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Passing data between view controllers
(45 answers)
Closed 8 years ago.
I have 2 views, ParentViewController and ChildViewController; I want to nest ChildViewController inside ParentViewController. I have designed ParentViewController and ChildViewController in Storyboard. ParentViewController.m contains the logic for the parent and ChildViewController.m contains the logic for the child. In ParentViewController.m I add the child like so:
ChildViewController *childVC = [self.storyboard instantiateViewControllerWithIdentifier:#"ChildSBI"];
[self addChildViewController:childVC];
My Question: How can I receive information (like an NSString) from the child back to the parent? Should I do this through delegation?
A common pattern would be to have the child as a property of the parent, and have the parent be a delegate of the child. The first thing you need to do is make your own protocol.
// MyProtocol.h
- (void)heyParentSomethingHappened:(Something)something;
Next, make the child a property of the parent so they can talk through delegation.
// ParentVC.m
#interface ParentVC()
#property (nonatomic) ChildVC *child
#end
Now that the parent has the child as a property, they need some way to talk. This is where the delegation comes in. Have the parent conform to MyProtocol.
// ParentVC.h
#interface ParentVC : UIViewController <MyProtocol>
Now that the parent conforms to your special protocol, have the child make it a delegate.
//ChildVC.h
#interface ChildVC : UIViewController
#property (nonatomic) id <MyProtocol> delegate.
#end
Now that the child has the delegate property, set it to the parent and you are good to go.
// ParentVC.m
- (id)init {
// do your init
self.child.delegate = self // both (child.delegate, self) conform to <MyProtocol>, so no type mismatch.
}
Now when your child needs to alert your parent of something, they have a formal way of talking through the protocol + delegation.
// ChildVC.m
- (void)someFunc {
[self.delegate heyParentSomethingHappend:[Something new]];
}
Remember to always include the protocol file when using it.
Related
EDIT: edited for clarity
Disclaimer: I'm new and pretty bad. But I have tried very hard and read lots of stuff to figure this out, but I have not...
I think my whole delegate pattern would work, except I can't figure out how to set the delegate property of ViewController to self in the MatchLetter class. The reason is because I can't figure out how to call code there. It's not a view controller, so viewDidLoad or prepareForSegue won't work.
This is what I've got:
ViewController.h
#import <UIKit/UIKit.h>
#class ViewController;
#protocol letterMatchProtocol <NSObject>
- (BOOL) isLetterMatch:(char) firstLetter;
#end
#interface ViewController : UIViewController
#property (nonatomic, weak) id <letterMatchProtocol> delegate;
#end
ViewController.m
#import "ViewController.h"
#interface ViewController ()
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
char c = 'a';
// This is the method I want to delegate to MatchLetter, to have a BOOL returned
BOOL returnValue = [self.delegate isLetterMatch:c];
}
#end
MatchLetter.h
#import <Foundation/Foundation.h>
#import "ViewController.h"
#interface Delegate : NSObject <letterMatchProtocol>
#end
MatchLetter.m
#import "MatchLetter.h"
#implementation Delegate
// this is the code I think I need to run here, to set the delegate property...
// ViewController *viewController = [ViewController new];
// viewController.delegate = self;
// ... so that isLetterMatch can be run here from ViewController.m
// But I don't know where to put this code, or how to get it to run before the ViewController
// especially since there are no segues or views to load.
- (BOOL) isLetterMatch:(char)firstLetter {
if (firstLetter == 'a') {
return YES;
}
else {
return NO;
}
}
#end
Can somebody please tell me the best way to proceed? Thanks for reading
You asked "Where to set delegate = self? Or should I just use a different design pattern?".
Answer: Don't. An object should never be it's own delegate.
Your code is quite a mess.
Don't name a class "Delegate". A delegate is a design pattern. The whole point of a delegate is that any object that conforms to a particular protocol ("speaks the language") can serve as the delegate. You don't need to know what class of object is serving as the delegate, but only that it speaks the language you need.
An analogy: When you call the operator, you don't care who is working the operator desk. You don't care about his/her gender, religion, ethnic background, how tall they are, etc. You just care that they speak your language.
Likewise, when you set up a delegate, it doesn't matter what type of object gets set as the delegate. All that matters is that the object that is the delegate conforms to the protocol for that delegate.
A table view can have ANY object serve as it's delegate, as long as that object conforms to the UITableViewDelegate protocol. You usually make you view controller be the table view's delegate, but you don't have to. You could create a custom class that manages your table views, and have it be the delegate. There is no "TableViewDelegate" object class. There is instead a UITableViewDelegate protocol, and any object that conforms to the protocol can act as a table view's delegate.
Edit: Your question is confusing. I think what you're proposing is that your Delegate class would create a view controller and make itself the delegate for the view controller.
If that's what you are talking about, your thinking is backwards. The view controller is using the Delegate class as a helper class. Any given instance of a view controller class can create an instance of the Delegate class and set it as it's delegate if it desires. You might have 3 instances of ViewController at one time, each with it's own instance of your Delegate class.
