Prevent direct test failure during init if the screen is not loaded (#8)

Author: mokagio

Sources/ScreenObject/ScreenObject.swift

@@ -6,6 +6,10 @@ import XCTest
 // and those are obviously specific to each screen, hence should be added by each subclass.
 open class ScreenObject {
 
+    public enum WaitForScreenError: Equatable, Error {
+        case timedOut
+    }
+
     /// The `XCUIApplication` instance this screen is part of. This is the value passed at
     /// initialization time.
     public let app: XCUIApplication
@@ -34,17 +38,31 @@ open class ScreenObject {
 
     @discardableResult
     func waitForScreen() throws -> Self {
-        XCTContext.runActivity(named: "Confirm screen \(self) is loaded") { (activity) in
-            let result = waitFor(element: expectedElement, predicate: "isEnabled == true", timeout: 20)
-            XCTAssert(result, "Screen \(self) is not loaded.")
+        try XCTContext.runActivity(named: "Confirm screen \(self) is loaded") { (activity) in
+            let result = waitFor(
+                element: expectedElement,
+                predicate: "isEnabled == true",
+                timeout: self.waitTimeout
+            )
+
+            guard result == .completed else { throw WaitForScreenError.timedOut }
         }
         return self
     }
 
-    private func waitFor(element: XCUIElement, predicate: String, timeout: Int = 5) -> Bool {
-        let elementPredicate = XCTNSPredicateExpectation(predicate: NSPredicate(format: predicate), object: element)
-        let result = XCTWaiter.wait(for: [elementPredicate], timeout: TimeInterval(timeout))
-
-        return result == .completed
+    private func waitFor(
+        element: XCUIElement,
+        predicate: String,
+        timeout: TimeInterval
+    ) -> XCTWaiter.Result {
+        XCTWaiter.wait(
+            for: [
+                XCTNSPredicateExpectation(
+                    predicate: NSPredicate(format: predicate),
+                    object: element
+                )
+            ],
+            timeout: timeout
+        )
     }
 }

Tests/TestAppUITests/TestAppUITests.swift

@@ -3,17 +3,26 @@ import XCTest
 
 class TestAppUITests: XCTestCase {
 
+    let app = XCUIApplication()
+
     override func setUpWithError() throws {
+        app.launch()
         continueAfterFailure = false
     }
 
-    func testExample() throws {
-        let app = XCUIApplication()
-        app.launch()
-
+    func testIsLoadedReturnsTrueWhenScreenIsLoaded() throws {
         let screen = try HelloWorldScreen()
         XCTAssertTrue(screen.isLoaded)
     }
+
+    func testScreenInitThrowsWhenScreenIsNotLoaded() throws {
+        do {
+            _ = try MissingScreen(app: app)
+            XCTFail("Expected `ScreenObject` `init` to throw, but it didn't")
+        } catch {
+            XCTAssertEqual(error as? ScreenObject.WaitForScreenError, .timedOut)
+        }
+    }
 }
 
 final class HelloWorldScreen: ScreenObject {
@@ -22,3 +31,15 @@ final class HelloWorldScreen: ScreenObject {
         try super.init(expectedElementGetter: { $0.staticTexts["Hello, world!"] })
     }
 }
+
+/// A screen that doesn't exist. Use it to test the init failure behavior.
+class MissingScreen: ScreenObject {
+
+    init(app: XCUIApplication) throws {
+        try super.init(
+            expectedElementGetter: { $0.staticTexts["this screen does not exist"] },
+            app: app,
+            waitTimeout: 1
+        )
+    }
+}