ios - iOS从方法返回 block 值

Question

我如何从方法和块外返回变量“myDoub(例如= 65)”？

- (double)executeRequestUrlString:(NSString *)urlString withBlock:(double (^)(double myDoub))block {
    [NSURLConnection sendAsynchronousRequest:request
                                       queue:[NSOperationQueue mainQueue]
                           completionHandler:^(NSURLResponse *response,
                                               NSData *data,
                                               NSError *error) {
                                   if (!error){
                                      //data = 65; for example
                                       block(data);
                                   }
                               }];

    return block(data);  // ???????? This NOT work
}


- (void)viewDidLoad
{
    [super viewDidLoad];

    //
    .......
    //
    double myNewDouble =  [self executeRequestUrlString:fullUrlImage withBlock:^double(double myDoub) {
        return myDoub;
    }];

    // how i can get here variable "myDoub=65" ????

}

Answer 1

根本问题是网络请求正在异步运行(即，要“返回”的值直到方法返回后才完全可用)，因此您不能像概述那样使用它。您唯一可以做到的是使请求同步，这是一个非常糟糕的主意。

相反，您应该采用异步模式。具体来说，不要尝试在double方法之后使用executeRequestUrlString(因为到达该位置时您将不会检索到该值)，而应仅在withBlock块的上下文中使用它。因此，将所有取决于该double的代码放入块内:

- (void)executeRequestUrlString:(NSString *)urlString withBlock:(void (^)(double myDoub))block 
{
    [NSURLConnection sendAsynchronousRequest:request
                                       queue:[NSOperationQueue mainQueue]
                           completionHandler:^(NSURLResponse *response,
                                               NSData *data,
                                               NSError *error) {
                                   if (!error){
                                       double val = ... // extract the double from the `data`, presumably
                                       block(val);
                                   }
                               }];
}    

- (void)viewDidLoad
{
    [super viewDidLoad];

    //
    .......
    //
    [self executeRequestUrlString:fullUrlImage withBlock:^(double myDoub) {
        // put all of your code that requires `myDoub` in here
    }];

    // but because the above is running asynchronously, do not try to use `myDoub` here
}