I can't get data with help of NSURLSession - objective-c

Question

I wrote some test code
-(void)downloadFile
{
[self getFile:^(id result) {
if([result isKindOfClass:[NSData class]])
{
self.myData=result;
}
}];
NSLog(#"%lu",(unsigned long)[self.myData length]);
}
-(void)getFile:(void(^)(id result))completionHandler
{
void(^partCompetionHandle)(NSURL *, NSURLResponse *, NSError *)=^(NSURL *url, NSURLResponse *response, NSError *error){
if(response==nil){
completionHandler(error);
return;
}
NSData *data=[NSData dataWithContentsOfURL:url];
completionHandler(data);
};
self.session=[NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
NSURL *url=[NSURL URLWithString:#"http://upload.wikimedia.org/wikipedia/commons/7/7f/Williams_River-27527.jpg"];
NSURLSessionDownloadTask *dataTask =[_session downloadTaskWithURL:url completionHandler:^(NSURL* data, NSURLResponse * response, NSError * error) {
partCompetionHandle(data, response, error);
}];
[dataTask resume];
}
My datas are zero, when I downloaded theirs
this line shows 0
NSLog(#"%lu",(unsigned long)[self.myData length]);

Answer 1

The classic : As the task is performed asynchronously, the logging of the result is much earlier than the data are downloaded.
Put the log line in the completion block
-(void)downloadFile
{
[self getFile:^(id result) {
if([result isKindOfClass:[NSData class]]) {
self.myData=result;
NSLog(#"%lu",(unsigned long)[self.myData length]);
}
}];
}

Answer 2

I changed code. But I don't see data
-(void)downloadFile
{
[self getFile:^(id result) {
if([result isKindOfClass:[NSData class]])
{
self.myData=result;
NSLog(#"%lu",(unsigned long)[self.myData length]);
}
}];
}
I called method downloadFile from main function
int main(int argc, const char * argv[]) {
#autoreleasepool {
DownloadFile *donload=[DownloadFile new];
[donload downloadFile];
}
return 0;
}
How can I get data?