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.
和Grails编写断点续传服务器端的测试数据一致，因此是正确的。