在执行其他代码之前等待异步请求

Question

我正在使用[NSURLSession sharedSession] dataTaskWithRequest从Web服务器获取数据，并将其显示在标签&根据它设置按钮图像但我的问题是，首先执行标签代码，它们显示为空，然后完成异步块代码。

if(![self connected])
{
NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
arrMyres = [[NSMutableArray alloc] initWithArray:[prefs objectForKey:@"Myres"]];
}
else
{
// POSTrequest with URL to get data from server
[[[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:
^(NSData * _Nullable responseData,
NSURLResponse * _Nullable urlResponse,
NSError * _Nullable error) {
if (error) {
//Display error
}
else
{
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)urlResponse;
if([httpResponse statusCode ]>=200 && [httpResponse statusCode]<300)
{
dispatch_async(dispatch_get_main_queue(), ^{
NSDictionary *dataDictionary = [NSJSONSerialization JSONObjectWithData:responseData options:0 error:&error];

NSArray *array=[[dataDictionary objectForKey:@"GetExistingFavorites"] isKindOfClass:[NSNull class]]? nil:[dataDictionary objectForKey:@"GetExistingFavorites"];

arrMyres=[[NSMutableArray alloc]initWithArray:array];
});
}
}
}] resume];
}
//This block of code executes first and displays empty label
if([arrMyres count]>0 )
{
//Set Button Image and display label 
}
else
{ 
// Do something else
} 

如何等待异步请求完成执行并在它之后的某个地方使用它的结果？在研究过程中，我发现了调度组和完成处理程序。但无法理解如何实现

任何建议都会很有帮助。

Answer 1

使用主线程更新异步代码内部的firstLabelUI代码。参考以下代码

if(![self connected])
{
NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
arrMyres = [[NSMutableArray alloc] initWithArray:[prefs objectForKey:@"Myres"]];
[self updateUI];
}
else
{
// POSTrequest with URL to get data from server
[[[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:
^(NSData * _Nullable responseData,
NSURLResponse * _Nullable urlResponse,
NSError * _Nullable error) {
if (error) {
//Display error
}
else
{
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)urlResponse;
if([httpResponse statusCode ]>=200 && [httpResponse statusCode]<300)
{
dispatch_async(dispatch_get_main_queue(), ^{
NSDictionary *dataDictionary = [NSJSONSerialization JSONObjectWithData:responseData options:0 error:&error];
NSArray *array=[[dataDictionary objectForKey:@"GetExistingFavorites"] isKindOfClass:[NSNull class]]? nil:[dataDictionary objectForKey:@"GetExistingFavorites"];
arrMyres=[[NSMutableArray alloc]initWithArray:array];
//This block of code executes first and displays empty label
if([arrMyres count]>0 )
{
[self updateUI];
}
}
}
}] resume];
}
-(void)updateUI {
dispatch_async(dispatch_get_main_queue(), ^{
//update UI in main thread.
//Add your ui updates
});
}