[iOS 多线程 & 网络

A.需求

1. 边下边写入硬盘
2. 显示下载进度
3. 暂停/恢复 下载
4. 解压文件
5. 多线程下载

 

B.基本知识

1.小文件下载

如果文件比较小，下载方式会比较多
直接用NSData的+ (id)dataWithContentsOfURL:(NSURL *)url;
利用NSURLConnection发送一个HTTP请求去下载
如果是下载图片，还可以利用SDWebImage框架

 

2.HTTP的Range头信息

通过设置请求头Range可以指定每次从网路下载数据包的大小
Range示例
bytes=0-499 从0到499的头500个字节
bytes=500-999 从500到999的第二个500字节
bytes=500- 从500字节以后的所有字节

bytes=-500 最后500个字节
bytes=500-599,800-899 同时指定几个范围
Range小结
- 用于分隔
前面的数字表示起始字节数
后面的数组表示截止字节数，没有表示到末尾
, 用于分组，可以一次指定多个Range，不过很少用

 

3.第三方解压框架SSZipArchive

下载地址：https://github.com/samsoffes/ssziparchive
注意：需要引入libz.dylib框架 

// Unzipping
NSString *zipPath = @"path_to_your_zip_file";
NSString *destinationPath = @"path_to_the_folder_where_you_want_it_unzipped";
[SSZipArchive unzipFileAtPath:zipPath toDestination:destinationPath];

// Zipping
NSString *zippedPath = @"path_where_you_want_the_file_created";
NSArray *inputPaths = [NSArray arrayWithObjects:
                       [[NSBundle mainBundle] pathForResource:@"photo1" ofType:@"jpg"],
                       [[NSBundle mainBundle] pathForResource:@"photo2" ofType:@"jpg"]
                       nil];
[SSZipArchive createZipFileAtPath:zippedPath withFilesAtPaths:inputPaths];

 

 

 

C.实现

1.使用NSFileHandle实现边下边写入硬盘

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    NSLog(@"开始接收");
   
    // 获取存放路径
    NSString *cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    NSString *filePath = [cachePath stringByAppendingPathComponent:@"0627.zip"];
   
    // 创建一个空的文件，用来存放接收的数据
    NSFileManager *manager = [NSFileManager defaultManager];
    [manager createFileAtPath:filePath contents:nil attributes:nil];
   
    // 把文件句柄用写入方式指向文件
    self.handle = [NSFileHandle fileHandleForWritingAtPath:filePath];
   
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    NSLog(@"正在接收: %d byte", data.length);
   
    // 移动句柄到上次写入数据的末位置
    [self.handle seekToEndOfFile];
   
    // 写入数据
    [self.handle writeData:data];
}

 

2.显示下载进度条

使用ProgressView

从response得到文件大小，再累加每次接收到的数据，计算出完成百分比

 

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    NSLog(@"开始接收");
   
    // 获取存放路径
    NSString *cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    NSString *filePath = [cachePath stringByAppendingPathComponent:@"0627.zip"];
   
    // 创建一个空的文件，用来存放接收的数据
    NSFileManager *manager = [NSFileManager defaultManager];
    [manager createFileAtPath:filePath contents:nil attributes:nil];
   
    // 把文件句柄用写入方式指向文件
    self.handle = [NSFileHandle fileHandleForWritingAtPath:filePath];
   
    // 复位进度条
    self.progressView.progress = 0.0;
   
    // 复位当前下载量
    self.currentDataLength = 0;
   
    // 得到总的文件大小
    self.totalDataLength = response.expectedContentLength;
   
   
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    NSLog(@"正在接收: %d byte", data.length);
    // 移动句柄到上次写入数据的末位置
    [self.handle seekToEndOfFile];
   
    // 写入数据
    [self.handle writeData:data];
   
    // 下载量累加
    self.currentDataLength += data.length;
   