Thus, the ViewController object is the one that should create and set up an instance of Delegate if it needs one:
- (void) viewDidLoad;
{
self.delegate = [[Delegate alloc] init];
//other setup here
}
When the views are simple, their IBActions and IBoutlets are in viewcontroller, viewcontrollers assigns respective models to be loaded and viewcontroller get notified when models are prepared.
As My project contains lot of custom views for each viewcontroller, I want to implement actions in custom view itself and set data from controller (ViewController).
I should be able to use the same controllers and models for both iPhone and iPad where only UI changes.
I am concerned about how to pass data from view to viewcontroller and displaying data back on view when model changes?
Can anyone please suggest me to pass data between views <---> viewcontroller (controller) <---> model?
To do this I use Delegate design-pattern. It looks like this :
MyView.h
#protocol MyViewDelegate <NSObject>
- (void)customViewDidSomething;
#end
#interface MyView : UIView
#property (nonatomic, assign) id<MyViewDelegate> delegate
#end
MyView.m
- (void)userDidSomething {
[_delegate customViewDidSomething];
}
MyViewController.h
#import "MyView.h"
// ViewController has to implement the protocol
#interface MyViewController <MyViewDelegate>
#property (nonatomic, retain) IBOutlet MyView myView;
MyViewController.m
- (void)viewDidLoad { // Set the delegate somewhere
_myView.delegate = self
}
- (void)customViewDidSomething {
// Ok VC is aware that something happened
// Do something (tell subview to do something ?)
}
Instead of using different custom views, try using a UIViewController and then use the viewcontroller's view to display your UI. Also, this will also ensure that you will be able to communicate between the views and controller efficiently without confusion.
I have a UIViewController which contains a UIView Subclass, from the subclass I want to call a method defined in the UIViewController which contains it. I do not want to instantiate a new instance of the view controller, because it contains information that I need within the method I am attempting to call. Here is a diagram trying to further clarify:
(UIViewController) MainView --> has method updateView
(UIView) SubView ---> Has Button that plays animation and has completion block
I want to call UpdateView in the completion block
I think you can set up a protocol in your Subview, which can be implemented by your ViewController
Your SubView.h
#class SubView;
#protocol SubViewDelegate <NSObject>
- (void)actionToPromoteToViewController: (NSString *)exampleString isSelected:(BOOL)exampleBool;
#end
Then, in your ViewController.h:
#interface MainViewController : UIViewController <SubViewDelegate>
and your ViewController.m, implement the method.
- (void)actionToPromoteToViewController: (NSString *)exampleString isSelected:(BOOL)exampleBool{
// Method Implementation
}
For 'correct' implementation you need a reference to view controller in your UIView Subclass:
#interface UIViewSubclass : UIView
...
#property UIViewControllerSubclass *viewController;
#end
Then set this viewController reference to your view controller and use it in completion block.
If you want a local solution (and not to extend UIViewSubclass with property) take a look at this answer: https://stackoverflow.com/a/3732812/326017
The title is what I think I need but i will go back one step. I want to create a class which handles certain things in an iOS app. This class might be called by multiple UIViewcontrollers in an iOS app. The class may need to show a UIView at some stage for user input. So my question is how can I show a UIView when I don't know which subclass of UIViewController is calling it? To what can I add the UIView from this class?
I suppose there are two possible answers either the class finds the current UIViewController or the calling subclass of UIViewController passes itself to the class so the class knows.
How is this supposed to be done.
Thanks guys for your help.
I'm going to expand on #ericleaf's comment regarding using a protocol and subclasses. It sounds like you are asking the following:
How can I create a resusable, generic class that presents a view
within a UIViewController subclass?
A great way to do this is to define a protocol in your generic class and have your view controller subclasses support this protocol. The protocol defines an interface for your custom class to comunicate with it's delegate, in this case a UIViewController subclass. Other than the protocol, the objects don't need to know anything else about the implementation of each other.
Any information your custom object needs to be able to present views within it's delegate would be passed via protocol methods. The specifics of the protocol are up to you based on your needs. You could have the custom object "ask" the delegate for information (e.g. what view should I put a subview in?) or you could have the protocol provide information to the delegate and let the delegate deal with it (e.g. here is a subview you can put wherever you want).
There is a lot of great documentation on protocols available on SO and elsewhere. This is long enough already so I kept the example fairly simple.
custom class .h file with protocol definition
// my custom class that adds adds a view to a view controller that supports it's protocol
// forward class definition for the protocol
#class MyAwesomeObject;
#protocol MyAweseomeObjectDelegate <NSObject>
- (UIView *)viewForMyAwesomeObject:(MyAwesomeObject *)awesomeObject;
#end
// this could be defined such that the delegate *must* be a UIViewController. I've left it generic.
