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[英]Receiving “Thread 1: signal SIGABRT” after I add a new View Controller. How can I resolve this?
[英]How can I signal one thread to wait until a download is finished and after that pass the data to waiting thread
我有一個單例類從Web下載一些數據。 我從GCD隊列中的其他類'A'調用單例類的下載方法,並開始下載。 同時我也從GCD隊列中的類'B'執行此下載方法。 在這種情況下,我想通知班級'B'等到下載完成。 下載完成后,還將下載數據的副本提供給“B”類。 這里我試圖從兩個A和B類下載相同的文件,否則我的實現沒有問題。 這怎么可能?
這意味着我在不同的線程中調用相同的方法。 那么如何向線程B發出信號,告知線程A中正在進行相同的文件下載,並且在完成時也將數據傳遞給線程B.
- (NSData *)Download:(NSString *)urlString{
// here I am doing all the downloading operations
return data;
}
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
// 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
希望這很清楚
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