我的应用程序中有以下方法,它使用NSURLSession从 JSON格式的Web服务中提取电影数据:
- (void) downloadMovieData {
//this is just a visual cue to show that processing is being done
[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];
//creating the request
NSURL *url = [NSURL URLWithString:kMovieURL];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
//creating the session
self.config = [NSURLSessionConfiguration defaultSessionConfiguration];
self.session = [NSURLSession sessionWithConfiguration:self.config];
//the object that makes the call to the web service using the request and the session, and returns a response or an error
NSURLSessionDataTask *task = [self.session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
//At this point a response has been received, so we can turn the indicator off
[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
//I am casting the response to an NSHTTPURLResponse so I can check the status code of the response
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
//a status code of 200 means a successful connection with the web service
if (httpResponse.statusCode == 200) {
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(@"Success!");
//I send the data that was received from the response to the method so that the JSON data is extracted
[self populateArray:data];
});
} else {
NSString *result = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSLog(@"Received HTTP %ld: %@", (long)httpResponse.statusCode, result);
}
}];
[task resume];
}
- (void) populateArray: (NSData *)data {
NSError *jsonError;
NSDictionary *response =[NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:&jsonError];
if (response) {
self.movieObjects = response[@"movies"];
NSLog(@"The movie objects are: %@", self.movieObjects);
[self.tableView reloadData];
} else {
NSLog(@"ERROR: %@", jsonError);
}
}
上面的代码工作正常.没有问题.但是,我现在要做的是重构我的代码,以便在包含我的UITableView委托方法的类中包含所有网络代码,我想将代码移动到一个单独的类中,该类使用Singleton方法更好地分离代码我有以下框架代码,如下所示:
#import "Networker.h"
@implementation Networker
+ (NSURLSession *)dataSession {
static NSURLSession *session = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
});
return session;
}
+ (void)fetchContentsOfURL:(NSURL *)url completion:(void (^)(NSData *data, NSError *error)) completionHandler {
NSURLSessionDataTask *dataTask = [[self dataSession] dataTaskWithURL:url
completionHandler: ^(NSData *data, NSURLResponse *response, NSError *error) {
if (completionHandler == nil) return;
if (error) {
completionHandler(nil, error);
return;
}
completionHandler(data, nil);
}];
[dataTask resume];
}
我完全理解Singleton类没有问题.这不是问题.我的问题是了解如何理解此方法的“完成处理程序”部分:
+ (void)fetchContentsOfURL:(NSURL *)url completion:(void (^)(NSData *data, NSError *error)) completionHandler {}
我想做的是将我在方法中的代码“downloadMovieData”移动到“fetchContentsOfURL”中,然后返回一个NSData对象,然后我可以使用它来填充我的调用类中的UITableView.但是,在这样做的过程中,我想确保我理解这个新方法的“completionHandler”部分发生了什么.我怎样才能做到这一点?
最佳答案 你不能移动它以便它返回,因为它是异步的.从技术上讲你可以,但你在等待时阻止主线程这是坏事.
相反,您只需用对单例的调用替换当前代码,在完成块中调用其他方法来处理数据.
有几点需要注意:
>在调用某人完成块之前调出main中的main是个好主意,这样你就不需要函数的所有用户记住这样做了
>它不需要是单例,您可以在每次需要时实例化类的副本或使用依赖注入,这两者通常都提供更好的体系结构