut.perform(selector, with: observer)
+            }
+        }
+
+        XCTAssertEqual(sut.applicationState, .active)
+    }
 
     private class TestApplicationDelegate: NSObject, UIApplicationDelegate {
         var window: UIWindow?