func race(fulfilled:) for Objective-C

Sources/fwd.h

@@ -13,6 +13,7 @@ extern NSString * __nonnull const PMKErrorDomain;
 #define PMKOperationFailed 8l
 #define PMKTaskError 9l
 #define PMKJoinError 10l
+#define PMKNoWinnerError 11l
 
 
 #ifdef __cplusplus
@@ -160,6 +161,11 @@ extern AnyPromise * __nonnull dispatch_promise_on(dispatch_queue_t __nonnull que
 */
 extern AnyPromise * __nonnull PMKRace(NSArray * __nonnull promises) NS_REFINED_FOR_SWIFT;
 
+/**
+ Returns a new promise that resolves with the value of the first fulfilled promise in the provided array of promises.
+*/
+extern AnyPromise * __nonnull PMKRaceFulfilled(NSArray * __nonnull promises) NS_REFINED_FOR_SWIFT;
+
 #ifdef __cplusplus
 }   // Extern C
 #endif

Sources/race.m

@@ -1,4 +1,9 @@
 #import "AnyPromise+Private.h"
+#import <libkern/OSAtomic.h>
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+// ^^ OSAtomicDecrement32 is deprecated on watchOS
 
 AnyPromise *PMKRace(NSArray *promises) {
     return [AnyPromise promiseWithResolverBlock:^(PMKResolver resolve) {
@@ -7,3 +12,31 @@
         }
     }];
 }
+
+/**
+ Waits for one promise to fulfill
+
+ @note If there are no fulfilled promises, the returned promise is rejected with `PMKNoWinnerError`.
+ @param promises The promises to fulfill.
+ @return The promise that was fulfilled first.
+*/
+AnyPromise *PMKRaceFulfilled(NSArray *promises) {
+    __block int32_t countdown = (int32_t)[promises count];
+
+    return [AnyPromise promiseWithResolverBlock:^(PMKResolver resolve) {
+        for (__strong AnyPromise* promise in promises) {
+            [promise __pipe:^(id value) {
+                if (IsError(value)) {
+                    if (OSAtomicDecrement32(&countdown) == 0) {
+                        id err = [NSError errorWithDomain:PMKErrorDomain code:PMKNoWinnerError userInfo:@{NSLocalizedDescriptionKey: @"PMKRaceFulfilled(nil)"}];
+                        resolve(err);
+                    }
+                } else {
+                    resolve(value);
+                }
+            }];
+        }
+    }];
+}
+
+#pragma GCC diagnostic pop

Tests/CoreObjC/AnyPromiseTests.m

@@ -799,6 +799,44 @@ - (void)test_race {
     [self waitForExpectationsWithTimeout:1 handler:nil];
 }
 
+- (void)test_race_fullfilled {
+    id ex = [self expectationWithDescription:@""];
+    NSArray* promises = @[
+        PMKAfter(1).then(^{ return dummyWithCode(1); }),
+        PMKAfter(2).then(^{ return dummyWithCode(2); }),
+        PMKAfter(5).then(^{ return @1; }),
+        PMKAfter(4).then(^{ return @2; }),
+        PMKAfter(3).then(^{ return dummyWithCode(3); })
+    ];
+    PMKRaceFulfilled(promises).then(^(id obj){
+        XCTAssertEqual(2, [obj integerValue]);
+        [ex fulfill];
+    }).catch(^{
+        XCTFail();
+        [ex fulfill];
+    });
+    [self waitForExpectationsWithTimeout:10 handler:nil];
+}
+
+- (void)test_race_fullfilled_with_no_winner {
+    id ex = [self expectationWithDescription:@""];
+    NSArray* promises = @[
+        PMKAfter(1).then(^{ return dummyWithCode(1); }),
+        PMKAfter(2).then(^{ return dummyWithCode(2); }),
+        PMKAfter(3).then(^{ return dummyWithCode(3); })
+    ];
+    PMKRaceFulfilled(promises).then(^(id obj){
+        XCTFail();
+        [ex fulfill];
+    }).catch(^(NSError *e){
+        XCTAssertEqual(e.domain, PMKErrorDomain);
+        XCTAssertEqual(e.code, PMKNoWinnerError);
+        XCTAssertEqualObjects(e.userInfo[NSLocalizedDescriptionKey], @"PMKRaceFulfilled(nil)");
+        [ex fulfill];
+    });
+    [self waitForExpectationsWithTimeout:10 handler:nil];
+}
+
 - (void)testInBackground {
     id ex = [self expectationWithDescription:@""];
     PMKAfter(0.1).thenInBackground(^{ [ex fulfill]; });