Updates `flatMapFirst` and adds it to `Driver` unit.

RxCocoa/Common/CocoaUnits/Driver/Driver+Operators.swift

@@ -84,6 +84,25 @@ extension DriverConvertibleType {
     }
 }
 
+extension DriverConvertibleType {
+
+    /**
+     Projects each element of an observable sequence to an observable sequence and merges the resulting observable sequences into one observable sequence.
+     If element is received while there is some projected observable sequence being merged it will simply be ignored.
+
+     - parameter selector: A transform function to apply to element that was observed while no observable is executing in parallel.
+     - returns: An observable sequence whose elements are the result of invoking the one-to-many transform function on each element of the input sequence that was received while no other sequence was being calculated.
+     */
+    @warn_unused_result(message="http://git.io/rxs.uo")
+    public func flatMapFirst<R>(selector: (E) -> Driver<R>)
+        -> Driver<R> {
+        let source: Observable<R> = self
+            .asObservable()
+            .flatMapFirst(selector)
+        return Driver<R>(source)
+    }
+}
+
 extension DriverConvertibleType {
     
     /**

RxSwift/Observables/Observable+StandardSequenceOperators.swift

@@ -206,10 +206,11 @@ extension ObservableType {
 extension ObservableType {
 
     /**
-    Projects each element of an observable sequence to an observable sequence and merges the resulting observable sequences into one observable sequence.
-    
-    - parameter selector: A transform function to apply to each element.
-    - returns: An observable sequence whose elements are the result of invoking the one-to-many transform function on each element of the input sequence.
+     Projects each element of an observable sequence to an observable sequence and merges the resulting observable sequences into one observable sequence.
+     If element is received while there is some projected observable sequence being merged it will simply be ignored.
+
+    - parameter selector: A transform function to apply to element that was observed while no observable is executing in parallel.
+    - returns: An observable sequence whose elements are the result of invoking the one-to-many transform function on each element of the input sequence that was received while no other sequence was being calculated.
     */
     @warn_unused_result(message="http://git.io/rxs.uo")
     public func flatMapFirst<O: ObservableConvertibleType>(selector: (E) throws -> O)

RxTests/RxSwiftTests/Tests/Driver+Test.swift

@@ -278,6 +278,50 @@ extension DriverTest {
     }
 }
 
+// MARK: flatMapFirst
+extension DriverTest {
+    func testAsDriver_flatMapFirst() {
+        let hotObservable = BackgroundThreadPrimitiveHotObservable<Int>()
+        let hotObservable1 = MainThreadPrimitiveHotObservable<Int>()
+        let hotObservable2 = MainThreadPrimitiveHotObservable<Int>()
+        let errorHotObservable = MainThreadPrimitiveHotObservable<Int>()
+
+        let drivers: [Driver<Int>] = [
+            hotObservable1.asDriver(onErrorJustReturn: -2),
+            hotObservable2.asDriver(onErrorJustReturn: -3),
+            errorHotObservable.asDriver(onErrorJustReturn: -4),
+        ]
+
+        let driver = hotObservable.asDriver(onErrorJustReturn: 2).flatMapFirst { drivers[$0] }
+
+        let results = subscribeTwiceOnBackgroundSchedulerAndOnlyOneSubscription(driver) {
+            XCTAssertTrue(hotObservable.subscriptions == [SubscribedToHotObservable])
+
+            hotObservable.on(.Next(0))
+            hotObservable.on(.Next(1))
+
+            hotObservable1.on(.Next(1))
+            hotObservable1.on(.Next(2))
+            hotObservable1.on(.Error(testError))
+
+            hotObservable2.on(.Next(10))
+            hotObservable2.on(.Next(11))
+            hotObservable2.on(.Error(testError))
+
+            hotObservable.on(.Error(testError))
+
+            errorHotObservable.on(.Completed)
+            hotObservable.on(.Completed)
+
+            XCTAssertTrue(hotObservable.subscriptions == [UnsunscribedFromHotObservable])
+        }
+
+        XCTAssertEqual(results, [
+            1, 2, -2,
+            ])
+    }
+}
+
 // MARK: doOn
 extension DriverTest {
     func testAsDriver_doOn() {