iOS Http断点续传浅析

iOS iOS

http实现断点续传的关键地方就是在httprequest中加入“Range”头。

//设置Range头,值:bytes=x-y;x:开始字节,y:结束字节,不指定则为文件末尾
[request addValue:@"bytes=500-" forHTTPHeaderField:@"Range"];

如果服务器正确响应的话,就可以顺利续传;如果服务器不支持,那就只能用其它方法了。

经过测试,服务器的不支持分为两种情况:

1.完全没响应

如果不处理会导致文件无法下载。

测试地址:http://dl_dir.qq.com/qqfile/qq/QQforMac/QQ_V2.4.2.dmg

发送请求后,过一段时间直接进入了didFailWithError的delegate;错误信息为time out。

针对这种情况可以做出的处理是:增加一个是否支持断点续传的标志。

具体:

第一次请求,开始字节为0,不用发送Range头,可以正常下载;

当下载中断,开始第二次请求,开始字节不为0,发送range头;

如果进入didFailWithError的delegate,就标明此链接不可以断点续传,每次请求前都清除缓存,保证开始的字节为0,不发送Range头。

2.无论发送Range的值是多少,服务器都会重新下载。

如果不处理,会导致续传过的文件出错。

测试地址:https://github.com/CocoaPods/CocoaPods/archive/master.zip

这种情况的处理方案是:

第一次收到响应的时候,就把文件的总大小记录下来;

以后每次收到响应的时候都比较一下下载长度和总大小是不是一样;

如果一样而且又存在缓存;就表明属于这种情况了;直接删掉缓存,重新下载。

下面是用NSURLConnection实现http断点续传的实例:

针对上面两种做了简单的处理,回调函数还有待添加

MXDownload.h文件:

#import <Foundation/Foundation.h>

@interface MXDownload : NSObject

//文件名路径
@property (nonatomic, readonly) NSString *filePath;

//是否正在下载的标志
@property (nonatomic, readonly) BOOL downloading;

//初始化
- (id)initWithUrlString:(NSString *)urlString;

//两个状态
- (void)start;
- (void)stop;

//清除缓存
- (void)clearCache;

@end

MXDownload.m文件:

#import "MXDownload.h"
#import "NSString+MX.h"

#define FILE_INFO_PLIST [NSString   pathWithName:@"MXDownload/fileInfo.plist"   directory:NSCachesDirectory]

@interface MXDownload (){
    NSURLConnection *_urlConnection;
    NSString *_urlString;
    BOOL _downloading,_didAddRange,_shouldResume;

    NSString *_fileName,*_filePath, *_tempFilePath;
    NSFileHandle        *_fileHandle;
    unsigned long long  _fileOffset,_fileSize;
}

@end

@implementation MXDownload
@synthesize downloading = _downloading;
@synthesize filePath = _filePath;

//初始化,顺便设置下载文件和下载临时文件路径
- (id)initWithUrlString:(NSString *)urlString{
    self = [super init];
    if (self){
        _urlString = urlString;
        _shouldResume = YES;
        if (_urlString) {
            _fileName = [_urlString MD5];
            _filePath = [NSString pathWithName:[NSString stringWithFormat:@"MXDownload/ %@",_fileName] directory:NSCachesDirectory];
            _tempFilePath = [NSString stringWithFormat:@"%@.temp",_filePath];
        }
    }
    return self;
}

//开始下载
- (void)start{
    //如果正在下载,中断
    if (_downloading) return;
    //没有url,也中断
    if (!_urlString) return;

    //临时文件句柄
    _fileHandle = [NSFileHandle fileHandleForWritingAtPath:_tempFilePath];
    //获取本次请求下载开始的位置,如果文件不存在,就是0
    _fileOffset = _fileHandle ? [_fileHandle seekToEndOfFile] : 0;

    //初始化请求
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL  URLWithString:_urlString]];
    //设置缓存策略,很重要,因为文件是自己储存的,和缓存无关,所以要忽略缓存
    //要不然第二次请求会出错
    [request setCachePolicy:NSURLRequestReloadIgnoringLocalCacheData];

    //最关键地方,设置Range头,值:bytes=x-y;x:开始字节,y:结束字节,不指定则为文件末尾
    _didAddRange = NO;
    if (_fileOffset != 0 && _shouldResume) {
        [request addValue:[NSString stringWithFormat:@"bytes=%llu-",_fileOffset]    forHTTPHeaderField:@"Range"];
        _didAddRange = YES;
    }

