断点离线下载

• 以下代码只是提供一种思路，具体开发中还是需要自己去封装
• 存储文件总长度的方式还需要改进

// 需要下载的文件的URL
#define YWFileURL @"下载地址"

// 文件名(沙盒中的文件名，下载地址的MD5的字符串)
#define YWFileName YWFileURL.md5String

#define YWFileFullPath [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:YWFileName]

// 存储文件总长度的文件路径
#define YWTotalLengthFullPath [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:@"totalLength.plist"]

// 文件已下载的长度
#define YWDownloadLength [[[NSFileManager defaultManager] attributesOfItemAtPath:YWFileFullPath error:nil][NSFileSize] integerValue]


#import "ZLViewController.h"
#import "NSString+Hash.h"

@interface ZLViewController ()<NSURLSessionDataDelegate>

/**
 下载任务
 */
@property(nonatomic, strong) NSURLSessionDataTask *task;

/**
 NSURLSession
 */
@property(nonatomic, strong) NSURLSession *session;
/**
 写文件的流对象
 */
@property(nonatomic, strong) NSOutputStream *stream;
/**
 文件的总长度
 */
@property(nonatomic, assign) NSInteger totalLength;

@end

@implementation ZLViewController

- (NSURLSession *)session
{
    if(_session == nil){
        _session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:[[NSOperationQueue alloc] init]];
    }
    return _session;
}

- (NSOutputStream *)stream
{
    if(_stream == nil){
        _stream = [NSOutputStream outputStreamToFileAtPath:YWFileFullPath append:YES];
    }
    return _stream;
}

- (NSURLSessionDataTask *)task
{
    if(_task == nil){
        NSInteger totalLength = [[NSDictionary dictionaryWithContentsOfFile:YWTotalLengthFullPath][YWFileName] integerValue];
        if(totalLength && YWDownloadLength == totalLength){
            // 文件已经下载过了
            return nil;
        }
        
        // 创建请求
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:YWFileURL]];
        
        // 设置请求头
        NSString *range = [NSString stringWithFormat:@"bytes=%zd-", YWDownloadLength];
        [request setValue:range forHTTPHeaderField:@"Range"];
        
        // 创建一个Data任务
        _task = [self.session dataTaskWithRequest:request];
    }
    return _task;
}


- (void)viewDidLoad {
    [super viewDidLoad];
    NSLog(@"%@",YWFileFullPath);
}


/**
 开始下载
 */
- (IBAction)start:(id)sender {
    // 启动任务
    [self.task resume];
}


/**
 暂停下载
 */
- (IBAction)pause:(id)sender {
    [self.task suspend];
}

#pragma mark - NSURLSessionDataDelegate

/**
 接收到响应
 */
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSHTTPURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition))completionHandler
{
    // 打开流
    [self.stream open];
    
    // 获得服务器这次请求，返回数据的总长度
    self.totalLength = [response.allHeaderFields[@"Content-Length"] integerValue] + YWDownloadLength;
    
    // 存储总长度
    NSMutableDictionary *dict = [NSMutableDictionary dictionaryWithContentsOfFile:YWTotalLengthFullPath];
    if(dict == nil){
        dict = [NSMutableDictionary dictionary];
    }
    dict[YWFileName] = @(self.totalLength);
    [dict writeToFile:YWTotalLengthFullPath atomically:YES];
    
    // 接收这个请求，允许接收服务器数据
    completionHandler(NSURLSessionResponseAllow);
}


/**
 接收到服务器返回的数据(这个方法可能被调用多次)
 */
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data
{
    // 写入数据
    [self.stream write:data.bytes maxLength:data.length];
    
    // 下载进度
    NSLog(@"%f", 1.0 * YWDownloadLength / self.totalLength);
}


/**
 请求完毕
 */
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
    [self.stream close];
    self.stream = nil;
    
    self.task = nil;
}


- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}

@end