    // 刷新进度条
    CGFloat completedProgress = (double)self.currentDataLength / self.totalDataLength;
    NSLog(@"completed percentage: %f%%", completedProgress * 100);
    self.progressView.progress = completedProgress;
}

 

3.暂停/恢复下载

使用成员属性标记下载状态

请求得到文件头信息，根据当前下载量，设置请求request头信息，从上次下载完成的地方开始继续下载

注意：NSURLConnection 取消cancel之后不能恢复，只能再创建

//
//  BigFileDownloadViewController.m
//  BigFileDownloadDemo
//
//  Created by hellovoidworld on 15/1/26.
//  Copyright (c) 2015年 hellovoidworld. All rights reserved.
//

#import "BigFileDownloadViewController.h"

@interface BigFileDownloadViewController ()

/** 进度条 */
@property (weak, nonatomic) IBOutlet UIProgressView *progressView;

/** 文件句柄 */
@property(nonatomic, strong) NSFileHandle *handle;

/** 当前完成的下载量 */
@property(nonatomic, assign) long long currentDataLength;

/** 总的文件大小 */
@property(nonatomic, assign) long long totalDataLength;

/** 当前下载连接 */
@property(nonatomic, strong) NSURLConnection *conn;

/** 下载状态标识 */
@property(nonatomic, assign, getter=isDownloading) BOOL downloading;

/** 下载/暂停 按钮事件 */
- (IBAction)download:(UIButton *)button;

@end

@implementation BigFileDownloadViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
}


- (IBAction)download:(UIButton *)button {
    if (self.isDownloading) { // 如果正在下载中，暂停下载

        self.downloading = NO;
        [button setTitle:@"开始" forState:UIControlStateNormal];
       
        //  取消连接，不能恢复
        [self.conn cancel];
        self.conn = nil;
    } else { // 如果是暂停中，恢复下载
       
        self.downloading = YES;
        [button setTitle:@"暂停" forState:UIControlStateNormal];
       
        // 从上次下载完成的地方继续下载，初始就是0
        NSURL *url = [NSURL URLWithString:@"http://192.168.0.21:8080/MyTestServer/videos/0627.zip"];
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
       
        // 设置request头信息，指明要从文件哪里开始下载
        NSString *value = [NSString stringWithFormat:@"bytes=%lld-", self.currentDataLength];
        [request setValue:value forHTTPHeaderField:@"Range"];
       
        self.conn = [NSURLConnection connectionWithRequest:request delegate:self];
    }
}

#pragma mark - NSURLConnectionDataDelegate
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    NSLog(@"失败");
    NSLog(@"%@", error);
}

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    NSLog(@"开始接收");
   
    // 如果是继续下载，就不要再重复创建文件了
    if (self.totalDataLength) return;
   
    // 获取存放路径
    NSString *cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    NSString *filePath = [cachePath stringByAppendingPathComponent:@"0627.zip"];
   
    // 创建一个空的文件，用来存放接收的数据
    NSFileManager *manager = [NSFileManager defaultManager];
    if ([manager fileExistsAtPath:filePath]) {
        [manager removeItemAtPath:filePath error:nil];
    }
   
    // 刚创建默认是0字节
    [manager createFileAtPath:filePath contents:nil attributes:nil];
   
   
    // 把文件句柄用写入方式指向文件
    self.handle = [NSFileHandle fileHandleForWritingAtPath:filePath];
   
    // 得到总的文件大小
    self.totalDataLength = response.expectedContentLength;
   
   
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    NSLog(@"正在接收: %d byte", data.length);

    // 下载量累加
    self.currentDataLength += data.length;
   
    // 刷新进度条
    CGFloat completedProgress = (double)self.currentDataLength / self.totalDataLength;
    NSLog(@"completed percentage: %f%%", completedProgress * 100);
    self.progressView.progress = completedProgress;
   
    // 移动句柄到上次写入数据的末位置
    [self.handle seekToEndOfFile];
   
    // 写入数据
    [self.handle writeData:data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    NSLog(@"接收完毕");
   