#interface MyAwesomeClassObject : NSObject
#property (nonatomic, weak) id <MyAwesomeObjectDelegate> delegate;
#end
custom class .m file
// MyAwesomeObject.m
#import "MyAwesomeObject.h"
#implementation MyAwesomeObject
// this is a dumb example, but shows how to get the view from the delegate
// and add a subview to it
- (void)presentViewInDelegate
{
UIView *containingView = [self.delegate viewForMyAwesomeObject:self];
if (containingView) {
UIView *subview = [[UIView alloc] initWithFrame:containingView.bounds];
subview.backgroundColor = [UIColor redColor];
subview.autoresizingMask = UIViewAutoresizingFlexibleWidth|UIViewAutoresizingFlexibleHeight;
[containingView addSubview:subview];
}
}
MyViewController .h using the custom object
// MyViewController.h
#import "MyAwesomeObject.h"
#interface MyViewController : UIViewController <MyAwesomeObjectDelegate>
#property (nonatomic, strong) MyAwesomeObject *awesomeObject;
#end
MyViewController .m using the custom object
// MyViewController.m
#import "MyViewController.h"
#implementation MyViewController
- (void)init
{
self = [super init];
if (self) {
_awesomeObject = [[MyAwesomeObject alloc] init];
_awesomeObject.delegate = self;
}
return self;
}
// MyAwesomeObjectDelegate
- (UIView *)viewForMyAwesomeObject:(MyAwesomeObject *)awesomeObject
{
return self.view;
}
You can get the class into a string and do a compare.
For example, lets assume your custom UIViewController subclass is CustomViewCon and the UIViewController object reference is myUnknownClassObject, then:
NSString *classString = NSStringFromClass([myUnknownClassObject class]);
Then you can:
if([classString isEqualToString:#"CustomViewCon"]){
//do something like maybe present a particular view
myUnknownClassObject.view = myCustomView; //or anything..
}
Similarly you can check for any class.
Edit: According to the suggestions from comments, you could also do the following(better way):
if([[myUnknownClassObject class] isKindOfClass:[CustomViewCon class]]){
//same as before
}
Why wont you use a block for this?
BaseViewController.h:
#property (copy) void (^addViewBlock)();
- (IBAction)showViewWhenNeeded;
BaseViewController.m:
- (IBAction)showViewWhenNeeded
{
if (self.addViewBlock)
self.addViewBlock();
}
And in your child class, set that block's actions, and call the method when you feel like you should put up a view.
ChildViewController.m
// within some method, propably init or smth
[self setAddViewBlock:^{
[self.vied addSubView:...];
}];
// when need to actually add the view
[self showViewWhenNeeded];
I have a container view controller with 3 child UIViewController subclasses (added with addChildViewController). I want one of my child view controllers to do something when something is dropped from my container view controller onto it. I'm having trouble grasping how this communication should happen. If I try making a delegate, I get an error in my child view controller because I would both subclasses to import each other.
It sounds like you're having a problem compiling your app because of mutual .h files importing each other, right?
Edit: upon reading your question again, I'm not 100% clear on which view controller needs to call which other one. If I mixed up the
roles of parent and child view controller in my solution, just switch
them. The techniques below let you communicate between any two view
controllers (parent and child, sibling and sibling, etc.)
There's a number of ways to handle this. If you want to stay with a delegate pattern, you could simply rewrite the header to avoid the #import in one of the .h files:
ParentViewController.h:
#import "ChildViewController.h"
#interface ParentViewController: UIViewController {
#private
ChildViewController* childVc;
}
- (void) doSomething;
ChildViewController.h
#class ParentViewController; // NOT #import!
#interface ChildViewController: UIViewController {
#private
ParentViewController* parentVc;
}
ChildViewController.m
#import "ParentViewController.h"
This should avoid the circular dependency that keeps your app from compiling.
Now, although the above works, I might choose another solution, for the sake of cleanliness. Use a protocol. The parent can implement the protocol and then the child only needs to have a delegate that implements the protocol:
#import "MyProtocol.h"
#interface ParentViewController: UIViewController<MyProtocol> {
}
- (void) doSomething;
In MyProtocol.h:
#protocol MyProtocol
- (void) doSomething;
#end
Then in ChildViewController.h
#import "MyProtocol.h"
#interface ChildViewController: UIViewController {
#private
id<MyProtocol> delegate;
}
#property (nonatomic, assign) id<MyProtocol> delegate;
And in ChildViewController.m:
[delegate doSomething];
Or, you could avoid using delegates altogether, and communicate between the controllers using NSNotificationCenter, which decouples them a bit, and avoids your compiler circularity (bidirectional dependency).
Here are the Apple docs on NSNotificationCenter
Couldn't you just go:
MyChildViewController *myChildViewController = (MyChildViewController *)[self.childViewControllers objectAtIndex:0];
[myChildViewController doWhatever];
?
That should let you message the child view controller at the first index of the array childViewControllers (which is a property on UIViewController).