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.

Tools/Scripts/Templates/SimpleScreenExample/Tests/Unit/TemplateScreenViewModelTests.swift

@@ -22,13 +22,31 @@ class TemplateScreenViewModelTests: XCTestCase {
     }
 
     func testInitialState() {
-        XCTAssertFalse(context.viewState.placeholder.isEmpty)
         XCTAssertFalse(context.composerText.isEmpty)
+        XCTAssertEqual(context.viewState.counter, 0)
     }
 
-    func testCounter() async throws {
+    func testTextField() async throws {
         context.composerText = "123"
         context.send(viewAction: .textChanged)
         XCTAssertEqual(context.composerText, "123")
     }
+    
+    func testCounter() async throws {
+        var deferred = deferFulfillment(context.observe(\.viewState.counter)) { $0 == 1 }
+        context.send(viewAction: .incrementCounter)
+        try await deferred.fulfill()
+        XCTAssertEqual(context.viewState.counter, 1)
+        
+        deferred = deferFulfillment(context.observe(\.viewState.counter)) { $0 == 3 }
+        context.send(viewAction: .incrementCounter)
+        context.send(viewAction: .incrementCounter)
+        try await deferred.fulfill()
+        XCTAssertEqual(context.viewState.counter, 3)
+        
+        deferred = deferFulfillment(context.observe(\.viewState.counter)) { $0 == 2 }
+        context.send(viewAction: .decrementCounter)
+        try await deferred.fulfill()
+        XCTAssertEqual(context.viewState.counter, 2)
+    }
 }