iPhone协议委托传递数据

Question

I am passing a string through a delegate and protocol. The string is received to the class correctly, but this occurs after the viewDidLoad method is called. The viewDidLoad method needs that string that is passed.

Any ideas on what I can do to get the delegate method to be called before viewDidLoad? I thought this was the idea of the delegate/protocol data passing.

Method where new view is created and pushed:

ViewControllerTwo *two = [[ViewControllerTwo alloc] initWithNibName:@"ViewControllerTwo" bundle:nil];
two.delegate = self;
[two setString:theString];
[self.navigationController pushViewController:two animated:YES];
[two release]; 

In ViewControllerTwo:

- (void)setString:(NSString *)str
{
    self.myString = str;
}

Edit: Thanks for the input. However, I do understand how to pass this data through the init method. I have been testing protocols and delegates lately, and I wanted to see if there was a way to do this. I have successfully passed data like this in another class and it worked. The protocol method was called first setting the string. It seemed like a much cleaner way to handle passing data.

Answer 1

I think you might be a little confused about what Delegation is used for and why.

For example you might want to make a protocol in a UIViewController subclass if you were doing some kind of action in that ViewController and needed to inform another subclass that that action is being taken, or of the result of that action.

Now in order for the subclass that wants to know about the action(the receiver), it has to conform to that protocol in it's header file.

You also must "set" the delegate to the receiving class/controller .

There are many ways to get a reference to the receiving controller/class to set it as the delegate but a common mistake is allocating and initializing a new instance of that class to set it as the delegate, when that class has already been created.What that does is set your newly created class as the delegate instead of the class that's already been created and waiting for a message.

What your trying to do is just pass a value to a Newly created class. Since your just creating this UIViewController class all that needed for that is a Property in the receiver(ViewControllerTwo) .

In your case a NSString :

 @Property (nonatiomic, retain) NSString *string; //goes in ViewControllerTwo.h

and of course don't forget in the main:

  @synthesize string; //Goes in ViewControllerTwo.m

Now there is no need for a setter in your ViewControllerTwo .

 - (void)setString:(NSString *)str  //This Method can be erased
    {                                  //The setter is created for free
        self.myString = str;          // when you synthesized the property
    }   

The setter and Getters are free when you use the @synthesize . Just Pass the value over to the ViewController . The implementation is identical to your code except for the delegate:

  ViewControllerTwo *two = [[ViewControllerTwo alloc] initWithNibName:@"ViewControllerTwo" bundle:nil];
    [two setString:theString];
    [self.navigationController pushViewController:two animated:YES];
    [two release];

Answer 2

if you need to pass that string in the initialization phase, you could pass it in the init method.

So in your controller you need to create a property and an additional init method like this:

.h

@property (nonatomic, copy) NSString* myString;

-(id)initWithString:(NSString*)theString;

.m

@synthesize myString;

-(id)initWithString:(NSString*)theString {
    self = [super initWithNibName:@"ViewController" bundle:nil]; // I prefer to set the name of the controller internally
    if(self) {
        myString = [theString copy];
    }
    return self;
}

then, use self.myString wherever you want.

If you don't use ARC, remember to release.

- (void)dealloc
{
   [myString release];
   [super dealloc];
}

Answer 3

Can you create a custom initializer for ViewControllerTwo , eg

- (id)initWithString:(NSString *)aString;
{
    self = [super initWithNibName:@"ViewControllerTwo" bundle:nil];
    if( !self ) return nil;

    self.myString = aString;
    // other initialization

    return self;
}

You initialization in ViewControllerOne would just be:

ViewControllerTwo *vc = [[ViewControllerTwo alloc] initWithString:theString];
[[self navigationController] pushViewController:vc animated:YES];
[vc release];

Answer 4

I wouldn't use a protocol for this. To pass data from the parent view controller to the child, use a custom initialiser method.

In the ViewControllerTwo.h:

-(id)initWithString:(NSString *)str;

And then implement that in your ViewControllerTwo.m:

-(id)initWithString:(NSString *)str {
    self = [super self];
    if(self) {
        self.myString = str;
    }
    return self;

And then to call that in your ViewController one:

ViewControllerTwo *two = [[ViewControllerTwo alloc] initWithString:@"Cool String"];

Answer 5

Placing the data in an init method or a viewDidLoad won't work because the users can switch back and forth without unloading the view or reinitializing the view controller.

The best place to retrieve changing data is in the viewWillAppear controller method. That way the data will be updated every time the user switches.

The another best way is using MVC architecture. If you have separate model class which will just hold data and you may write/update from any controller.