最后一个块返回时的触发方法

Question

当我确信两个块代码都已返回时，我如何触发方法？像这样......

// Retrieve Messages Array from Parse
[ParseManager retrieveAllMessagesForShredderUser:(ShredderUser *)[PFUser currentUser] withCompletionBlock:^(BOOL success, NSError *error, NSArray *objects){
        self.messagesArray = objects;
    }];

// Retrieve MessagesPermissions Array from Parse
[ParseManager retrieveAllMessagePermissionsForShredderUser:(ShredderUser *)[PFUser currentUser] withCompletionBlock:^(BOOL success, NSError *error, NSArray *objects){
        self.messagePermissionsArray = objects;
    }];

-(void)methodToRunWhenBothBlocksHaveReturned{
}

Answer 1

如果你可以保证在同一个线程（即UI线程）上执行这些块，那么使用__block变量就可以很简单了。

-(void)yourMethod {
    __block int count = 0;
    [ParseManager retrieveAllMessagesForShredderUser:(ShredderUser *)[PFUser currentUser] withCompletionBlock:^(BOOL success, NSError *error, NSArray *objects){
        self.messagesArray = objects;
        count++;
        if (count == 2) {
             [self methodToRunWhenBothBlocksHaveReturned];
        }
    }];

    [ParseManager retrieveAllMessagePermissionsForShredderUser:(ShredderUser *)[PFUser currentUser] withCompletionBlock:^(BOOL success, NSError *error, NSArray *objects){
        self.messagePermissionsArray = objects;
        count++;
        if (count == 2) {
             [self methodToRunWhenBothBlocksHaveReturned];
        }
    }];
}

-(void)methodToRunWhenBothBlocksHaveReturned{
}

如果你没有相同的线程保证，你可以使用lock来确保变量的增量（和2的比较）是原子的。