我正在尝试从 XML 提要中存储的 URL 下载多个图像。从 XML 获取图像 url 工作正常。但是，NSURLConnection 正在创建空文件，但数据已按照 NSLog 中的说明接收。在connectionDidFinishLoading:(NSURLConnection *)connection，收到数据和正确的字节，问题是如何使收到的数据写入正确的文件。
半工作代码：
-(void)parsingComplete:(XMLDataSource*)theParser
{
/* iterate through the Categories and create the
sub-directory if it does not exist
*/
for (int i = 0; i < [categories count]; i++) {
NSString *cat = [NSString stringWithFormat:@"%@/%@",BASE_DIR,[[categories objectAtIndex:i] objectForKey:@"name"]];
NSString *catName = [[categories objectAtIndex:i] objectForKey:@"name"];
NSArray *catArray = [[categories objectAtIndex:i] objectForKey:@"images"];
/* create the sub-direcotry naming it the #category# key */
if (![FILEMANAGER fileExistsAtPath:cat]) {
[FILEMANAGER createDirectoryAtPath:cat withIntermediateDirectories:NO attributes:nil error:nil];
}
//NSLog(@"\n\nCategory: %@",cat);
for (int x = 0; x < [catArray count]; x++) {
//NSLog(@"Image: %@",[[catArray objectAtIndex:x] objectForKey:@"imageUrl"]);
/* download each file to the corresponding category sub-directory */
fileOut = [NSString stringWithFormat:@"%@/%@_0%i.jpg",cat,catName,x];
NSURLRequest *imageRequest =
[NSURLRequest requestWithURL:[NSURL URLWithString:[[catArray objectAtIndex:x] objectForKey:@"imageUrl"]]
cachePolicy:NSURLRequestReloadIgnoringLocalCacheData timeoutInterval:30.0];
NSURLConnection *imageConnection = [[NSURLConnection alloc] initWithRequest:imageRequest delegate:self];
int counter = 0;
//BOOL result = NO;
if(imageConnection)
{
NSLog(@"Counter: %i",counter++);
receivedData = [[NSMutableData data] retain];
/*result = */[receivedData writeToFile:fileOut atomically:YES];
}
/*
if (!result) NSLog(@"Failed"); else NSLog(@"Successful");
*/
}
}
}
#pragma mark NSURLConenction
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
[receivedData setLength:0];
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
[receivedData appendData:data];
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
// release the connection, and the data object
[connection release];
// receivedData is declared as a method instance elsewhere
[receivedData release];
// inform the user
NSLog(@"Connection failed! Error - %@ %@",
[error localizedDescription],
[[error userInfo] objectForKey:NSErrorFailingURLStringKey]);
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
// do something with the data
// receivedData is declared as a method instance elsewhere
NSLog(@"Succeeded! Received %d bytes of data",[receivedData length]);
NSString *aStr = [[NSString alloc] initWithData:receivedData encoding:NSASCIIStringEncoding];
// release the connection, and the data object
//[receivedData release];
}
您必须等到连接告诉您连接已完成，然后才能写入数据。连接在另一个线程上处理；如果您尝试立即在原始线程上访问数据，那么其中将不会有任何内容。
你应该移动writeToFile:调用到末尾connectionDidFinishLoading:，或从那里调用的另一个方法。这是您知道数据已全部收集的第一点。
我还建议创建NSMutableData实例在didRecieveResponse:，以便您知道它在正确的时间可用。这将更具可读性/易于理解。您可以将委托方法视为一个集体“范围”——数据仅在其中使用，因此应该在其中一个方法内部创建。
回复您的评论：
一种可能性是，由于您有很多需要围绕这一下载完成的工作，并且似乎没有触及 GUI，因此运行整个parsingComplete:后台线程上的方法，并使用+[NSURLConnection sendSynchronousRequest:returningResponse:error:]。这样，您的代码将只等待数据返回，并且您可以在数据返回后立即编写它sendSynchronous...呼叫返回。
NSError * err;
NSURLResponse * response;
NSData * receivedData = [NSURLConnection sendSynchronousRequest:imageRequest
returningResponse:&response
error:&err];
if( !receivedData ){
/* Handle error */
}
/* Check response */
BOOL result = [receivedData writeToFile:fileOut atomically:YES];
/* check result, etc. */
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