Update the template screen to use the new(ish) Observation framework. (#4077)

* Update the template screen to use the new(ish) Observation framework.

* Add a variant of deferFulfillment that supports observables.

* Update snapshot fulfilment to work with either a publisher or a stream.

ElementX/Sources/Other/Extensions/XCTestCase.swift

@@ -48,6 +48,39 @@ extension XCTestCase {
         }
     }
     
+    /// XCTest utility that assists in observing an async stream, deferring the fulfilment and results until some condition has been met.
+    /// - Parameters:
+    ///   - asyncStream: The stream to wait on.
+    ///   - timeout: A timeout after which we give up.
+    ///   - message: An optional custom expectation message
+    ///   - until: callback that evaluates outputs until some condition is reached
+    /// - Returns: The deferred fulfilment to be executed after some actions and that returns the result of the publisher.
+    func deferFulfillment<Value>(_ asyncStream: AsyncStream<Value>,
+                                 timeout: TimeInterval = 10,
+                                 message: String? = nil,
+                                 until condition: @escaping (Value) -> Bool) -> DeferredFulfillment<Value> {
+        var result: Result<Value, Error>?
+        let expectation = expectation(description: message ?? "Awaiting stream")
+        var hasFulfilled = false
+        
+        let task = Task {
+            for await value in asyncStream {
+                if condition(value), !hasFulfilled {
+                    result = .success(value)
+                    expectation.fulfill()
+                    hasFulfilled = true
+                }
+            }
+        }
+        
+        return DeferredFulfillment<Value> {
+            await self.fulfillment(of: [expectation], timeout: timeout)
+            task.cancel()
+            let unwrappedResult = try XCTUnwrap(result, "Awaited stream did not produce any output")
+            return try unwrappedResult.get()
+        }
+    }
+    
     /// XCTest utility that assists in subscribing to a publisher and deferring the fulfilment and results until some other actions have been performed.
     /// - Parameters:
     ///   - publisher: The publisher to wait on.