在不使用GCD下載情況：
- (void)btnPress:(id)sender{
self.labContent.text = @"";
self.indicator.hidden = NO;
[self.indicator startAnimating];
NSOperationQueue *que = [[NSOperationQueue alloc] init];
NSInvocationOperation *op = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(download) object:nil];
[que addOperation:op];
self.queue = que;
}
- (void)download{
NSURL *url = [NSURL URLWithString:@"http://www.baidu.com"];
NSError *error;
NSString *strData = [NSString stringWithContentsOfURL:url
encoding:NSUTF8StringEncoding
error:&error];
if (strData && strData != nil) {
[self performSelectorOnMainThread:@selector(download_completed:) withObject:strData waitUntilDone:NO];
}else {
NSLog(@"error when download:%@", error);
}
}
- (void)download_completed:(NSString *)strData{
NSLog(@"call back");
[self.indicator stopAnimating];
self.indicator.hidden = YES;
self.labContent.text = strData;
}
使用GCD後代碼簡潔多了：
- (void)btnPress:(id)sender{
self.labContent.text = @"";
self.indicator.hidden = NO;
[self.indicator startAnimating];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSURL *url = [NSURL URLWithString:@"http://www.baidu.com"];
NSError *error;
NSString *strData = [NSString stringWithContentsOfURL:url
encoding:NSUTF8StringEncoding
error:&error];
if (strData && strData != nil) {
dispatch_async(dispatch_get_main_queue(), ^{
[self.indicator stopAnimating];
self.indicator.hidden = YES;
self.labContent.text = strData;
});
}else {
NSLog(@"error when download:%@", error);
}
});
}
GCD參考資料：http://mobile.51cto.com/iphone-403490.htm