다운로드가 끝날 때까지 대기 할 스레드를 신호로 알릴 수 있습니까?

Question

웹에서 데이터를 다운로드하는 싱글 톤 클래스가 있습니다. 나는 GCD 대기열 안에서 다른 클래스 'A'로부터 싱글 톤 클래스의 다운로드 메서드를 호출하고 다운로드가 시작된다. 같은 시간에 GCD Queue의 'B'클래스에서이 다운로드 메소드를 실행하고 있습니다. 이 상황에서 클래스 'B'에 다운로드가 완료 될 때까지 기다리라고 알리고 싶습니다. 그리고 다운로드가 완료되면 다운로드 된 데이터의 사본을 클래스 'B'에 제공하십시오. 여기에 두 클래스 A와 B에서 동일한 파일을 다운로드하려고합니다. 그렇지 않으면 구현시 아무런 문제가 없습니다. 그게 어떻게 가능해?

의미 다른 스레드에서 같은 메서드를 호출합니다. 어떻게 내가 같은 파일 다운로드가 스레드 A에서 진행하고 완료되면 B도

- (NSData *)Download:(NSString *)urlString{ 

    // here I am doing all the downloading operations 
    return data; 
} 

Answer 1

Downloader.h

// Signature of a block that is called with the downloaded data from a URL 
// when the download is complete 
typedef (void)(^)(NSData *) DownloadCompletedBlock; 

// Public interface of a class that downloads data from URLs 
// Downloads take place on a private dispatch queue, which 
// downloads URLs one at a time 
// Previously downloaded URLs are cached in a dictionary 
// Every so often the cache should be processed to discard old 
// entries. This will stop the cache from growing too large. 
// Since all downloads happen on the same dispatch queue, 
// accesses to the cache are naturally serialized without the need 
// for a lock 
@interface Downloader : NSObject 

// Download the contents of a URL 
// When the download is complete downloadCompleted will 
// be executed on the callbackQueue to pass the downloaded 
// data as a result 
// This is the method that thread A should call 
- (void)download:(NSString *)URLString 
    calbackQueue:(dispatch_queue_t)callbackQueue 
completionBlock:(DownloadCompletedBlock)downloadCompleted; 

// Download the contents of a URL blocking the thread that calls the 
// method 
- (NSData *)downloadBlocking:(NSString *)URLString; 

@end 

Downloader.m

스레드에 데이터를 전달하는 스레드 B 신호를 보낼 수

// Private implementation interface 
@interface Downloader() 

// The thread/queue on which all downloads take place 
@property (readwrite, atomic) dispatch_queue_t downloadQueue; 
// A cache of previously downloaded URLs 
@property (readwrite, atomic) NSMutableDictionary *cachedDownloads; 

// Download the contents of a URL and cache them 
- (NSData *)downloadAndCacheUrl:(NSString *)URLString; 

@end 

// Implementation 
@implementation Downloader 

// Create the download queue and cache 
- (id)init { 
    self = [super init]; 
    if (self) { 
    downloadQueue = dispatch_queue_create("downloadQueue", NULL); 
    self.cachedDownloads = [NSMutableDictionary dictionary]; 
    } 
    return self; 
} 

// Download the URL aynchronously on the download queue. 
// When the download completes pass the result to the callback queue 
// by calling downloadCompleted on the callback queue 
- (void)download:(NSString *)URLString 
    calbackQueue:(dispatch_queue_t)callbackQueue 
completionBlock:(DownloadCompletedBlock)downloadCompleted { 
    dispatch_async(self.downloadQueue, ^{ 
    NSData *downloadedData = [self downloadAndCacheUrl:URLString]; 
    dispatch_async(callbackQueue, ^{ 
     downloadCompleted(downloadedData); 
    }); 
    }); 
} 

// Download the data blocking the calling thread 
// Use a block variable to store the result 
// Since the downloaded data is immutable, we do not need 
// to worry about synchronizing it or copying it 
// Use dispatch_sync to wait until the result is available 
// If the data is already in the cache because thread A downloaded it 
// then the cached data is used. 
// Since downloads happen serially, there is only ever one download happening 
// at a time so the download will only happen once 
- (NSData *)downloadBlocking:(NSString *)URLString { 
    __block NSData *downloadedData = nil; 
    dispatch_sync(self.downloadQueue, ^{ 
    downloadedData = [self downloadAndCacheUrl:URLString]; 
    }); 
    return downloadedData; 
} 

// Download the content of a URL. If the data has already been 
// downloaded and cached, then use the cached value 
- (NSData *)downloadAndCacheUrl:(NSString *)URLString { 
    NSURL *URL = [NSURL URLWithString:*)URLString]; 
    NSData *downloadedData = [self.cachedDownloads objectForKey:URL]; 
    if (downloadedData) { 
    return downloadedData; 
    } 
    downloadedData = [NSData dataWithContentsOfURL:URL]; 
    if (downloadedData) { 
    [self.cachedDownloads setObject:downloadedData forKey:URL]; 
    } 
} 

@end 

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