    // 复位当前下载量
    self.currentDataLength = 0;
   
    // 复位下载文件大小
    self.totalDataLength = 0;
   
    // 关闭连接
    [self.handle closeFile];
    self.handle = nil;
}


@end

 

4.文件解压/压缩

使用ssziparchive-master框架

注意：需要引入libz.dylib框架

（1）解压

使用block存储下载完成后的操作：解压

//     完成下载后block
    self.downloader.didFinishHandler = ^{
        // 解压文件
        [SSZipArchive unzipFileAtPath:filePath toDestination:cachePath];
        NSLog(@"解压完成!");
    };

 

#mark: 我使用大小为1.4G，仅包含一个视频的压缩文件进行解压，没有解出来; 解压包含了几张图片的压缩包能正常工作。

 

（2）压缩

a.准备实验文件

 

b.使用SSZipArchive归档解压

    // 获得所有png图片，注意这里是不能获得沙盒中的文件的，只能是项目中的
    NSArray *pngs = [[NSBundle mainBundle] pathsForResourcesOfType:@"PNG" inDirectory:nil];
    NSLog(@"%@", pngs);
   
    // 压缩文件存放路径
    NSString *cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    NSString *filePath = [cachePath stringByAppendingPathComponent:@"pngs.zip"];
   
    // 压缩文件
    [SSZipArchive createZipFileAtPath:filePath withFilesAtPaths:pngs];
  
    NSLog(@"压缩完成!");

 

 

5.下载器的封装

将下载功能封装成一个类，传入请求url和文件存放地址，调用接口开启、暂停线程

//
//  FileDownloader.h
//  BigFileDownloadDemo
//
//  Created by hellovoidworld on 15/1/26.
//  Copyright (c) 2015年 hellovoidworld. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface FileDownloader : NSObject

/** 请求路径 */
@property(nonatomic, strong) NSURL *url;

/** 存放路径 */
@property(nonatomic, strong) NSString *filePath;

/** 下载状态 */
@property(nonatomic, readonly, getter=isDownloading) BOOL downloading;

/** 更新进度block */
@property(nonatomic, copy) void(^progressHandler)(double progress);

/** 完成下载后block */
@property(nonatomic, copy) void(^didFinishHandler)();

- (void) startDownloading;
- (void) pauseDownloading;

@end

 

//
//  FileDownloader.m
//  BigFileDownloadDemo
//
//  Created by hellovoidworld on 15/1/26.
//  Copyright (c) 2015年 hellovoidworld. All rights reserved.
//

#import "FileDownloader.h"
#import <UIKit/UIKit.h>

@interface FileDownloader() <NSURLConnectionDataDelegate>

/** 文件句柄 */
@property(nonatomic, strong) NSFileHandle *handle;

/** 当前完成的下载量 */
@property(nonatomic, assign) long long currentDataLength;

/** 总的文件大小 */
@property(nonatomic, assign) long long totalDataLength;

/** 当前下载连接 */
@property(nonatomic, strong) NSURLConnection *conn;

@end

@implementation FileDownloader

/** 开始下载 */
- (void) startDownloading {
    _downloading = YES;
   
    // 从上次下载完成的地方继续下载，初始就是0
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:self.url];
   
    // 设置request头信息，指明要从文件哪里开始下载
    NSString *value = [NSString stringWithFormat:@"bytes=%lld-", self.currentDataLength];
    [request setValue:value forHTTPHeaderField:@"Range"];
   
    self.conn = [NSURLConnection connectionWithRequest:request delegate:self];
}

/** 暂停下载 */
- (void) pauseDownloading {
    _downloading = NO;
   
    //  取消连接，不能恢复
    [self.conn cancel];
    self.conn = nil;
}

#pragma mark - NSURLConnectionDataDelegate
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    NSLog(@"失败");
    NSLog(@"%@", error);
}

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    NSLog(@"开始接收");
   
    // 如果是继续下载，就不要再重复创建文件了
    if (self.totalDataLength) return;
   
