Xcode, UIImage is NIL when i trying to implement Load image asynchronously - xcode

Question

I trying to implement Load image asynchronously.
NSURL *url = [NSURL URLWithString:_posterImg];
NSURLRequest* request = [NSURLRequest requestWithURL:url];
[NSURLConnection sendAsynchronousRequest:request
queue:[NSOperationQueue mainQueue]
completionHandler:^(NSURLResponse * response,
NSData * data,
NSError * error) {
if (!error){
UIImage *getImg = [[UIImage alloc] initWithData:data];
// do whatever you want with image
}
}];
But when i put this code, getImg will get warning "Unused Variable". i had check "response", "data" and "error", it all look fine but the getImg is NIL. is that i had write any thing wrong? thanks.

Answer 1

The affected variable is response. Although you use data and error, response is only declared as a parameter but nowhere used in your completion handler!
NSURL *url = [NSURL URLWithString:_posterImg];
NSURLRequest* request = [NSURLRequest requestWithURL:url];
[NSURLConnection sendAsynchronousRequest:request
queue:[NSOperationQueue mainQueue]
completionHandler:^(NSURLResponse * response,
NSData * data,
NSError * error) {
NSString *errorMsg = nil;
UIImage *getImg = nil;
if (!error){
getImg = [[UIImage alloc] initWithData:data];
}
else
{
errorMsg = [NSString stringWithFormat:#"Failed to load image. Error Message: %#", [error localizedString]];
}
[self handleImageRequestWithResponse:response image:getImg andErrorMessage:errorMsg];
}];
// Image hasn't load yet here since the request is asynchronously!
//if(getImg != nil && errorMsg == nil)
// NSLog(#"Image is available!");
//else
// NSLog(#"Loading the image asynchronously failed! %#", errorMsg);
// In addition now provide the following method.
- (void) handleImageRequestWithResponse:(NSURLResponse*)response image:(UIImage*)img andErrorMessage:(NSString*)err
{
if(img!= nil && err == nil)
NSLog(#"Image is available!");
else
NSLog(#"Loading the image asynchronously failed! %#", err);
// Handle image
};
EDIT: My bad! Since the code executes asynchronously getImg was of course nil when you checked as before
EDIT:
Using NSData dataWithContentsOfURL is synchronous,i.e. if executed on the main thread your application is blocked.
See this official documentation: https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSData_Class/Reference/Reference.html#//apple_ref/occ/clm/NSData/dataWithContentsOfURL:
Most important:
Important: Do not use this synchronous method to request network-based URLs. For network-based URLs, this method can block the current thread for tens of seconds on a slow network, resulting in a poor user experience, and in iOS, may cause your app to be terminated.
Going for a completionHandler and a handler method called after the requested raw data has been handled/prepared is better for your performance and does not violate the official recommendation!