ios - 返回 NSString 时如何解决不兼容的 block 指针类型编译器错误？

Question

我试图用这个方法返回一个变量 fifeScore，但我一直收到错误:

Incompatible block pointer types sending 'NSString *(^)(FIRDataSnapshot *__strong)' to parameter of type 'void (^ _Nonnull)(FIRDataSnapshot * _Nonnull __strong)'

+(NSString *)getFifeScoreForUserWithUID:(NSString *)uid {

    FIRDatabaseReference *scoreRef = [[[[FIRDatabase database] reference] child:@"Scores"] child:uid];
    [scoreRef observeSingleEventOfType:FIRDataEventTypeValue withBlock:^(FIRDataSnapshot *snapshot) {

        NSString * fifeScore;

        if(snapshot.exists){
            NSString * tempScore = [snapshot.value objectForKey:@"fifeScore"];
            fifeScore = [NSString stringWithFormat:@"%@",tempScore];
        } else {
            fifeScore = @"0";
        }
        return fifeScore;
    }];

}

当我注释掉 return 语句时，错误消失了。这是我第一次尝试编写返回字符串的方法。这里有什么问题，我该如何解决错误？

Answer 1

在这种情况下你应该使用 block 。

+(void)getFifeScoreForUserWithUID:(NSString *)uid completionBlock:(void(^)(NSString *score))completion {

  FIRDatabaseReference *scoreRef = [[[[FIRDatabase database] reference] child:@"Scores"] child:uid];
  [scoreRef observeSingleEventOfType:FIRDataEventTypeValue withBlock:^(FIRDataSnapshot *snapshot) {

    NSString * fifeScore;

    if(snapshot.exists){
      NSString * tempScore = [snapshot.value objectForKey:@"fifeScore"];
      fifeScore = [NSString stringWithFormat:@"%@",tempScore];
    } else {
      fifeScore = @"0";
    }

    if (completion) {
      completion(fifeScore)
    }
  }];
}

用法:

[YOUR_CLASS getFifeScoreForUserWithUID:YOUR_UID completionBlock:^(NSString *score) {
  NSLog(@"%@", score);
}];