    // 创建一个空的文件，用来存放接收的数据
    NSFileManager *manager = [NSFileManager defaultManager];
    if ([manager fileExistsAtPath:self.filePath]) {
        [manager removeItemAtPath:self.filePath error:nil];
    }
   
    // 刚创建默认是0字节
    [manager createFileAtPath:self.filePath contents:nil attributes:nil];
   
   
    // 把文件句柄用写入方式指向文件
    self.handle = [NSFileHandle fileHandleForWritingAtPath:self.filePath];
   
    // 得到总的文件大小
    self.totalDataLength = response.expectedContentLength;
   
   
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    NSLog(@"正在接收: %d byte", data.length);
   
    // 下载量累加
    self.currentDataLength += data.length;
   
    // 刷新进度条
    CGFloat completedProgress = (double)self.currentDataLength / self.totalDataLength;
    NSLog(@"completed percentage: %f%%", completedProgress * 100);
    if (self.progressHandler) {
        self.progressHandler(completedProgress);
    }
   
    // 移动句柄到上次写入数据的末位置
    [self.handle seekToEndOfFile];
   
    // 写入数据
    [self.handle writeData:data];
   
    NSLog(@"现在的线程:%@", [NSThread currentThread]);
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    NSLog(@"接收完毕");
   
    // 复位当前下载量
    self.currentDataLength = 0;
   
    // 复位下载文件大小
    self.totalDataLength = 0;
   
    // 关闭连接
    [self.handle closeFile];
    self.handle = nil;
   
    if (self.didFinishHandler) {
        self.didFinishHandler();
    }
}

@end

 

//
//  BigFileDownloadViewController.m
//  BigFileDownloadDemo
//
//  Created by hellovoidworld on 15/1/26.
//  Copyright (c) 2015年 hellovoidworld. All rights reserved.
//

#import "BigFileDownloadViewController.h"
#import "FileDownloader.h"
#import "SSZipArchive.h"

@interface BigFileDownloadViewController ()

/** 进度条 */
@property (weak, nonatomic) IBOutlet UIProgressView *progressView;

/** 下载器 */
@property(nonatomic, strong) FileDownloader *downloader;

/** 下载/暂停 按钮事件 */
- (IBAction)download:(UIButton *)button;

@end

@implementation BigFileDownloadViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
   
    // 设置下载器请求路径
    self.downloader = [[FileDownloader alloc] init];
    self.downloader.url = [NSURL URLWithString:@"http://192.168.0.21:8080/MyTestServer/videos/0627.zip"];
//    self.downloader.url = [NSURL URLWithString:@"http://192.168.0.21:8080/MyTestServer/images/images.zip"];
   
    // 设置存放路径
    NSString *cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    NSString *filePath = [cachePath stringByAppendingPathComponent:@"0627.zip"];
//    NSString *filePath = [cachePath stringByAppendingPathComponent:@"images.zip"];
    self.downloader.filePath = filePath;
   
   
    // 刷新进度条的block
    typeof(self) vc = self;
    self.downloader.progressHandler = ^(double progress) {
        vc.progressView.progress = progress;
    };
   
//     完成下载后block
    self.downloader.didFinishHandler = ^{
        // 解压文件
        [SSZipArchive unzipFileAtPath:filePath toDestination:cachePath];
        NSLog(@"解压完成!");
    };
}


- (IBAction)download:(UIButton *)button {
    if (self.downloader.isDownloading) { // 如果正在下载中，暂停下载
        [button setTitle:@"开始" forState:UIControlStateNormal];
        [self.downloader pauseDownloading];
    } else { // 如果是暂停中，恢复下载
        [button setTitle:@"暂停" forState:UIControlStateNormal];
        [self.downloader startDownloading];
    }
}


- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
   
    // 获得所有png图片
    NSArray *pngs = [[NSBundle mainBundle] pathsForResourcesOfType:@"PNG" inDirectory:nil];
    NSLog(@"%@", pngs);
   
