http://stackoverflow.com/questions/5210535/passing-data-between-view-controllers
This question seems to be very popular here on stackoverflow so I thought I would try and give a better answer to help out people starting in the world of iOS like me.
I hope this answer is clear enough for people to understand and that I have not missed anything.
Passing Data Forward
Passing data forward to a view controller from another view controller. You would use this method if you wanted to pass an object/value from one view controller to another view controller that you may be pushing on to a navigation stack.
For this example we will have ViewControllerA
and ViewControllerB
To pass a BOOL
value from ViewControllerA
to ViewControllerB
we would do the following.
-
in
ViewControllerB.h
create a property for theBOOL
@property(nonatomic) BOOL *isSomethingEnabled;
-
in
ViewControllerA
you need to tell it aboutViewControllerB
so use an#import "ViewControllerB.h"
Then where you want to load the view eg.
didSelectRowAtIndex
or someIBAction
you need to set the property inViewControllerB
before you push it onto nav stack.ViewControllerB*viewControllerB =[[ViewControllerB alloc] initWithNib:@"ViewControllerB" bundle:nil]; viewControllerB.isSomethingEnabled = YES;[self pushViewController:viewControllerB animated:YES];
This will set
isSomethingEnabled
inViewControllerB
toBOOL
valueYES
.
Passing Data Forward using Segues
If you are using Storyboards you are most likely using segues and will need this procedure to pass data forward. This is similar to the above but instead of passing the data before you push the view controller, you use a method called
-(void)prepareForSegue:(UIStoryboardSegue*)segue sender:(id)sender
So to pass a BOOL
from ViewControllerA
to ViewControllerB
we would do the following:
-
in
ViewControllerB.h
create a property for theBOOL
@property(nonatomic) BOOL *isSomethingEnabled;
-
in
ViewControllerA
you need to tell it aboutViewControllerB
so use an#import "ViewControllerB.h"
-
Create a the segue from
ViewControllerA
toViewControllerB
on the storyboard and give it an identifier, in this example we'll call it"showDetailSegue"
-
Next we need to add the method to
ViewControllerA
that is called when any segue is performed, because of this we need to detect which segue was called and then do something. In our example we will check for"showDetailSegue"
and if thats performed we will pass ourBOOL
value toViewControllerB
-(void)prepareForSegue:(UIStoryboardSegue*)segue sender:(id)sender{if([segue.identifier isEqualToString:@"showDetailSegue"]){ViewControllerB*controller =(ViewControllerB*)segue.destinationViewController; controller.isSomethingEnabled = YES;}}
If you have your views embedded in a navigation controller you need to change the method above slightly to the following
-(void)prepareForSegue:(UIStoryboardSegue*)segue sender:(id)sender{if([segue.identifier isEqualToString:@"showDetailSegue"]){UINavigationController*navController =(UINavigationController*)segue.destinationViewController;ViewControllerB*controller =(ViewControllerB*)navController.topViewController; controller.isSomethingEnabled = YES;}}
This will set
isSomethingEnabled
inViewControllerB
toBOOL
valueYES
.
Passing Data Back
To pass data back from ViewControllerB
to ViewControllerA
you need to use Protocols and Delegates or Blocks, the latter can be used as a loosely coupled mechanism for callbacks.
To do this we will make ViewControllerA
a delegate of ViewControllerB
. This allows ViewControllerB
to send a message back to ViewControllerA
enabling us to send data back.
For ViewControllerA
to be delegate of ViewControllerB
it must conform to ViewControllerB
's protocol which we have to specify. This tells ViewControllerA
which methods it must implement.
-
In
ViewControllerB.h
, below the#import
, but above@interface
you specify the protocol.@classViewControllerB;@protocolViewControllerBDelegate<NSObject>-(void)addItemViewController:(ViewControllerB*)controller didFinishEnteringItem:(NSString*)item;@end
-
next still in the
ViewControllerB.h
you need to setup adelegate
property and synthesize inViewControllerB.m
@property(nonatomic, weak) id <ViewControllerBDelegate>delegate;
-
In
ViewControllerB
we call a message on thedelegate
when we pop the view controller.NSString*itemToPassBack =@"Pass this value back to ViewControllerA";[self.delegate addItemViewController:self didFinishEnteringItem:itemToPassBack];
-
That's it for
ViewControllerB
. Now inViewControllerA.h
, tellViewControllerA
to importViewControllerB
and conform to its protocol.#import "ViewControllerB.h"@interfaceViewControllerA:UIViewController<ViewControllerBDelegate>
-
In
ViewControllerA.m
implement the following method from our protocol-(void)addItemViewController:(ViewControllerB*)controller didFinishEnteringItem:(NSString*)item {NSLog(@"This was returned from ViewControllerB %@",item);}
-
Before pushing
viewControllerB
to navigation stack we need to tellViewControllerB
thatViewControllerA
is its delegate, otherwise we will get an error.ViewControllerB*viewControllerB =[[ViewControllerB alloc] initWithNib:@"ViewControllerB" bundle:nil]; viewControllerB.delegate= self [[self navigationController] pushViewController:viewControllerB animated:YES];
References
- Using Delegation to Communicate With Other View Controllers in the View Controller Programming Guide
- Delegate Pattern
Further Help
There are many answers to this questions offering many different ways to perform view controller communication that would indeed work, but I don't see anywhere mentioned which one are actually best to use and which ones to avoid.
In practice, in my opinion only a few solutions are recommended:
- To pass data forward:
- override the
prepare(for:sender:)
method ofUIViewController
when using a storyboard and segues - pass data through an initializer or through properties when performing view controller transitions thtough code
- override the
- To pass data backwards
- update the app shared state (which you can pass forward between view controllers with either one of the methods above)
- use delegation
- use an unwind segue
Solutions I recommend NOT to use:
- Referencing the previous controller directly instead of using delegation
- Sharing data through a singleton
- Passing data through the app delegate
- Sharing data through the user defaults
- Passing data through notifications
These solutions, although working in the short term, introduce too many dependencies that will garble the architecture of the app and create more problems later.
For those interested, I wrote some articles that address these points more in depth and highlight the various drawbacks: