I want to use [FBPlacePickerViewController loadData], and do some logic on the data received, before using the view.
How can I see the location information received in the loadData method?
You may look into using the FBPlacePickerDelegate: https://developers.facebook.com/docs/reference/ios/3.1/protocol/FBPlacePickerDelegate
It's not quite clear what you want to do, but if you're looking to filter the places, you can check out the placePickerViewController:shouldIncludePlace: delegate method.
Related
In my app delegate I save(persist) an object when the applicationWillResignActive: method is called. I have a view controller which responds to UIApplicationWillResignActiveNotification and makes a modification to the previously mentioned object but doesn't save(persist) the object itself.
What I need to know is if that change will be persisted trough the applicationWillResignActive: method or if I should explicitly save it in the view controller just to be sure?
Edit: NSLog test result is: 1st is the delegate method, 2nd it is the Notification method.
I wasn't sure the test would guarantee me that the order will be the same every time because I don't know how notification center works. That is why I posted the question before testing and giving results.
The documentation for applicationWillResignActive: says, "After calling this method, the app also posts a UIApplicationWillResignActiveNotification notification to give interested objects a chance to respond to the transition."
The "after" makes me believe that you will not get the changed value inside applicationWillResignActive:.
Of course, inserting a couple of NSLog lines would let you prove the order of operation for yourself.
I have the following code below that loops through an array. I need to check if the finish or fail selector has been called iterating to the next object in my dataArray.
for (id object in dataArray) {
[client setDidFinishSelector:#selector(getDataFinish:)];
[client setDidFailSelector:#selector(getDataFail:)];
[client getData:object];
}
In my getDataFinish method I assign values and I am trying to keep it in order. If I use the above method, the values can get out of order since the client response time can be different for each request..
I see two possible solutions, depending on what you're actually trying to do. It sounds like you're making calls to the internet, so yes you will get varied response time (or no response at all). Because of this, I would recommend using NSNotification. See this answer for more information about that.
Another option is making a flag in your code (AKA a BOOL) that you set to YES when your method has completed. Again, if you're making calls to the web I would not recommend this method as you are setting yourself up for an infinite loop if the user has no service and the BOOL never changes.
If you are still having trouble let me know and I can provide a more detailed answer.
I'm dealing with a design Problem right now. I'm writing a Swift App, wich has a Storyboard with multiple ViewControllers and a Class HTTPRequests, wich performs GET and POST Requests on a RESTful Webservice.
My Problem right now is as following. From a view controller, I have to perform a a GET Request on the Webservice to fetch data. The received Data should be displayed trough this view controller on the specific view.
I see two possibilities to achieve this right now:
Put the Request function into the ViewController. Perform an asynchronus request and update the view in this asynchronus task as soon as data are received.
I don't really like this approach for two reasons. First, I don't want the Request function inside the ViewController, because I need the same function in multiple ViewControllers and this leads to code duplication. Second, I don't want to update the view inside the asynchronus task.
Put the Request function into the HTTPRequests (Singleton) Class. Call the requests from a ViewController and get a return value back. With this value, the ViewController is able to update its view.
I like this approach, because it's in design concerns "cleaner". My only problem is the return value. How can I return a value from an asynchronus task in Swift?
If possible please with code examples.
Or is there a better approach than those two?
Edit:
Thanks to your answers and this Blog Post I could successfully implement approach 2 with delegates.
You don't have to use a singleton, but rather you can have a separate class (model) that does all the web requests. You can implement a protocol in the model and set you view controller to be delegates. Sorry I'm on mobile so I can't include code, but I'd be glad to add it later.
I am making an object that goes to download stuff for all of my view controllers. The object is singleton instance and has a callback method with received data once the download is completed. It also has a delegate property so that it knows which object to call back to after the download is done.
There are multiple controllers that use this shared instance, and my question is how to call back to the correct view controller that requested the download.
My approach is to use delegation, but the problem is that since other view controllers are also its delegate, the download object could call back to every object and this will be hard to track.
I've worked on projects where people have attempted to use multiple delegates and it's basically a bad idea. The delegate pattern is about a 1 to 1 relationship between a class and it's delegate. Whilst it is possible to achieve some level of multiple delegation through switching the delegates in and out, it's more likely to lead to unpredictable behaviour and bugs.
My recommendation would be to change how you are thinking about this. You have two options as I see it:
Switch to an Observer pattern where you can register multiple observers which your main class can interact with. This is useful where your observers all implement the same protocol and where your main class wants to be aware of the observers and interaction with them.
Broadcast NSNotifications to indicate state changes and events. Here is a more decoupled approach because the main class does not need to know who is listening and does not directly interact with them. Other can start and stop being notified at their leisure. It also has the advantage that you do not need to create or implement a separate protocol. Instead you register the classes that need to know about changes with the NSNotificationCenter which in turns handles all the routing of notifications for you.
It actually sounds like the delegate pattern might not be the best approach here.
I would look into NSNotificationCenter instead.
The basic idea is that your singleton doing the net connection posts a notification (with something like postNotificationName:object:userInfo:) , saying that new data is available. Within this notification, you can pass a dictionary object (userInfo) that holds the data you've fetched, or info on what parts of your Model contain updated data.
Then, your other view controllers can register themselves to 'observe' these notifications by calling addObserver:selector:name:object:. Generally speaking, when a vc becomes visible I call addObserver, and removeObserver when it's being hidden or transitioned out.
Good luck!
