Test values should default to preview before live (#30)

If you define a live value and preview value, we should probably default
to the preview value in tests to avoid accidentally hitting the live
implementation. The test will fail when the dependency is accessed
regardless.

Sources/Dependencies/DependencyKey.swift

@@ -163,8 +163,8 @@ extension DependencyKey {
   /// which will take precedence over this implementation.
   public static var previewValue: Value { Self.liveValue }
 
-  /// A default implementation that provides the ``liveValue`` to XCTest runs, but will trigger test
-  /// failure to occur if accessed.
+  /// A default implementation that provides the ``previewValue`` to XCTest runs (or ``liveValue``,
+  /// if no preview value is implemented), but will trigger a test failure when accessed.
   ///
   /// To prevent test failures, explicitly override the dependency in any tests in which it is
   /// accessed:
@@ -183,7 +183,7 @@ extension DependencyKey {
   /// which will take precedence over this implementation.
   public static var testValue: Value {
     guard !DependencyValues.isSetting
-    else { return Self.liveValue }
+    else { return Self.previewValue }
 
     var dependencyDescription = ""
     if let fileID = DependencyValues.currentDependency.fileID,
@@ -228,7 +228,7 @@ extension DependencyKey {
       requirement of the 'DependencyKey' protocol.
       """
     )
-    return Self.liveValue
+    return Self.previewValue
   }
 }
 

Tests/DependenciesTests/DependencyKeyTests.swift

@@ -88,7 +88,7 @@ final class DependencyKeyTests: XCTestCase {
 
     #if DEBUG && !os(Linux)
       XCTExpectFailure {
-        XCTAssertEqual(42, Key.testValue)
+        XCTAssertEqual(1729, Key.testValue)
       } issueMatcher: { issue in
         issue.compactDescription == """
           A dependency has no test implementation, but was accessed from a test context: