// Objective-C
[[self saveAsync:obj] continueWithBlock:^id(BFTask *task) {
    if (task.isCancelled) {
    // the save was cancelled.
    } else if (task.error) {
    // the save failed.
    } else {
    // the object was saved successfully.
    PFObject *object = task.result;
    }
    return nil;
}];
 
// Swift
self.saveAsync(obj).continueWithBlock {
    (task: BFTask!) -> BFTask in
    if task.isCancelled() {
    // the save was cancelled.
    } else if task.error() {
    // the save failed.
    } else {
    // the object was saved successfully.
    var object = task.result() as PFObject
  }
}

  // Objective-C
  //异步获取一个NSString。
  - (BFTask *)getStringAsync {
    //我们假设getNumberAsync返回BFTask，其结果是一个NSNumber。
    return [[self getNumberAsync] continueWithBlock:^id(BFTask *task) {
       //这个延续块返回的NSNumber BFTask作为输入，
       //并提供一个NSString作为输出。
        NSNumber *number = task.result;
        return [NSString stringWithFormat:@"%@", number];
        )];
    }

  // Swift
  //Gets an NSString asynchronously.
  func getStringAsync() -> BFTask {
    //Let's suppose getNumberAsync returns a BFTask whose result is an NSNumber.
    return self.getNumberAsync().continueWithBlock {
        (task: BFTask!) -> NSString in
        // This continuation block takes the NSNumber BFTask as input,
        // and provides an NSString as output.
        let number = task.result() as NSNumber
        return NSString(format:"%@", number)
    }
}

 // Objective-C
[[self saveAsync:obj] continueWithSuccessBlock:^id(BFTask *task) {
    // the object was saved successfully.
    return nil;
}];
 
 // Swift
self.saveAsync(obj).continueWithSuccessBlock {
    (task: BFTask!) -> AnyObject! in
    // the object was saved successfully.
    return nil
}

// Objective-C
PFQuery *query = [PFQuery queryWithClassName:@"Student"];
[query orderByDescending:@"gpa"];
[[[[[self findAsync:query] continueWithSuccessBlock:^id(BFTask *task) {
    NSArray *students = task.result;
    PFObject *valedictorian = [students objectAtIndex:0];
    [valedictorian setObject:@YES forKey:@"valedictorian"];
    return [self saveAsync:valedictorian];
}] continueWithSuccessBlock:^id(BFTask *task) {
    PFObject *valedictorian = task.result;
    return [self findAsync:query];
}] continueWithSuccessBlock:^id(BFTask *task) {
    NSArray *students = task.result;
    PFObject *salutatorian = [students objectAtIndex:1];
    [salutatorian setObject:@YES forKey:@"salutatorian"];
    return [self saveAsync:salutatorian];
}] continueWithSuccessBlock:^id(BFTask *task) {
    // Everything is done!
    return nil;
}];

// Swift
var query = PFQuery(className:"Student")
query.orderByDescending("gpa")
findAsync(query).continueWithSuccessBlock {
    (task: BFTask!) -> BFTask in
    let students = task.result() as NSArray
    var valedictorian = students.objectAtIndex(0) as PFObject
    valedictorian["valedictorian"] = true
    return self.saveAsync(valedictorian)
}.continueWithSuccessBlock {
    (task: BFTask!) -> BFTask in
    var valedictorian = task.result() as PFObject
    return self.findAsync(query)
}.continueWithSuccessBlock {
    (task: BFTask!) -> BFTask in
    let students = task.result() as NSArray
    var salutatorian = students.objectAtIndex(1) as PFObject
    salutatorian["salutatorian"] = true
    return self.saveAsync(salutatorian)
}.continueWithSuccessBlock {
    (task: BFTask!) -> AnyObject! in
    // Everything is done!
    return nil
}
 

// Objective-C
  PFQuery *query = [PFQuery queryWithClassName:@"Student"];
  [query orderByDescending:@"gpa"];
  [[[[[self findAsync:query] continueWithSuccessBlock:^id(BFTask *task) {
      NSArray *students = task.result;
      PFObject *valedictorian = [students objectAtIndex:0];
      [valedictorian setObject:@YES forKey:@"valedictorian"];
      //强制此回调失败。
      return [BFTask taskWithError:[NSError errorWithDomain:@"example.com"
                                                       code:-1
                                                   userInfo:nil]];
  }] continueWithSuccessBlock:^id(BFTask *task) {
      //现在，这个continue将被跳过。
      PFQuery *valedictorian = task.result;
      return [self findAsync:query];
  }] continueWithBlock:^id(BFTask *task) {
      if (task.error) {
          //这个错误处理程序将被调用。
          // 该错误将是上面返回的error.
          //让我们通过返回一个新值处理错误。
          //该任务将以nil作为其值返回。
          return nil;
      }
      //这里也将被跳过。
      NSArray *students = task.result;
      PFObject *salutatorian = [students objectAtIndex:1];
      [salutatorian setObject:@YES forKey:@"salutatorian"];
      return [self saveAsync:salutatorian];
  }] continueWithSuccessBlock:^id(BFTask *task) {
      //一切都完成了！这里被调用。
      //任务的结果是nil
      return nil;
  }];
  
