http://blog.csdn.net/daiyelang/article/category/1377418
ASIHTTPRequest可以实现断点续传。网上有一些介绍类似使用：
[request setAllowResumeForFileDownloads:YES];
方法的。但是它不是真正意义的断点续传。它只能让应用在下载过程中，暂停和继续。如果退出应用再进入是无效的。
不过，通过ASIHTTPRequest的异步请求以及delegate还是可以实现断点续传的。
本文还是以Grails编写断点续传服务器端为例。
异步请求的代码：
-(void) doSimpleGetBinary{
NSURL *url = [NSURL URLWithString:@"http://localhost:8080/BookProto/book/image"];
ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
[request setRequestMethod:@"GET"];
//[request addRequestHeader:@"Range" value:@"bytes=3-"];
[request setDelegate:self];
[request startAsynchronous];
}
这里设置了Delegate, 要在头文件中实现相应的protocol：
@interface CFHttpDemoViewController : UIViewController<ASIProgressDelegate> {
本例中使用到了delegate的如下方法。
requestFinished：
- (void)requestFinished:(ASIHTTPRequest *)request{
NSLog(@"response status code: %i",[request responseStatusCode]);
NSLog(@"response content length: %@",[[request responseHeaders] objectForKey:@"Content-Length" ]);
NSLog(@"request finished.");
label.text=@"request finished.";
}
这个方法在异步请求结束后调用。
下面的方法，是当缓冲区接收到部分数据后调用，看起来是每间隔一定的毫秒，就调用一下，并传入缓冲区的NSData对象。
-(void)request:(ASIHTTPRequest *)request didReceiveData:(NSData *)data{
NSLog(@"did receive data, data length: %i",[data length]);
//复制到字节数组中
Byte *byteData=(Byte *)malloc([data length]);
memcpy(byteData,[data bytes],[data length]);
for (int i=0; i<=10; i++) {
NSLog(@"%i: %i",i+1,byteData[i]);
}
free(byteData);
[request cancel];
label.text=@"canceled.";
}
运行代码，屏蔽：
[request addRequestHeader:@"Range" value:@"bytes=3-"];
和取消屏蔽，数据分别如下：
2011-07-12 14:17:13.497 CFHttpDemo[2647:207] did receive data, data length: 10172
2011-07-12 14:17:13.514 CFHttpDemo[2647:207] 1: 137
2011-07-12 14:17:13.515 CFHttpDemo[2647:207] 2: 80
2011-07-12 14:17:13.516 CFHttpDemo[2647:207] 3: 78
2011-07-12 14:17:13.516 CFHttpDemo[2647:207] 4: 71
2011-07-12 14:17:13.517 CFHttpDemo[2647:207] 5: 13
2011-07-12 14:17:13.518 CFHttpDemo[2647:207] 6: 10
2011-07-12 14:17:13.518 CFHttpDemo[2647:207] 7: 26
2011-07-12 14:17:13.519 CFHttpDemo[2647:207] 8: 10
2011-07-12 14:17:13.520 CFHttpDemo[2647:207] 9: 0
2011-07-12 14:17:13.520 CFHttpDemo[2647:207] 10: 0
2011-07-12 14:17:13.521 CFHttpDemo[2647:207] 11: 0
2011-07-12 14:17:13.522 CFHttpDemo[2647:207] response status code: 200
2011-07-12 14:17:13.523 CFHttpDemo[2647:207] response content length: 10172
2011-07-12 14:17:13.523 CFHttpDemo[2647:207] request finished.
2011-07-12 14:02:24.551 CFHttpDemo[2578:207] did receive data, data length: 10169
2011-07-12 14:02:24.553 CFHttpDemo[2578:207] byteData ok.
2011-07-12 14:02:24.554 CFHttpDemo[2578:207] 1: 71
2011-07-12 14:02:24.554 CFHttpDemo[2578:207] 2: 13
2011-07-12 14:02:24.555 CFHttpDemo[2578:207] 3: 10
2011-07-12 14:02:24.555 CFHttpDemo[2578:207] 4: 26
2011-07-12 14:02:24.556 CFHttpDemo[2578:207] 5: 10
2011-07-12 14:02:24.556 CFHttpDemo[2578:207] 6: 0
2011-07-12 14:02:24.557 CFHttpDemo[2578:207] 7: 0
2011-07-12 14:02:24.557 CFHttpDemo[2578:207] 8: 0
2011-07-12 14:02:24.558 CFHttpDemo[2578:207] 9: 13
2011-07-12 14:02:24.558 CFHttpDemo[2578:207] 10: 73
2011-07-12 14:02:24.560 CFHttpDemo[2578:207] 11: 72
2011-07-12 14:02:24.561 CFHttpDemo[2578:207] response status code: 206
2011-07-12 14:02:24.561 CFHttpDemo[2578:207] response content length: 10169
2011-07-12 14:02:24.562 CFHttpDemo[2578:207] request finished.
2.
NSUrlConnection实现断点续传的关键是自定义http request的头部的range域属性。
Range头域
Range头域可以请求实体的一个或者多个子范围。例如，
表示头500个字节：bytes=0-499
表示第二个500字节：bytes=500-999
表示最后500个字节：bytes=-500
表示500字节以后的范围：bytes=500-
第一个和最后一个字节：bytes=0-0,-1
同时指定几个范围：bytes=500-600,601-999
但是服务器可以忽略此请求头，如果无条件GET包含Range请求头，响应会以状态码206（PartialContent）返回而不是以200（OK）。
在ios中使用NSMutableURLRequest来定义头部域
NSURL *url1=[NSURL URLWithString:@"下载地址";
NSMutableURLRequest* request1=[NSMutableURLRequest requestWithURL:url1];
[request1 setValue:@"bytes=20000-" forHTTPHeaderField:@"Range"];
[request1 setCachePolicy:NSURLRequestReloadIgnoringLocalCacheData];
NSData *returnData1 = [NSURLConnection sendSynchronousRequest:request1 returningResponse:nil error:nil];
[self writeToFile:returnData1 fileName:@"SOMEPATH"];
-(void)writeToFile:(NSData *)data fileName:(NSString *) fileName
{
NSString *filePath=[NSString stringWithFormat:@"%@",fileName];
if([[NSFileManager defaultManager] fileExistsAtPath:filePath] == NO){
NSLog(@"file not exist,create it...");
[[NSFileManager defaultManager] createFileAtPath:filePath contents:nil attributes:nil];
}else {
NSLog(@"file exist!!!");
}
FILE *file = fopen([fileName UTF8String], [@"ab+" UTF8String]);
if(file != NULL){
fseek(file, 0, SEEK_END);
}
int readSize = [data length];
fwrite((const void *)[data bytes], readSize, 1, file);
fclose(file);
}