初始化Objective c中另一个类的属性

Question

Hi guys I'm new to objective c and i was searching for an c++ or java like way to initialize an objects properties from another UIViewController class.

So I used the -init function which seems to work terrific, but the problem is that after some debugging I did I found that even though my property (an NSString ) is successfully initialize in -init when the viewDidLoad starts it deletes (initialize) everything I did in init so I can't use my property ! I want to play a video link but I found that the string that goes to playTheVideo method is null. Also note that when I initialize the stream inside viewDidLoad my video plays right away.

Here is my code:

ButtonsController.m

#import "ButtonsController.h"
#import "PlayVideoController.h"

@interface ButtonsController ()

@end

@implementation ButtonsController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    **UPDATE:[self createButton];**
}

-(void)createButton {

PlayVideoController *newObject = [[PlayVideoController alloc] initWithString:@"--somestring--" ];
[newObject playTheVideo];

}

PlayVideoController.m

#import "PlayVideoController.h"

@interface PlayVideoController ()

@end

@implementation PlayVideoController

@synthesize stream;
@synthesize player;


- (void)viewDidLoad
{
    [super viewDidLoad];

    [self playTheVideo];
}


- (id) initWithString: (NSString*) theStream {
    self = [super init];
    if (self) {
        stream = theStream;
    }
    return self;
}

- (void) playTheVideo {

    NSURL *url = [NSURL URLWithString:stream];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    [player loadRequest:request];
}

PlayVideoController.h

#import <UIKit/UIKit.h>

@interface PlayVideoController : UIViewController

@property NSString *stream;

@property (weak, nonatomic) IBOutlet UIWebView *player;
- (void) playTheVideo;
- (id) initWithString: (NSString*) theStream;

@end

I create an object in main class just to start it quickly

EDIT: deleted actions in main.m and left the defaults

Answer 1

The issue is that you're creating this controller in your main function, but don't do anything with it and it will fall out of scope (and if using ARC, will be deallocated). Regardless, when the view controller is instantiated during the standard flow of the app, it will create a second instance, this time without having createButton called. So your calling of createButton in main is for naught. This concept of "I create an object in main class just to start it quickly" doesn't pass muster. The creating of the view controller in CreateButtons suffers from an analogous problem.

You really need to set the stream property somewhere else (eg in the viewDidLoad of PlayVideoController or in the prepareForSegue of the view controller that is segueing to PlayVideoController ). But you almost certainly will never call initWithString for PlayVideoController (and you probably can retire that method).

If, for example, you're transitioning from ButtonsController view controller to PlayVideoController view controller via a segue with a storyboard identifier of, say playVideo , then you could use prepareForSegue in ButtonsController like so:

#import "PlayVideoController.h"

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if([segue.identifier isEqualToString:@"playVideo"]){
        PlayVideoController *controller = (PlayVideoController *)segue.destinationViewController;
        controller.stream = @"--somestring--";
    }
}

(See the "Passing data forward using segues" section of this answer about passing data between controllers .)

If you want to do something at startup, eg initialize some model object structure (but not view controllers), you can do that in your app delegate's didFinishLaunchingWithOptions , but there's nothing shared with us thus far that seems to suggest that this is necessary. But if you do this, make sure you do the necessary memory management to keep it around (eg make this model object a property of the app delegate, make it a singleton, etc.). But if you create a local variable and configure it and let it fall out of scope, you'll lose the work you did on that.

Answer 2

Your property should specify attributes for its memory management and whether it is atomic or nonatomic.

So instead of @property NSString *stream;

You should write

@property (copy, nonatomic) NSString *stream;

You no longer need to write @synthesize as that is taken care of for you.

Lastly, in your initializer you need to access the iVar directly by using the underscore.

So it should say _stream = [theStream copy];

EDIT:

Change your getter to self.stream

Answer 3

You should update your PlayVideoController.h file as below

#import <UIKit/UIKit.h>

@interface PlayVideoController : UIViewController

@property (strong, nonatomic) NSString *stream;
@property (weak, nonatomic) IBOutlet UIWebView *player;

- (void) playTheVideo;
- (id) initWithString: (NSString*) theStream;

@end

It should work.