  // Swift
  var query = PFQuery(className:"Student")
  query.orderByDescending("gpa")
  findAsync(query).continueWithSuccessBlock {
      (task: BFTask!) -> BFTask in
      let students = task.result() as NSArray
      var valedictorian = students.objectAtIndex(0) as PFObject
      valedictorian["valedictorian"] = true
      //Force this callback to fail.
      return BFTask(error:NSError(domain:"example.com",
                                  code:-1, userInfo: nil))
  }.continueWithSuccessBlock {
      (task: BFTask!) -> AnyObject! in
      //Now this continuation will be skipped.
      var valedictorian = task.result() as PFObject
      return self.findAsync(query)
  }.continueWithBlock {
      (task: BFTask!) -> AnyObject! in
      if task.error() {
          // This error handler WILL be called.
          // The error will be the NSError returned above.
          // Let's handle the error by returning a new value.
          // The task will be completed with nil as its value.
          return nil
      }
      // This will also be skipped.
      let students = task.result() as NSArray
      var salutatorian = students.objectAtIndex(1) as PFObject
      salutatorian["salutatorian"] = true
      return self.saveAsync(salutatorian)
  }.continueWithSuccessBlock {
      (task: BFTask!) -> AnyObject! in
      // Everything is done! This gets called.
      // The tasks result is nil.
      return nil
  }

 

- (BFTask *) fetchAsync:(PFObject *)object {
       BFTaskCompletionSource *task = [BFTaskCompletionSource taskCompletionSource];
       [object fetchInBackgroundWithBlock:^(PFObject *object, NSError *error) {
           if (!error) {
               [task setResult:object];
           } else {
               [task setError:error];
           }
       }];
       return task.task;
   }
   
   // Swift
   func fetchAsync(object: PFObject) -> BFTask {
       var task = BFTaskCompletionSource()
       object.fetchInBackgroundWithBlock {
           (object: PFObject?, error: NSError?) -> Void in
           if error == nil {
               task.setResult(object)
           } else {
               task.setError(error)
           }
       }
       return task.task
   } 

// Objective-C
  PFQuery *query = [PFQuery queryWithClassName:@"Comments"];
  [query whereKey:@"post" equalTo:@123];
  
  [[[self findAsync:query] continueWithBlock:^id(BFTask *task) {
      NSArray *results = task.result;
      
      / /创建一个简单的完成任务的基本情况。
      BFTask *task = [BFTask taskWithResult:nil];
      for (PFObject *result in results) {
          //对于每个对象，继续任务与函数删除的项目。
          task = [task continueWithBlock:^id(BFTask *task) {
              //返回一个已完成的被标记的任务，当删除完成的时候。
              
              return [self deleteAsync:result];
          }];
      }
      return task;
  }] continueWithBlock:^id(BFTask *task) {
      //每个评论都被删除。
      
      return nil;
  }];
  
  // Swift
  var query = PFQuery(className:"Comments")
  query.whereKey("post", equalTo:123)
  findAsync(query).continueWithBlock {
      (task: BFTask!) -> BFTask in
      let results = task.result() as NSArray
      
      // Create a trivial completed task as a base case.
      let task = BFTask(result:nil)
      for result : PFObject in results {
          // For each item, extend the task with a function to delete the item.
          task = task.continueWithBlock {
              (task: BFTask!) -> BFTask in
              return self.deleteAsync(result)
          }
      }
      return task
  }.continueWithBlock {
      (task: BFTask!) -> AnyObject! in
      // Every comment was deleted.
      return nil
  }
  
  // Swift
  var query = PFQuery(className:"Comments")
  query.whereKey("post", equalTo:123)
  findAsync(query).continueWithBlock {
      (task: BFTask!) -> BFTask in
      let results = task.result() as NSArray
      
      // Create a trivial completed task as a base case.
      let task = BFTask(result:nil)
      for result : PFObject in results {
          // For each item, extend the task with a function to delete the item.
          task = task.continueWithBlock {
              (task: BFTask!) -> BFTask in
              return self.deleteAsync(result)
          }
      }
      return task
  }.continueWithBlock {
      (task: BFTask!) -> AnyObject! in
      // Every comment was deleted.
   }

