使用NSURLConnection实现大文件断点下载

//
//  YXDownloadNetwork.m
//  Download
//
//  http://home.cnblogs.com/u/YouXianMing/
//
//  Copyright (c) 2014年 Y.X. All rights reserved.
//

#import "YXDownloadNetwork.h"

@interface YXDownloadNetwork ()<NSURLConnectionDelegate, NSURLConnectionDataDelegate>

@property (nonatomic, assign) unsigned long long   totalLength;      // 文件总大小
@property (nonatomic, assign) unsigned long long   startDataLength;  // 本地存在文件的大小
@property (nonatomic, assign) unsigned long long   expectedLength;   // 从服务器期望文件的大小
@property (nonatomic, assign) unsigned long long   cacheCapacity;    // 缓存文件容量,以k为单位

@property (nonatomic, strong) NSURLConnection     *dataConncetion;   // 网络连接
@property (nonatomic, strong) NSDictionary        *responseHeaders;  // 网络连接头部信息
@property (nonatomic, strong) NSFileHandle        *file;             // 文件操作句柄
@property (nonatomic, strong) NSMutableData       *cacheData;        // 用于缓存的data数据

@end

@implementation YXDownloadNetwork

- (instancetype)initWithUrlString:(NSString *)urlString cacheCapacity:(unsigned long long)capacity
{
    self = [super init];
    
    if (self)
    {
        // 获取缓存容量
        if (capacity <= 0)
        {
            _cacheCapacity = 100 * 1024;
        }
        else
        {
            _cacheCapacity = capacity * 1024;
        }
        
        // 获取用于缓存的数据
        _cacheData = [NSMutableData new];
        
        // 获取文件名以及文件路径
        NSString *fileName = [urlString lastPathComponent];
        NSString *filePath = \
            fileFromPath([NSString stringWithFormat:@"/Documents/%@", fileName]);
        
        // 记录文件起始位置
        if ([[NSFileManager defaultManager] fileExistsAtPath:filePath])
        {
            // 从文件中读取出已经下载好的文件的长度
            _startDataLength = [[NSData dataWithContentsOfFile:filePath] length];
        }
        else
        {
            // 不存在则创建文件
            _startDataLength = 0;
            [[NSFileManager defaultManager] createFileAtPath:filePath
                                                    contents:nil
                                                  attributes:nil];
        }
        
        // 打开写文件流
        _file = [NSFileHandle fileHandleForWritingAtPath:filePath];
        
        // 创建一个网络请求
        NSMutableURLRequest* request = \
        [NSMutableURLRequest requestWithURL:[NSURL URLWithString:urlString]];
        
        // 禁止读取本地缓存
        [request setCachePolicy:NSURLRequestReloadIgnoringLocalCacheData];
        
        // 设置断点续传(需要服务器支持)
        [request setValue:[NSString stringWithFormat:@"bytes=%llu-", _startDataLength]
       forHTTPHeaderField:@"Range"];
        
        // 开始创建连接
        self.dataConncetion = \
        [[NSURLConnection alloc] initWithRequest:request
                                        delegate:self
                                startImmediately:NO];
    }
    
    return self;
}

- (void)start
{
    [self.dataConncetion start];
}

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    if([response isKindOfClass:[NSHTTPURLResponse class]])
    {
        NSHTTPURLResponse *r = (NSHTTPURLResponse *)response;
        
        // 如果能获取到期望的数据长度就执行括号中的方法
        if ([r expectedContentLength] != NSURLResponseUnknownLength)
        {
            // 获取剩余要下载的
            _expectedLength  = [r expectedContentLength];
            
            // 计算出总共需要下载的
            _totalLength = _expectedLength + _startDataLength;

            // 获取头文件
            _responseHeaders = [r allHeaderFields];
        }
        else
        {
            NSLog(@"不支持断点下载");
        }
    }
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)theData
{
    // 追加缓存数据
    [_cacheData appendData:theData];
    
    // 如果该缓存数据的大小超过了指定的缓存大小
    if ([_cacheData length] >= _cacheCapacity)
    {
        // 移动到文件结尾
        [_file seekToEndOfFile];
        
        // 在文件末尾处追加数据
        [_file writeData:_cacheData];
        
        // 清空缓存数据
        [_cacheData setLength:0];
    }
    
    // 当前已经下载的所有数据的总量
    _startDataLength += [theData length];
    
    // 如果指定了block
    if (_downloadProgress)
    {
        _downloadProgress(_startDataLength, _totalLength);
    }
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    // 移动到文件结尾
    [_file seekToEndOfFile];
    
    // 在文件末尾处追加最后的一点缓存数据
    [_file writeData:_cacheData];
    
    // 清空缓存
    [_cacheData setLength:0];
    
    NSLog(@"下载完成哦");
}

NS_INLINE NSString * fileFromPath(NSString *filePath)
{
    return [NSHomeDirectory() stringByAppendingString:filePath];
}

@end