Present calls using window manager.

Signal/src/call/CallService.swift

@@ -1687,11 +1687,9 @@ protocol CallServiceObserver: class {
             return
         }
 
-        let frontmostViewController = UIApplication.shared.frontmostViewControllerIgnoringAlerts
+        if !OWSWindowManager.shared().hasCall() {
-
-        guard nil != frontmostViewController as? CallViewController else {
             OWSProdError(OWSAnalyticsEvents.callServiceCallViewCouldNotPresent(), file: #file, function: #function, line: #line)
-            owsFail("\(self.logTag) in \(#function) Call terminated due to call view presentation delay: \(frontmostViewController.debugDescription).")
+            owsFail("\(self.logTag) in \(#function) Call terminated due to missing call view.")
             self.handleFailedCall(failedCall: call, error: CallError.assertionError(description: "Call view didn't present after \(kMaxViewPresentationDelay) seconds"))
             return
         }

Signal/src/util/OWSWindowManager.h

@@ -30,6 +30,7 @@ extern const UIWindowLevel UIWindowLevel_Background;
 - (void)endCall:(UIViewController *)callViewController;
 - (void)leaveCallView;
 - (void)returnToCallView;
+- (BOOL)hasCall;
 
 @end
 

Signal/src/util/OWSWindowManager.m

@@ -160,13 +160,10 @@ const int kReturnToCallWindowHeight = 40.f;
     window.hidden = YES;
     window.windowLevel = UIWindowLevel_CallView();
     window.opaque = YES;
-    // TODO:
+    window.backgroundColor = [UIColor ows_materialBlueColor];
-    window.backgroundColor = UIColor.ows_materialBlueColor;
-    window.backgroundColor = [UIColor yellowColor];
 
     UIViewController *viewController = [OWSWindowRootViewController new];
-    viewController.view.backgroundColor = UIColor.ows_materialBlueColor;
+    viewController.view.backgroundColor = [UIColor ows_materialBlueColor];
-    viewController.view.backgroundColor = [UIColor yellowColor];
 
     UINavigationController *navigationController =
         [[UINavigationController alloc] initWithRootViewController:viewController];
@@ -196,7 +193,9 @@ const int kReturnToCallWindowHeight = 40.f;
 
     self.callViewController = callViewController;
     // Attach callViewController from window.
-    [self.callViewWindow.rootViewController.navigationController pushViewController:callViewController animated:NO];
+    OWSAssert([self.callViewWindow.rootViewController isKindOfClass:[UINavigationController class]]);
+    UINavigationController *navigationController = (UINavigationController *)self.callViewWindow.rootViewController;
+    [navigationController pushViewController:callViewController animated:NO];
     self.isCallViewActive = YES;
 
     [self ensureWindowState];
@@ -214,7 +213,9 @@ const int kReturnToCallWindowHeight = 40.f;
     }
 
     // Dettach callViewController from window.
-    [self.callViewWindow.rootViewController.navigationController popToRootViewControllerAnimated:NO];
+    OWSAssert([self.callViewWindow.rootViewController isKindOfClass:[UINavigationController class]]);
+    UINavigationController *navigationController = (UINavigationController *)self.callViewWindow.rootViewController;
+    [navigationController popToRootViewControllerAnimated:NO];
     self.callViewController = nil;
     self.isCallViewActive = NO;
 
@@ -243,6 +244,13 @@ const int kReturnToCallWindowHeight = 40.f;
     [self ensureWindowState];
 }
 
+- (BOOL)hasCall
+{
+    OWSAssertIsOnMainThread();
+
+    return self.callViewController != nil;
+}
+
 #pragma mark - Window State
 
 - (void)ensureWindowState