iOS处理多线程异步Block中的UI操作

多线程方法的completionHandlerblock可能运行在非主线程上。两种处理方法：

1. 在block里手动加上dispatch_async(dispatch_get_main_queue(), ^{});

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];

NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration];//no delegateQueue
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:url] cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:30.0];

NSURLSessionDownloadTask *task = [session downloadTaskWithRequest:imgRequest completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
    dispatch_async(dispatch_get_main_queue(), ^{/* do UI things */});
    //or
    [self performSelectorOnMainThread:@selector(doUIthings) withObject:nil waitUntilDone:NO];
}];

[task resume];

2. 带有delegateQueue等参数的方法，可传入主线程队列，然后blockcompletionHandler便运行在主线程了。

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];

NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:[NSOperationQueue mainQueue]];//get the mainQueue
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:url] cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:30.0];

NSURLSessionDownloadTask *task = [session downloadTaskWithRequest:imgRequest completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
/* do UI things */
}];

[task resume];