fix: Reads UI Windows in the main thread while fetching view hierarchy

CHANGELOG.md

@@ -6,6 +6,10 @@
 
 - Gather profiling timeseries metrics for CPU usage and memory footprint, and thermal and memory pressure events (#2493)
 
+### Fixes
+
+- Always fetch view hierarchy on the main thread (#2629)
+
 ## 8.0.0
 
 ### Features

Sources/Sentry/SentryViewHierarchy.m

@@ -48,11 +48,12 @@ - (BOOL)saveViewHierarchy:(NSString *)filePath
 
 - (NSData *)fetchViewHierarchy
 {
-    NSArray<UIWindow *> *windows = [SentryDependencyContainer.sharedInstance.application windows];
-
     __block NSMutableData *result = [[NSMutableData alloc] init];
 
     void (^save)(void) = ^{
+        NSArray<UIWindow *> *windows =
+            [SentryDependencyContainer.sharedInstance.application windows];
+
         if (![self processViewHierarchy:windows
                             addFunction:writeJSONDataToMemory
                                userData:(__bridge void *)(result)]) {

Tests/SentryTests/SentryViewHierarchyTests.swift

@@ -172,14 +172,37 @@ class SentryViewHierarchyTests: XCTestCase {
         wait(for: [ex], timeout: 1)
     }
 
+    func test_fetch_usesMainThread() {
+        let sut = TestSentryViewHierarchy()
+        let window = UIWindow(frame: CGRect(x: 0, y: 0, width: 10, height: 10))
+        fixture.uiApplication.windows = [window]
+
+        let ex = expectation(description: "Running on background Thread")
+        let dispatch = DispatchQueue(label: "background")
+        dispatch.async {
+            let _ = sut.fetch()
+            ex.fulfill()
+        }
+
+        wait(for: [ex], timeout: 1)
+        XCTAssertTrue(fixture.uiApplication.calledOnMainThread, "fetchViewHierarchy is not using the main thread to get UI windows")
+    }
+
     class TestSentryUIApplication: SentryUIApplication {
         private var _windows: [UIWindow]?
+        private var _calledOnMainThread = true
+
+        var calledOnMainThread: Bool {
+            return _calledOnMainThread
+        }
 
         override var windows: [UIWindow]? {
             get {
+                _calledOnMainThread = Thread.isMainThread
                 return _windows
             }
             set {
+                _calledOnMainThread = Thread.isMainThread
                 _windows = newValue
             }
         }