Support XCTest assertions by forwarding messages to the current example

Allows the use of XCTest assertions in specs. This should include other
matcher libraries implemented in terms of XCTest assertions.

- Within `+buildExampleGroups`, defines a shadow `self` pointer that is
  of the type of the test case instance.
- Unrecognized selectors sent to the test case class are forwarded to
  the current example, and then to the current test case instance.
- Adds a test for the use of XCTest assertions within specs.

Classes/Core/KWExample.h

@@ -14,6 +14,8 @@
 #import "KWReporting.h"
 #import "KWExampleDelegate.h"
 
+#import <XCTest/XCTestCase.h>
+
 @class KWCallSite;
 @class KWExampleSuite;
 @class KWContextNode;
@@ -40,7 +42,7 @@
 
 #pragma mark - Running
 
-- (void)runWithDelegate:(id<KWExampleDelegate>)delegate;
+- (void)runWithDelegate:(XCTestCase<KWExampleDelegate> *)delegate;
 
 #pragma mark - Anonymous It Node Descriptions
 

Classes/Core/KWExample.m

@@ -32,7 +32,7 @@ @interface KWExample ()
 
 @property (nonatomic, readonly) NSMutableArray *verifiers;
 @property (nonatomic, readonly) KWMatcherFactory *matcherFactory;
-@property (nonatomic, weak) id<KWExampleDelegate> delegate;
+@property (nonatomic, weak) XCTestCase<KWExampleDelegate> *delegate;
 @property (nonatomic, assign) BOOL didNotFinish;
 @property (nonatomic, strong) id<KWExampleNode> exampleNode;
 @property (nonatomic, assign) BOOL passed;
@@ -71,6 +71,20 @@ - (NSString *)description {
   return [NSString stringWithFormat:@"<KWExample: %@>", self.exampleNode.description];
 }
 
+#pragma mark - Message forwarding
+
+- (id)forwardingTargetForSelector:(SEL)aSelector {
+    if ([self.delegate respondsToSelector:aSelector]) {
+        return self.delegate;
+    } else {
+        return [super forwardingTargetForSelector:aSelector];
+    }
+}
+
+- (BOOL)respondsToSelector:(SEL)aSelector {
+    return [super respondsToSelector:aSelector] || [self.delegate respondsToSelector:aSelector];
+}
+
 #pragma mark - Adding Verifiers
 
 - (id)addVerifier:(id<KWVerifying>)aVerifier {
@@ -105,7 +119,7 @@ - (id)addAsyncVerifierWithExpectationType:(KWExpectationType)anExpectationType c
 
 #pragma mark - Running examples
 
-- (void)runWithDelegate:(id<KWExampleDelegate>)delegate; {
+- (void)runWithDelegate:(XCTestCase<KWExampleDelegate> *)delegate {
     self.delegate = delegate;
     [self.matcherFactory registerMatcherClassesWithNamespacePrefix:@"KW"];
     [[KWExampleSuiteBuilder sharedExampleSuiteBuilder] setCurrentExample:self];

Classes/Core/KWSpec.m

@@ -81,6 +81,26 @@ + (NSInvocation *)invocationForExample:(KWExample *)example selector:(SEL)select
     return invocation;
 }
 
+#pragma mark - Message forwarding
+
+// Inside `SPEC_BEGIN`/`SPEC_END`, `self` references the spec class object but
+// ultimately needs to instead behave like an _instance_ of `KWSpec`. The current
+// example has a reference to the running test case via its delegate, and so
+// forwarding messages there enables messages sent to `self` from within an
+// example to behave just like the test case instance.
+//
+// This is useful, for example, because the `XCTAssert*` macros require a `self`
+// that is a kind of `XCTestCase`.
++ (id)forwardingTargetForSelector:(SEL)aSelector {
+    KWExample *example = [[KWExampleSuiteBuilder sharedExampleSuiteBuilder] currentExample];
+
+    if ([example respondsToSelector:aSelector]) {
+        return example;
+    } else {
+        return [super forwardingTargetForSelector:aSelector];
+    }
+}
+
 #pragma mark - Running Specs
 
 - (void)runExample {

Classes/Core/KiwiMacros.h

@@ -89,8 +89,14 @@
     + (NSString *)file { return @__FILE__; } \
     \
     + (void)buildExampleGroups { \
+        id _kw_test_case_class = self; \
+        { \
+            /* The shadow `self` must be declared inside a new scope to avoid compiler warnings. */ \
+            /* The receiving class object delegates unrecognized selectors to the current example. */ \
+            __unused name *self = _kw_test_case_class;
 
 #define SPEC_END \
+        } \
     } \
     \
     @end

Tests/KWObjCXCTestAssertionTests.m

@@ -3,7 +3,7 @@
 SPEC_BEGIN(KWObjCXCTestAssertionTests)
 
 describe(@"XCTest assertions in Objective C", ^{
-    pending(@"supports XCTAssert", ^{
+    it(@"supports XCTAssert", ^{
         XCTAssert(1 + 1 == 2);
     });
 });