问题
What I am trying to do is a Facebook wrapper for the Facebook iOS SDK. Basically the idea is that my ViewController should do nothing but showing ex. my friends that will be acquired with a simple call like
self.friends = [FacebookWrapper myFriends];
[self.tableView reloadData];
My wrapper myFriends method should look like this
+ (NSArray *)myFriends
{
__block NSArray *friends = nil;
[FBSession openActiveSessionWithReadPermissions:nil allowLoginUI:YES completionHandler:^(FBSession *session, FBSessionState status, NSError *error) {
if(FB_ISSESSIONOPENWITHSTATE(status)) {
[FBRequestConnection startForMyFriendsWithCompletionHandler:^(FBRequestConnection *connection, id data, NSError *error) {
CFRunLoopStop(CFRunLoopGetCurrent());
if(error) {
return;
}
NSArray *friendsData = (NSArray *)[data data];
NSMutableArray *fbFriends = [NSMutableArray array];
for(id friendData in friendsData) {
Friend *friend = [Friend friendWithDictionary:friendData];
fbFriends addObject:friend];
}
friends = [NSArray arrayWithArray:fbFriends];
}];
CFRunLoopRun();
}
}];
return friends;
}
The issue is that the openActiveSessionWithReadPermissions and startForMyFriendsWithCompletionHandler are asynchronous blocks so the method returns before the blocks complete their task.
Any help would be much appreciated.
回答1:
I created a similar wrapper in the past and my approach was passing a "completion block" when calling my wrapper method; this completion block is then triggered once all the asynchronous calls are done running, and it receives whatever data your method would return in a synchronous scenario (in your case, the array of friends).
To illustrate - you could have your "myFriends" method redefined as:
+ (void)myFriendsWithCompletionBlock:(void (^)(NSArray *friends))completionBlock;
Then in the implementation, right after the friends = [NSArray arrayWithArray:fbFriends]; line, you would add this:
if (completionBlock != nil) {
completionBlock(friends);
}
... and remove the return statement at the end.
Finally, on your view controller (or any object using the method, you would do something like this:
[FacebookWrapper myFriendsWithCompletionBlock:^(NSArray *friends){
// do what you need to do with the friends array
}];
Of course, this is still asynchronous - but there's no way around since that's how the Facebook SDK was build (and, to be fair, this is probably the best way to do it - waiting for requests to finish synchronous would be terrible!)
Edit: I noticed you're also returning from the wrapper method in case it fails; in that situation, instead of returning you would do something like this:
if (completionBlock != nil) {
completionBlock(nil);
}
That would cause the friends array to be nil when your completion block is called - you can then treat that error there however seems appropriate to you.
Hope this helped!
回答2:
If you are dispatching an asynchronouos block, you can communicate with your UIViewController subclass by calling back to it:
[self someSelectorWithCallbackData:stuffWhichYouWantToGiveBack];
This will call self to get captured by the block, and so will work as expected. From the relevant method you can refresh the view / reload the tableview / dance a jig as required.
Depending on the context, you may need to __block scope self, eg
__block UIViewController *bsself = self;
But if you do the latter, be careful to avoid a retain loop (the build and analysis tools are fairly good at pointing this out).
回答3:
Think you need to use a protol @class Webservice;
@protocol WebserviceDelegate
@optional
-(void)webservice:(Webservice *)webservice didFetchPosts:(NSArray *)posts;
-(void)webservice:(Webservice *)webservice didFetchComments:(NSArray *)comments forPostID:(NSString *)postID launchComments:(BOOL)launch;
-(void)webservice:(Webservice *)webservice didLoginWithUser:(User *)user;
-(void)webservice:(Webservice *)webservice didVoteWithSuccess:(BOOL)success forObject:(id)object direction:(BOOL)up;
@end
@interface Webservice : NSObject {
__weak id <WebserviceDelegate> delegate;
}
//Delegate
@property (weak) id <WebserviceDelegate> delegate;
-(void)getHomepage {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSURLResponse *response;
NSError *error;
// Create the URL Request
NSMutableURLRequest *request = [Webservice NewGetRequestForURL:[NSURL URLWithString:@"https://www.hnsearch.com/bigrss"]];
// Start the request
NSData *responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
//Handle response
//Callback to main thread
if (responseData) {
NSString *responseString = [[NSString alloc] initWithData:responseData encoding:NSStringEncodingConversionAllowLossy];
if (responseString.length > 0) {
dispatch_async(dispatch_get_main_queue(), ^{
[self parseIDsAndGrabPosts:responseString];
});
}
else {
dispatch_async(dispatch_get_main_queue(), ^{
[delegate webservice:self didFetchPosts:nil];
});
}
}
else {
dispatch_async(dispatch_get_main_queue(), ^{
[delegate webservice:self didFetchPosts:nil];
});
}
});
}
-(void)parseIDsAndGrabPosts:(NSString *)parseString {
// Parse String and grab IDs
NSMutableArray *items = [@[] mutableCopy];
NSArray *itemIDs = [parseString componentsSeparatedByString:@"<hnsearch_id>"];
for (int xx = 1; xx < itemIDs.count; xx++) {
NSString *idSubString = itemIDs[xx];
[items addObject:[idSubString substringWithRange:NSMakeRange(0, 13)]];
}
// Send IDs back to HNSearch for Posts
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSURLResponse *response;
NSError *error;
// Create Request String
NSString *requestString = @"http://api.thriftdb.com/api.hnsearch.com/items/_bulk/get_multi?ids=";
for (NSString *item in items) {
requestString = [requestString stringByAppendingString:[NSString stringWithFormat:@"%@,", item]];
}
// Create the URL Request
NSMutableURLRequest *request = [Webservice NewGetRequestForURL:[NSURL URLWithString:requestString]];
// Start the request
NSData *responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
//Handle response
//Callback to main thread
if (responseData) {
NSArray *responseArray = [NSJSONSerialization JSONObjectWithData:responseData options:NSJSONReadingAllowFragments error:&error];
if (responseArray) {
NSMutableArray *postArray = [@[] mutableCopy];
for (NSDictionary *dict in responseArray) {
[postArray addObject:[Post postFromDictionary:dict]];
}
NSArray *orderedPostArray = [self orderPosts:postArray byItemIDs:items];
dispatch_async(dispatch_get_main_queue(), ^{
[delegate webservice:self didFetchPosts:orderedPostArray];
// Update Karma for User
if ([HNSingleton sharedHNSingleton].User) {
[self reloadUserFromURLString:[NSString stringWithFormat:@"https://news.ycombinator.com/user?id=%@", [HNSingleton sharedHNSingleton].User.Username]];
}
});
}
else {
dispatch_async(dispatch_get_main_queue(), ^{
[delegate webservice:self didFetchPosts:nil];
});
}
}
else {
dispatch_async(dispatch_get_main_queue(), ^{
[delegate webservice:self didFetchPosts:nil];
});
}
});
}
来源：https://stackoverflow.com/questions/15134394/method-returning-value-from-asynchronous-block-with-facebooksdk