Update tracking branch

ObjectiveGit/GTBranch.h

@@ -79,6 +79,15 @@ typedef NS_ENUM(NSInteger, GTBranchType) {
 /// found, returns nil and sets `success` to YES.
 - (GTBranch *)trackingBranchWithError:(NSError **)error success:(BOOL *)success;
 
+/// Update the tracking branch.
+///
+/// trackingBranch - The tracking branch for the receiver. If nil, it unsets the
+///                  tracking branch.
+/// error          - The error if one occurred.
+///
+/// Returns whether it was successful.
+- (BOOL)updateTrackingBranch:(GTBranch *)trackingBranch error:(NSError **)error;
+
 /// Reloads the branch's reference and creates a new branch based off that newly
 /// loaded reference.
 ///

ObjectiveGit/GTBranch.m

@@ -205,6 +205,16 @@ - (GTBranch *)trackingBranchWithError:(NSError **)error success:(BOOL *)success
 	return [[self class] branchWithReference:[[GTReference alloc] initWithGitReference:trackingRef repository:self.repository] repository:self.repository];
 }
 
+- (BOOL)updateTrackingBranch:(GTBranch *)trackingBranch error:(NSError **)error {
+	int result = git_branch_set_upstream(self.reference.git_reference, trackingBranch.shortName.UTF8String);
+	if (result != GIT_OK) {
+		if (error != NULL) *error = [NSError git_errorFor:result description:@"Failed to update tracking branch for %@", self];
+		return NO;
+	}
+
+	return YES;
+}
+
 - (GTBranch *)reloadedBranchWithError:(NSError **)error {
 	GTReference *reloadedRef = [self.reference reloadedReferenceWithError:error];
 	if (reloadedRef == nil) return nil;

ObjectiveGitTests/GTBranchSpec.m

@@ -189,6 +189,47 @@
 	});
 });
 
+describe(@"-updateTrackingBranch:error:", ^{
+	__block GTBranch *masterBranch;
+	beforeEach(^{
+		masterBranch = [repository lookUpBranchWithName:@"master" type:GTBranchTypeLocal success:NULL error:NULL];
+		expect(masterBranch).notTo(beNil());
+	});
+
+	it(@"should set a tracking branch", ^{
+		GTBranch *branch = [repository lookUpBranchWithName:@"feature" type:GTBranchTypeLocal success:NULL error:NULL];
+		expect(branch).notTo(beNil());
+
+		BOOL success = NO;
+		GTBranch *trackingBranch = [branch trackingBranchWithError:NULL success:&success];
+		expect(trackingBranch).to(beNil());
+		expect(@(success)).to(beTruthy());
+
+		NSError *error;
+		success = [branch updateTrackingBranch:masterBranch error:&error];
+		expect(@(success)).to(beTruthy());
+		expect(error).to(beNil());
+
+		trackingBranch = [branch trackingBranchWithError:NULL success:&success];
+		expect(trackingBranch).notTo(beNil());
+		expect(@(success)).to(beTruthy());
+	});
+
+	it(@"should unset a tracking branch", ^{
+		BOOL success = NO;
+		GTBranch *trackingBranch = [masterBranch trackingBranchWithError:NULL success:&success];
+		expect(trackingBranch).notTo(beNil());
+		expect(@(success)).to(beTruthy());
+
+		success = [masterBranch updateTrackingBranch:nil error:NULL];
+		expect(@(success)).to(beTruthy());
+
+		trackingBranch = [masterBranch trackingBranchWithError:NULL success:&success];
+		expect(trackingBranch).to(beNil());
+		expect(@(success)).to(beTruthy());
+	});
+});
+
 // TODO: Test branch renaming, branch upstream
 //- (void)testCanRenameBranch {
 //