 // Objective-C
PFQuery *query = [PFQuery queryWithClassName:@"Comments"];
[query whereKey:@"post" equalTo:@123];

[[[self findAsync:query] continueWithBlock:^id(BFTask *results) {
    //收集执行删除任务的Task到一个数组。
    NSMutableArray *tasks = [NSMutableArray array];
    for (PFObject *result in results) {
        //开始立即删除，并添加到任务列表。
        [tasks addObject:[self deleteAsync:result]];
    }
    //返回将被标记为完成时，当所有的删除动作完成时.
    return [BFTask taskForCompletionOfAllTasks:tasks];
}] continueWithBlock:^id(BFTask *task) {
    // 每个评论都被删除
    return nil;
}];

// Swift
var query = PFQuery(className:"Comments")
query.whereKey("post", equalTo:123)

findAsync(query).continueWithBlock {
    (task: BFTask!) -> BFTask in
    // Collect one task for each delete into an array.
    var tasks = NSMutableArray.array()
    var results = task.result() as NSArray
    for result : PFObject! in results {
        // Start this delete immediately and add its task to the list.
        tasks.addObject(self.deleteAsync(result))
    }
    // Return a new task that will be marked as completed when all of the deletes
    // are finished.
    return BFTask(forCompletionOfAllTasks:tasks)
}.continueWithBlock {
    (task: BFTask!) -> AnyObject! in
    // Every comment was deleted.
    return nil
}

//创建一个使用主线程BFExecutor。
   BFExecutor *myExecutor = [BFExecutor executorWithBlock:^void(void(^block)()) {
       dispatch_async(dispatch_get_main_queue(), block);
   }];
   
   //而使用这样的主线程执行程序。执行程序仅适用于新的延续事件，通过continueWithBlock。
   [[self fetchAsync:object] continueWithExecutor:myExecutor withBlock:^id(BFTask *task) {
       myTextView.text = [object objectForKey:@"name"];
   }];
   
   一般的情况下，如在主线程上调用，我们提供BFExecutor的默认实现。这些实现包括defaultExecutor，immediateExecutor，mainThreadExecutor，executorWithDispatchQueue：和executorWithOperationQueue : 例如：
   
   //继续在主线程，采用了内置的执行者。
   [[self fetchAsync:object] continueWithExecutor:[BFExecutor mainThreadExecutor] withBlock:^id(BFTask *task) {
       myTextView.text = [object objectForKey:@"name"];
   }]; 

 - (void)doSomethingComplicatedAsync:(MYCancellationToken *)cancellationToken {
    [[self doSomethingAsync:cancellationToken] continueWithBlock:^{
        if (cancellationToken.isCancelled) {
            return [BFTask cancelledTask];
        }
        // Do something that takes a while.
        return result;
    }];
}

// Somewhere else.
MYCancellationToken *cancellationToken = [[MYCancellationToken alloc] init];
[obj doSomethingComplicatedAsync:cancellationToken];

//When you get bored...
[cancellationToken cancel];

 

let icon: UIImage?
    if let iconName = itemDesc["icon"] as? String {
    icon = UIImage(named: iconName)
} else {
    icon = nil
}

let iconName = itemDesc["icon"] as? String
let icon = UIImage(named: iconName ?? "")

let iconName = itemDesc["icon"] as? String
item.icon = iconName.map { imageName in UIImage(named: imageName) }

let iconName = itemDesc["icon"] as? String
item.icon = iconName.flatMap { imageName in UIImage(named: imageName) }

let iconName = itemDesc["icon"] as? String
item.icon = iconName.flatMap(UIImage.init)

struct ListItem {
var icon: UIImage?
var title: String
var url: NSURL

static func listItemsFromJSONData(jsonData: NSData?) -> [ListItem] {
    guard let jsonData = jsonData,
    let json = try? NSJSONSerialization.JSONObjectWithData(jsonData, options: []),
    let jsonItems = json as? Array<NSDictionary> else { return [] }
    
    return jsonItems.flatMap { (itemDesc: NSDictionary) -> ListItem? in
        guard let title = itemDesc["title"] as? String,
        let urlString = itemDesc["url"] as? String,
        let url = NSURL(string: urlString)
        else { return nil }
        let iconName = itemDesc["icon"] as? String
        let icon = iconName.flatMap { UIImage(named: $0) }
        return ListItem(icon: icon, title: title, url: url)
    }
  }
}