Delegation doesn't seem like the right solution to this problem. How about requiring the requesting view controller to provide an object (its self) and a selector for you to call as a completion notification? Of course, you'll need a place to store that object and selector until the download completes. Hopefully you have (or could create) an object for this.
i recommend to use one of these ways
observer:
when use data that you want to inform other object are near to primitive ones.for example when you are using 'NSMutableArray' you can not inform the change in one of object by the standard implemented pattern at least you need to implement one for your self that is not reusable that much
Notification
when your interaction with destination object (those need to be inform) is in one-way.it means you don't need any acknowledge or other data back from them.
delegate
when there is one object to inform at each time step.
note:block use for success and fail is not a pattern to broadcast data its about to queue task when you don't know when they are finishing or failing like network operations
EDIT:
how to create notification | multi delegate issues and implementation
While I agree with most of the answers here, if you did actually want to achieve multiple delegates you could potentially declare an array of delegates and send messages to all delegates within that array. If your protocol has optional delegate methods you safely check using responds(to aSelector: Selector!) -> Bool before invoking (being mindful of memory management, as those delegates would be strongly referenced in the array). Again I do agree that multiple delegates is likely a bad architectural idea and using blocks or notification center would suit your needs better.
One approach, which works for me if you only have one other object to forward messages to is to create a forwardingDelegate This does not end up with issues of hard to debug ordering of delegates and it does not unnecessarily create a dependency on the other object. Keep in mind, if you have many objects then this might not be the best approach, it is mainly for one additional object but this could be extended to support an array of objects so long as there is one that receives the SDK and forwards it to the other objects [1]. Note that every method that is needed for the forwarded object needs to pass it along, even if it is not used by the forwarding object.
For example, if I need to forward the messages coming from the mapView delegate:
- (void)mapView:(MKMapView *)mapView regionDidChangeAnimated:(BOOL)animated
{
// handle this object here.
if ([self.forwardingDelegate respondsToSelector:#selector(mapView:regionDidChangeAnimated:)])
{
[self.forwardingDelegate mapView:mapView regionDidChangeAnimated:animated];
}
// or handle this object here.
}
[self.forwardingDelegate mapView:mapView regionDidChangeAnimated:animated];
The forwarding property would be declared like this.
#property (nonatomic) id<MKMapViewDelegate> forwardingDelegate;
And the other object would adopt the protocol as if it were receiving the original message.
[1] The array approach for multiple delegates may get tricky because then you don't have as much control over what order the delegates get called, as was mentioned in other posts.
I've read that an object can only have one delegate at once.
But is that really true?
Let's say I make an object with a protocol and from that object I want to gather a lot of data from several other objects. I add every object that conforms to my protocol to an array. Then I just loop through it and call my methods on every delegate.
NSMutableArray *collectFromDelegates = [NSMutableArray alloc]init];
//in delegateArray I keep pointers to every delegate.
for(id delegate in delegateArray){
[collectFromDelegates addObject:[delegate someProtocolMethod]];
}
Is this wrong?
That's not really delegation.
Delegation is a simple and powerful pattern in which one object in a program acts on behalf of, or in coordination with, another object. The delegating object keeps a reference to the other object—the delegate—and at the appropriate time sends a message to it. The message informs the delegate of an event that the delegating object is about to handle or has just handled.
It doesn't make much sense to have more than one object handle an event for you, since it has already been handled. The only reason I could see to have multiple delegates is that if the first fails to handle an event, it can be passed to the next, continuing until some object handles it.
In your example, the objects are acting as data sources. This makes more sense than multiple delegates, but could easily be implemented by having a single data source combine data from multiple objects, which means the object asking for the data doesn't have to worry about how to combine it.
The other case where you would often want multiple objects is receiving notifications of an event. This is not delegation because the objects are not working for the object, just acting on something that happened to the object. This is better implemented using notifications or observing.
Apple's convention is to only have one "delegate" object. But you can set up your own class to have an array of delegates if that's what you need. You might want to call them something else for clarity.
In your example, calling them "dataSources" might be more appropriate.
A class only really needs one delegate, if you have more than one you are solving a different problem. The delegate pattern is used to modify the behaviour of a class. Say for instance we have a Dog class which can bark, but different types of dogs bark in different way. A delegate would be one way of changing the barking behaviour.
If you need more than one you are probably more interested in OBSERVING what your class is doing, it needs to NOTIFY others of current EVENTS. As several other classes might be interested in the behaviour of one you would need an array. In iOS SDK this is already done for you with notifications. This is called the Observer pattern.
Different use cases...
I've read that an object can only have one delegate at once. But it's that really true?
Where did you read that? No, it's not true. For instance, UITableView has two delegates, one to supply the data, the other to handle actions.
A delegate is just an abstract concept - you can have as many delegates as you want. However, this is rarely required and often a poor pattern.
Apple make good use of a source and delegate pattern. Source ivars (a form of delegate) provide data, while delegate ivars are invoked for logical responses. Perhaps this is a better solution?
Alternatively you can use NSNotification to inform many listeners of a single event.
Hope this helps!
Generally, when you want to message multiple classes that are interested in what you class does, you would use NSNotifications. That will however not allow them to return data unless you allow them to send a message to the object of the notification. I'm not sure if that would be a cleaner solution though.
One approach beside the mentioned Notifications could be, that your delegate implementation holds an array of objects conforming to the protocol and calls the protocols method on this as a wrapper.