    // 压缩文件存放路径
    NSString *cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    NSString *filePath = [cachePath stringByAppendingPathComponent:@"pngs.zip"];
   
    // 压缩文件
    [SSZipArchive createZipFileAtPath:filePath withFilesAtPaths:pngs];
   
    NSLog(@"压缩完成!");
}

@end

 

 

6.多线程下载

思路：由于调用NSURLConnection的代理方法进行服务器请求和接收，发送的是异步请求，所以可以认为利用几个NSURLConnection同时发送请求，下载一个文件的不同部分，就能实现多线程下载。

 

• 创建多线下载器类和单线下载器
• 多线下载器对象创建n个单线下载器来进行下载
• 创建一个跟实际文件大小一样的临时文件，用来辅助定位单线下载器的下载存储位置
• n个单线下载器负责一个文件的不同部分的下载
• 使用请求头设置Range来实现文件下载部分的不同

 

 

（1）基本下载器

用来声明了一些公用属性和方法

//
//  FileDownloader.h
//  BigFileDownloadDemo
//
//  Created by hellovoidworld on 15/1/26.
//  Copyright (c) 2015年 hellovoidworld. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface FileDownloader : NSObject
{
    @protected
    BOOL _downloading;
}

/** 请求路径 */
@property(nonatomic, strong) NSString *url;

/** 存放路径 */
@property(nonatomic, strong) NSString *filePath;

/** 下载状态 */
@property(nonatomic, readonly, getter=isDownloading) BOOL downloading;

/** 更新进度block */
@property(nonatomic, copy) void(^progressHandler)(double progress);

/** 完成下载后block */
@property(nonatomic, copy) void(^didFinishHandler)();

- (void) startDownloading;
- (void) pauseDownloading;

@end

 

（2）单线下载器

//
//  SingleDownloader.h
//  MultiDownloaderDemo
//
//  Created by hellovoidworld on 15/1/27.
//  Copyright (c) 2015年 hellovoidworld. All rights reserved.
//

#import "FileDownloader.h"

@interface SingleDownloader : FileDownloader

/** 开始下载的位置（字节) */
@property(nonatomic, assign) long long begin;

/** 结束下载的位置（字节) */
@property(nonatomic, assign) long long end;

@end

 

//
//  SingleDownloader.m
//  MultiDownloaderDemo
//
//  Created by hellovoidworld on 15/1/27.
//  Copyright (c) 2015年 hellovoidworld. All rights reserved.
//

#import "SingleDownloader.h"

@interface SingleDownloader() <NSURLConnectionDataDelegate>

/** 文件句柄 */
@property(nonatomic, strong) NSFileHandle *handle;

/** 当前下载连接 */
@property(nonatomic, strong) NSURLConnection *conn;

/** 当前下载量 */
@property(nonatomic, assign) long long currentLength;

@end

@implementation SingleDownloader

/** 初始化文件句柄 */
- (NSFileHandle *)handle {
    if (nil == _handle) {
        _handle = [NSFileHandle fileHandleForWritingAtPath:self.filePath];
    }
    return _handle;
}

/** 开始下载 */
- (void) startDownloading {
    // 从上次下载完成的地方继续下载，初始就是0
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:self.url]];
   
    // 设置request头信息，指明要从文件哪里开始下载
    NSString *value = [NSString stringWithFormat:@"bytes=%lld-%lld", self.begin, self.end];
    [request setValue:value forHTTPHeaderField:@"Range"];
   
    self.conn = [NSURLConnection connectionWithRequest:request delegate:self];
   
    _downloading = YES;
}

/** 暂停下载 */
- (void) pauseDownloading {
    //  取消连接，不能恢复
    [self.conn cancel];
    self.conn = nil;
   
    _downloading = NO;
}

#pragma mark - NSURLConnectionDataDelegate
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    NSLog(@"失败");
    NSLog(@"%@", error);
}

