ios - 从类方法中的 block 返回值

Question

我想在我的辅助类之一中创建一个返回 NSString 的简单方法，但我想不出返回值的正确方法。我在 if 语句中收到此错误。

Variable is not assignable (missing __block type specifier)

+ (NSString *) photoCount {

    NSString *numberOfPhoto = [[NSString alloc] init];

    PFQuery *photoQuery = [PFQuery queryWithClassName:@"PhotoContent"];
    [photoQuery whereKey:@"usr" equalTo:[PFUser currentUser]];
    [photoQuery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {

        if (objects) {

            numberOfPhoto = [NSString stringWithFormat:@"%d", [objects count]];

        }
    }];

    return numberOfPhoto;

}

我做错了什么？我试图直接从 block 中返回字符串，但没有帮助。

Answer 1

你正在调用异步方法，所以你不能立即返回值，而是你想采用异步完成 block 模式:

+ (void) photoCountWithCompletionHandler:(void (^)(NSInteger count, NSError *error))completionHandler {
    NSParameterAssert(completionHandler);

    PFQuery *photoQuery = [PFQuery queryWithClassName:@"PhotoContent"];
    [photoQuery whereKey:@"usr" equalTo:[PFUser currentUser]];
    [photoQuery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
        if (objects) {
            completionHandler([objects count], nil);
        } else {
            completionHandler(-1, error);
        }
    }];
}

然后当你调用它时，它会是这样的:

[MyClass photoCountWithCompletionHandler:^(NSInteger count, NSError *error) {
    if (error) {
        // handle the error here
        NSLog(@"photoCountWithCompletionHandler error: %@", error);
        self.textLabel.text = @"?";
    } else {
        // use `count` here
        self.textLabel.text = [NSString stringWithFormat:@"%ld", (long) count];
    }
}];

// do not use `count` here, as the above block is called later, asynchronously