我的应用程序中有以下方法,该方法使用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);
}
}
上面的代码运行良好。没有问题。然而,我现在想做的是重构我的代码,这样我就不想把所有的网络代码都放在包含UITableView
委托方法的类中,而是想把代码移到一个单独的类中。该类使用Singleton方法来更好地分离代码。我有下面的骨架代码,看起来如下:
#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];
}
我完全理解辛格尔顿的课。这不是问题所在。我的问题是理解如何理解这种方法的"完成处理程序"部分:
+ (void)fetchContentsOfURL:(NSURL *)url completion:(void (^)(NSData *data, NSError *error)) completionHandler {}
我想做的是,将方法中的代码"downloadMovieData"移动到"fetchContentsOfURL"中,并返回一个NSData
对象,然后我可以使用它来填充调用类中的UITableView。然而,在这样做的过程中,我想确保我了解这个新方法的"completionHandler"部分发生了什么。我该怎么做?
您不能移动它,所以它会返回,因为它是异步的。从技术上讲,你可以,但你会在等待时阻塞主线程,这很糟糕。
相反,您只需将当前代码替换为对单例的调用,并在完成块中调用其他方法来处理数据。
需要注意的几件事:
- 在singleton中调用某人完成块之前,最好先将其分派到main,这样就不需要该函数的所有用户都记得这样做了
- 它不需要是单例,您可以在每次需要时实例化类的副本,或者使用依赖注入,这两者通常都提供了更好的体系结构