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    NSLog(@"开始接收");
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    // 移动句柄到上次写入数据的末位置
    [self.handle seekToFileOffset:self.begin + self.currentLength];
   
    // 写入数据
    [self.handle writeData:data];
   
    // 已经下载的量
    self.currentLength += data.length;
 
    if (self.progressHandler) {
        double progress = (double)self.currentLength / (self.end - self.begin + 1);
        self.progressHandler(progress);
    }
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    NSLog(@"接收完毕");
   
    // 清空下载量
    self.currentLength = 0;
   
   
    // 关闭连接
    [self.handle closeFile];
    self.handle = nil;
   
    if (self.didFinishHandler) {
        self.didFinishHandler();
    }
}


@end

（3）总下载器

//
//  MultiDownloader.h
//  MultiDownloaderDemo
//
//  Created by hellovoidworld on 15/1/27.
//  Copyright (c) 2015年 hellovoidworld. All rights reserved.
//

#import "FileDownloader.h"

@interface MultiDownloader : FileDownloader

@end
 
//
//  MultiDownloader.m
//  MultiDownloaderDemo
//
//  Created by hellovoidworld on 15/1/27.
//  Copyright (c) 2015年 hellovoidworld. All rights reserved.
//

#import "MultiDownloader.h"
#import "SingleDownloader.h"

#define MaxMultilineCount 4

@interface MultiDownloader()

@property(nonatomic, strong) NSMutableArray *singleDownloaders;

@end

@implementation MultiDownloader

/** 初始化单线下载器 */
- (NSMutableArray *)singleDownloaders {
    if (!_singleDownloaders) {
        _singleDownloaders = [NSMutableArray array];
       
        long long fileSize = [self getFileSize];
        long long singleFileSize = 0; // 每条子线下载量
        if (fileSize % MaxMultilineCount == 0) {
            singleFileSize = fileSize / MaxMultilineCount;
        } else {
            singleFileSize = fileSize / MaxMultilineCount + 1;
        }
       
        for (int i=0; i<MaxMultilineCount; i++) {
            SingleDownloader *downloader = [[SingleDownloader alloc] init];
            downloader.url = self.url;
            downloader.filePath = self.filePath;
            downloader.begin = i * singleFileSize;
            downloader.end = downloader.begin + singleFileSize - 1;
            downloader.progressHandler = ^(double progress){
                NSLog(@"%d号单线下载器正在下载，下载进度:%f", i, progress);
            };
           
            [self.singleDownloaders addObject:downloader];
        }
       
        // 创建临时文件，文件大小要跟实际大小一致
        // 1.创建一个0字节文件
        [[NSFileManager defaultManager] createFileAtPath:self.filePath contents:nil attributes:nil];
       
        // 2.指定文件大小
        NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:self.filePath];
        [fileHandle truncateFileAtOffset:fileSize];
    }
   
    return _singleDownloaders;
}

/** 获得文件大小 */
- (long long) getFileSize {
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:self.url]];
    request.HTTPMethod = @"HEAD";// 请求得到头响应
   
    NSURLResponse *response = nil;
    [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:nil];
    return response.expectedContentLength;
}

/** 开始下载 */
- (void)startDownloading {
    [self.singleDownloaders makeObjectsPerformSelector:@selector(startDownloading)];
    _downloading = YES;
    NSLog(@"多线程下载开始");
}

/** 暂停下载 */
- (void)pauseDownloading {
    [self.singleDownloaders makeObjectsPerformSelector:@selector(pauseDownloading)];
    _downloading = NO;
    NSLog(@"多线程下载暂停!");
}

@end
 
（4）控制器调用
- (IBAction)startDownloading {
    MultiDownloader *downloader = [[MultiDownloader alloc] init];
    downloader.url = @"http://192.168.0.21:8080/MyTestServer/videos/0627.zip";
   
    // 设置存放路径
    NSString *cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    downloader.filePath = [cachePath stringByAppendingPathComponent:@"0627.zip"];
   
    // 开始下载
    [downloader startDownloading];
}