ASIHTTPRequest可以實現斷點續傳。網上有一些介紹類似使用：
[request setAllowResumeForFileDownloads:YES];
方法的。但是它不是真正意義的斷點續傳。它只能讓應用在下載過程中，暫停和繼續。如果退出應用再進入是無效的。
不過，通過ASIHTTPRequest的異步請求以及delegate還是可以實現斷點續傳的。
本文還是以Grails編寫斷點續傳服務器端為例。
異步請求的代碼：
-(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];
}
這裡設置了Delegate, 要在頭文件中實現相應的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.";
}
這個方法在異步請求結束後調用。
下面的方法，是當緩沖區接收到部分數據後調用，看起來是每間隔一定的毫秒，就調用一下，並傳入緩沖區的NSData對象。
-(void)request:(ASIHTTPRequest *)request didReceiveData:(NSData *)data{
NSLog(@"did receive data, data length: %i",[data length]);
//復制到字節數組中
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.";
}
運行代碼，屏蔽：
[request addRequestHeader:@"Range" value:@"bytes=3-"];
和取消屏蔽，數據分別如下：
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實現斷點續傳的關鍵是自定義http request的頭部的range域屬性。
Range頭域
Range頭域可以請求實體的一個或者多個子范圍。例如，
表示頭500個字節：bytes=0-499
表示第二個500字節：bytes=500-999
表示最後500個字節：bytes=-500
表示500字節以後的范圍：bytes=500-
第一個和最後一個字節：bytes=0-0,-1
同時指定幾個范圍：bytes=500-600,601-999
但是服務器可以忽略此請求頭，如果無條件GET包含Range請求頭，響應會以狀態碼206（PartialContent）返回而不是以200（OK）。
在ios中使用NSMutableURLRequest來定義頭部域
NSURL *url1=[NSURL URLWithString:@"下載地址";
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);
}
