iOS从方法返回块值

Question

How I can return variable "myDoub(for example=65)" out of method and out block ??

- (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

The fundamental issue is that the network request is running asynchronously (ie the value you want to "return" will simply not be available until well after the method returns), so you cannot use it like you've outlined. The only way you could do that is to make the request synchronous, which is a very bad idea.

Instead, you should embrace the asynchronous pattern. Specifically don't try to use the double after executeRequestUrlString method (because you won't have retrieved the value by the time you get there), but rather use it solely within the context of the withBlock block. So, put all the code contingent on that double inside the block:

- (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
}

Answer 2

You are sending an asynchronous request. That means that your block will be executed at some unknown time in the future, when the calling function is long gone. So the block cannot return anything to the function that is being called. Consider that a URL request could take long time, say a minute if you are out of luck. And consider that any URL request can go wrong and not succeed at all.

Instead, your block needs to deliver any data that it receives (at some point in the future) to whoever wants to process the data. Typically, a block will call something like

dispatch_async (dispatch_get_main_queue, ^ () {
    // Here you can do things in a thread-safe way. 
})); 

It looks like the method you are trying to create just cannot work.

Answer 3

if you want to access a variable outside a block you need to add __block prefix

__block double myDouble

Apple documentation about variables in blocks