多个文件串行下载

复习GCD信号量知识
https://www.jianshu.com/p/2d90fdf5de9e
依赖AFNetworking
.h文件

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

typedef void(^downloadpercent)(double percent);
typedef void(^filesavedPath)(NSURL *path);
typedef void(^errorReason)(NSString *error);

@interface xxx : NSObject {
/// 统计遍历到了第几个元素
    __block NSInteger downloadIndex_;
}
- (void)downloadStuffWithArray:(NSArray *)urlArray
             complitionHandler:(downloadpercent)complitionBlock
                 filesavedPath:(filesavedPath)pathBlock
                downloadError:(errorReason)errorBlock;
@end

NS_ASSUME_NONNULL_END

.m

#import "xxx.h"
#import "AFURLSessionManager.h"

@implementation xxx

- (instancetype)init
{
    self = [super init];
    if (self) {
        downloadIndex_ = 0;
    }
    return self;
}


- (void)dealDownload:(filesavedPath _Nonnull)pathBlock
            urlArray:(NSArray * _Nonnull)urlArray
   complitionHandler:(downloadpercent _Nonnull)complitionBlock
       downloadError:(errorReason)errorBlock {
    
    dispatch_group_t group = dispatch_group_create();
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    dispatch_queue_t queue = dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL);
    NSURL* URL = [NSURL URLWithString:urlArray[downloadIndex_]];
    dispatch_group_async(group, queue, ^{
        
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
        AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:configuration];
        NSURLRequest *request = [NSURLRequest requestWithURL:URL];
        
        NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request progress:^(NSProgress * _Nonnull downloadProgress) {
            
//            NSLog(@"%d %@ %f",__LINE__,NSStringFromClass(self.class),downloadProgress.fractionCompleted);
            complitionBlock([NSString stringWithFormat:@"%.2f",downloadProgress.fractionCompleted].doubleValue);

            
            if (downloadProgress.fractionCompleted == 1.0) {
                dispatch_semaphore_signal(semaphore);
                
            }
            
        }
                                                                      destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
            
            NSURL *documentsDirectoryURL = [[NSFileManager defaultManager] URLForDirectory:NSCachesDirectory
                                                                                  inDomain:NSUserDomainMask
                                                                         appropriateForURL:nil
                                                                                    create:NO
                                                                                     error:nil];
            pathBlock([documentsDirectoryURL URLByAppendingPathComponent:[response suggestedFilename]]);
            return [documentsDirectoryURL URLByAppendingPathComponent:[response suggestedFilename]];
        } completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
# if DEBUG
#else
            NSLog(@"%d %@ %ld",__LINE__,NSStringFromClass(self.class),self->downloadIndex_);
#endif

            /// 下载完一个，取下一个元素，继续下载。相当于遍历了一遍下载数组。
            self->downloadIndex_ += 1;
            errorBlock(error.debugDescription);
            if (self->downloadIndex_ == urlArray.count) {
                return;
            }
            [self dealDownload:pathBlock urlArray:urlArray complitionHandler:complitionBlock downloadError:errorBlock];
        }];
        [downloadTask resume];
        
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    });
    
    dispatch_group_notify(group, queue, ^{
        dispatch_async(dispatch_get_main_queue(), ^{
#if DEBUG
            NSLog(@"%d %@ %@",__LINE__,NSStringFromClass(self.class),@"下载完毕");
#else
#endif
        });
    });
}

- (void)downloadStuffWithArray:(NSArray *)urlArray
             complitionHandler:(downloadpercent)complitionBlock
                 filesavedPath:(filesavedPath)pathBlock
                downloadError:(errorReason)errorBlock {
    /// 有下载元素
    if (downloadIndex_ < urlArray.count) {
        
        [self dealDownload:pathBlock urlArray:urlArray complitionHandler:complitionBlock downloadError:errorBlock];
        
    }
    
}

@end

使用：

[xxx.new downloadStuffWithArray:list complitionHandler:^(double percent) {
        NSLog(@"%d %@ %f",__LINE__,NSStringFromClass(self.class),percent);
        } filesavedPath:^(NSURL * _Nonnull path) {
            NSLog(@"%d %@ savedPATH%@",__LINE__,NSStringFromClass(self.class),path);
        } downloadError:^(NSError * _Nonnull error) {
            NSLog(@"%d %@ %@",__LINE__,NSStringFromClass(self.class),error.debugDescription);
        }];