    _urlConnection = [[NSURLConnection alloc] initWithRequest:request delegate:self];
    [_urlConnection start];

    _downloading = YES;
}

//结束下载
- (void)stop{
    [_urlConnection cancel];
    _urlConnection = nil;
    [_fileHandle closeFile];
    _downloading = NO;
}

//清除文件
- (void)clearCache{
    if (_downloading) [self stop];
    [[NSFileManager defaultManager] removeItemAtPath:_filePath error:nil];
    [[NSFileManager defaultManager] removeItemAtPath:_tempFilePath error:nil];
}

#pragma mark -
#pragma mark NSURLConnectionDelegate

//接收到响应
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse  *)response{
    //本次请求回来的文件大小
    long long fileLength = response.expectedContentLength;
    if (fileLength == NSURLResponseUnknownLength) [self stop];

    NSData *existFileData = [[NSData alloc] initWithContentsOfFile:_filePath];

    //检查文件是否已下载完成
    if (existFileData && existFileData.length == fileLength) {
        NSLog(@"之前已经下载好了");
        [self stop];
    }
    else{
        //保存文件的总大小
        if (!_didAddRange){
            NSMutableDictionary *dic = [NSMutableDictionary new];
            [dic addEntriesFromDictionary:[NSDictionary     dictionaryWithContentsOfFile:FILE_INFO_PLIST]];
            [dic setValue:[NSNumber numberWithLongLong:fileLength]  forKey:_fileName];
            [dic writeToFile:FILE_INFO_PLIST atomically:YES];
        }

        NSFileManager *fileManager = [NSFileManager defaultManager];
        //先清除掉旧的文件
        [fileManager removeItemAtPath:_filePath error:nil];

        //如果此次请求回来的大小等于文件的总大小而且临时文件又存在,则删除临时文件
        //解决每次请求都是重新开始的问题
        NSDictionary *dic = [NSDictionary dictionaryWithContentsOfFile:FILE_INFO_PLIST];
        BOOL isTotalLength = fileLength == [[dic valueForKey:_fileName] longLongValue];
        if ([fileManager fileExistsAtPath:_tempFilePath] && isTotalLength){
            [fileManager removeItemAtPath:_tempFilePath error:nil];
        }

        //重新创建文件
        if (![fileManager fileExistsAtPath:_tempFilePath]){
            [fileManager createFileAtPath:_tempFilePath contents:nil attributes:nil];
            _fileHandle = [NSFileHandle fileHandleForWritingAtPath:_tempFilePath];
            _fileOffset = 0;
        }

        _fileSize = fileLength + _fileOffset;

        //用_fileOffset可以检查是重新下载还是继续下载
        NSLog(@"%@",_fileOffset ? @"继续下载" : @"开始下载");
    }
}

//不断接收到数据
-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)aData{
    //写入文件
    [_fileHandle writeData:aData];
    _fileOffset = [_fileHandle offsetInFile];
    NSLog(@"下载进度: %lld / %lld",_fileOffset,_fileSize);
}

-(void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error{
    [self stop];
    //如果不支持续传,删掉临时文件再试一次
    if (_shouldResume) {
        _shouldResume = NO;
        [[NSFileManager defaultManager] removeItemAtPath:_tempFilePath error:nil];
        [self start];
    }
}

//完成
-(void)connectionDidFinishLoading:(NSURLConnection *)connection{
    [[NSFileManager defaultManager] moveItemAtPath:_tempFilePath toPath:_filePath error:nil];
    NSLog(@"下载完成");
    [self stop];
}

@end

调用:

- (IBAction)startDownLoad:(id)sender{
    if (_downloader == nil){
//        _downloader = [[MXDownload alloc] initWithUrlString:@"https://github.com/ CocoaPods/CocoaPods/archive/master.zip"];
        _downloader = [[MXDownload alloc] initWithUrlString:@"http://dl_dir.qq.com/ qqfile/qq/QQforMac/QQ_V2.4.2.dmg"];
//        _downloader = [[MXDownload alloc] initWithUrlString:@"http:// 192.168.50.19:8080/vcont/wb.mp3"];
    }
    if (_downloader.downloading) {
        [_downloader stop];
    }
    else{
        [_downloader start];
    }
}