如何检测用户是否取消了NSSharingServiceNameComposeEmail请求？

Question

使用如下代码：

 -(void)testMail
{
_service = [NSSharingService sharingServiceNamed:NSSharingServiceNameComposeEmail];
_service.recipients = @[@"anEmailAddress"];
_service.subject = @"Test Mail";
NSArray* items = @[@"This is the body"];
_service.delegate = self;
[_service performWithItems:items];
}

如何检测到用户已取消/退出邮件而未发送电子邮件？