How to deal with a Data Flow between Controller1 and Controller2

by : stackoverflow

There are various ways by which a data can be received to a different class in iOS. For example -

1. Direct initialization after the allocation of another class.
2. Delegation - for passing data back
3. Notification - for broadcasting data to multiple classes at a single time
4. Saving in NSUserDefaults - for accessing it later
5. Singleton classes
6. Databases and other storage mechanisms like plist, etc.

But for the simple scenario of passing a value to a different class whose allocation is done in the current class, the most common and preferred method would be the direct setting of values after allocation. This is done as follows:-

We can understand it using two controllers - Controller1 and Controller2

Suppose in Controller1 class you want to create the Controller2 object and push it with a String value being passed. This can be done as this:-

- (void)pushToController2 {

    Controller2 *obj = [[Controller2 alloc] initWithNib:@"Controller2" bundle:nil];
    [obj passValue:@"String"];
    [self pushViewController:obj animated:YES];
}

In the implementation of the Controller2 class there will be this function as-

@interface Controller2  : NSObject

@property (nonatomic , strong) NSString* stringPassed;

@end

@implementation Controller2

@synthesize stringPassed = _stringPassed;

- (void) passValue:(NSString *)value {

    _stringPassed = value; //or self.stringPassed = value
}

@end

You can also directly set the properties of the Controller2 class in the similar way as this:

- (void)pushToController2 {

    Controller2 *obj = [[Controller2 alloc] initWithNib:@"Controller2" bundle:nil];
    [obj setStringPassed:@"String"];  
    [self pushViewController:obj animated:YES];
}

To pass multiple values you can use the multiple parameters like :-

Controller2 *obj = [[Controller2 alloc] initWithNib:@"Controller2" bundle:nil];
[obj passValue:@“String1” andValues:objArray withDate:date]; 

Or if you need to pass more than 3 parameters which are related to a common feature you can store the values to a Model class and pass that modelObject to the next class

ModelClass *modelObject = [[ModelClass alloc] init]; 
modelObject.property1 = _property1;
modelObject.property2 = _property2;
modelObject.property3 = _property3;

Controller2 *obj = [[Controller2 alloc] initWithNib:@"Controller2" bundle:nil];
[obj passmodel: modelObject];

So in-short if you want to -

1) set the private variables of the second class initialise the values by calling a custom function and passing the values.
2) setProperties do it by directlyInitialising it using the setter method.
3) pass more that 3-4 values related to each other in some manner , then create a model class and set values to its object and pass the object using any of the above process.

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Passing Data Forward using Segue's

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:

1. in ViewControllerB.h create a property for the BOOL

   @property(nonatomic) BOOL *isSomethingEnabled;

2. in ViewControllerA you need to tell it about ViewControllerB so use an

   #import "ViewControllerB.h"

3. Create a the segue from ViewControllerA to ViewControllerB on the storyboard and give it an identifier, in this example we'll call it "showDetailSegue"

4. 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 our BOOL value to ViewControllerB

   -(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 in ViewControllerB to BOOL value YES.

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.

1. In ViewControllerB.h, below the #import, but above @interface you specify the protocol.

   @class ViewControllerB;
   
   @protocol ViewControllerBDelegate <NSObject>
   - (void)addItemViewController:(ViewControllerB *)controller didFinishEnteringItem:(NSString *)item;
   @end

2. next still in the ViewControllerB.h you need to setup a delegate property and synthesize in ViewControllerB.m

   @property (nonatomic, weak) id <ViewControllerBDelegate> delegate;

3. In ViewControllerB we call a message on the delegate when we pop the view controller.

   NSString *itemToPassBack = @"Pass this value back to ViewControllerA";
   [self.delegate addItemViewController:self didFinishEnteringItem:itemToPassBack];

4. That's it for ViewControllerB. Now in ViewControllerA.h, tell ViewControllerA to import ViewControllerB and conform to its protocol.

   #import "ViewControllerB.h"
   
   @interface ViewControllerA : UIViewController <ViewControllerBDelegate>

5. In ViewControllerA.m implement the following method from our protocol

   - (void)addItemViewController:(ViewControllerB *)controller didFinishEnteringItem:(NSString *)item
   {
       NSLog(@"This was returned from ViewControllerB %@",item);
   }

6. The last thing we need to do is tell ViewControllerB that ViewControllerA is its delegate before we push ViewControllerB on to nav stack.

   ViewControllerB *viewControllerB = [[ViewControllerB alloc] initWithNib:@"ViewControllerB" bundle:nil];
   viewControllerB.delegate = self
   [[self navigationController] pushViewController:viewControllerB animated:YES];

7.