Xcode：如何从子类访问方法？

Question

When I try to access a method in my main ViewController from a SubView, my app crashes.

Please help.

I have a UIButton in my main ViewController that changes the text in a UILabel, which is also in my main View. I have a UIButton in a SubView that is supposed to change the string again, by accessing the same method from the main ViewController. But the app crashes.

Here's my code:

Main ViewController

ViewController.h

#import <UIKit/UIKit.h>
#import "SubViewController.h"

@interface ViewController : UIViewController

@property (nonatomic, strong) UILabel *label;
@property (nonatomic, retain) UIButton *mainClassButton;

- (void) setLabelTo:(NSString *)copy;

@end

ViewController.m

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

@synthesize label;
@synthesize mainClassButton;

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    self.label = [[UILabel alloc] initWithFrame:CGRectMake(30.0, 30.0, 200.0, 30.0)];
    self.label.text = @"Let's Go Mets!";
    [self.view addSubview:label];

    mainClassButton = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    mainClassButton.frame = CGRectMake(30.0, 70.0, 200.0, 30.0);
    [mainClassButton setTitle:@"Main Class Button" forState:UIControlStateNormal];
    [mainClassButton addTarget:self action:@selector(touchAction:) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:mainClassButton];

    SubViewController *subViewController = [[SubViewController alloc] init];
    subViewController.view.backgroundColor = [UIColor clearColor];
    [self.view addSubview:subViewController.view];

    [self.view bringSubviewToFront:self.mainClassButton];
}

- (IBAction)touchAction:(UIButton *) sender
{
    [self setLabelTo:@"Here we go Yankess!"];
}

- (void) setLabelTo:(NSString *)copy
{
    self.label.text = copy;
}

Sub ViewController

SubViewController.h

#import <UIKit/UIKit.h>
#import "ViewController.h"

@interface SubViewController : UIViewController

@property (nonatomic, retain) UIButton *subClassButton;

@end

SubViewController.m

@implementation SubViewController

@synthesize subClassButton;

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.

    subClassButton = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    subClassButton.frame = CGRectMake(30.0, 115.0, 200.0, 30.0);
    [subClassButton setTitle:@"Sub Class Button" forState:UIControlStateNormal];
    [subClassButton addTarget:self action:@selector(touchAction:) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:subClassButton];

}

- (IBAction)touchAction:(UIButton *) sender
{
    ViewController *vc = [[ViewController alloc] init];

    [vc setLabelTo:@"If you ask me, I prefer the White Sox."];
}

I'm still kind of new to Xcode and Objective-C and I would appreciate any help you could offer.

Thanks

Answer 1

You need to call -setLabelTo on your already existing instance of ViewController from within -[SubViewController touchAction:] --at the moment, you are creating a new instance of ViewController and calling -setLabelTo: on that new instance.

In order to call -setLabelTo on the ViewController , we need a reference to that ViewController .

UIViewController provides a property parentViewController which, under ideal circumstances, would provide your SubViewController a reference to your ViewController . However, in this case, it does not provide such a reference. You add your SubViewController 's view as a subview of your ViewController , but you do not add your SubViewController as a child view controller of your ViewController . (See also this question and the accepted answer .) To fix this issue, in -[ViewController viewDidLoad] , add the line

[self addChildViewController:subViewController];

after

SubViewController *subViewController = [[SubViewController alloc] init];

giving you

SubViewController *subViewController = [[SubViewController alloc] init];
[self addChildViewController:subViewController];
subViewController.view.backgroundColor = [UIColor clearColor];
[self.view addSubview:subViewController.view];

At this point, your SubViewController is a child view controller of ViewController and your SubViewController 's view is a subview of your ViewController 's view.

At this point, you can change your implementation of -[SubViewController touchAction:] to

- (IBAction)touchAction:(UIButton *) sender
{
    ViewController *vc = self.parentViewController;

    [vc setLabelTo:@"If you ask me, I prefer the White Sox."];
}

Which will call -setLabelTo: on your original ViewController rather than creating a new instance and causing a crash.

Answer 2

您必须为subviewcontroller获取父视图控制器： self.parentViewController在subViewController中的touchAction中完成的操作是创建一个新的View Controller。

Answer 3

Look at these lines:

ViewController *vc = [[ViewController alloc] init];
[vc setLabelTo:@"If you ask me, I prefer the White Sox."];

You are instantiating ViewController, but the viewDidLoad is not called. There you instantiate UILabel which you want to set the title on using setLabelTo method.

The viewDidXXXX and viewWillXXXX are called automatically when they are used in Storyboard or in some UINavigationController.