为什么会收到“未知班级”警告？

Question

I have a simple test app to help me learn how to persist data from a NSMutableArray to a plist. Everything seems to be working well, until I try to save the data by calling a ViewController method called "saveData" in my AppDelegate.m file:

- (void)applicationDidEnterBackground:(UIApplication *)application
{

    [ViewController saveData];
}

I get a "No known class method for selector 'saveData', although the method is clearly declared in ViewController.h, like so:

//
//  ViewController.h
//  PlistTest
//
//  Created by Tim Jones on 10/30/13.
//  Copyright (c) 2013 TDJ. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

@property (weak, nonatomic) IBOutlet UILabel *nameLabel;

@property (weak, nonatomic) IBOutlet UILabel *timeLabel;

@property NSMutableArray *mainActivityArray;



- (IBAction)buttonHit:(id)sender;

-(NSString *) getFilePath;
-(void) saveData;
-(void) loadData;


@end

and implemented in ViewController.m, thusly:

//
//  ViewController.m
//  PlistTest
//
//  Created by Tim Jones on 10/30/13.
//  Copyright (c) 2013 TDJ. All rights reserved.
//

#import "ViewController.h"
#import "DataClass.h"

@interface ViewController ()

@end

@implementation ViewController

-(NSString *) getFilePath
{
    NSArray *pathArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    return [[pathArray objectAtIndex:0] stringByAppendingPathComponent:@"PlistTestData"];
}

-(void) saveData
{
    [self.mainActivityArray writeToFile: self.getFilePath atomically:YES];
}

I imported the ViewController.h into AppDelegate.h.

I'm pretty green, so I expect the problem may be obvious to many here. Would sure appreciate some help.

Answer 1

Issue:

[ViewController saveData];

You are calling saveData method using class name ViewController .
But saveData is an instance method, not class method.

-(void) saveData;

Fixes:

1) Declare saveData as class method

+(void) saveData;

2) Call saveData using the object of ViewController .

ViewController *vControl = [[ViewController alloc] init];
[vControl saveData];

Answer 2

Previous answer (Midhun #2) can work, but I think you would be better off using the Application Did Enter Background Notification and skip the delegate.

Just add this to "view did load": it will call saveData whenever the app goes to background.

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(saveData) name:UIApplicationDidEnterBackgroundNotification object:nil];

Hope this helps.

Answer 3

As others have noted, your issue is that your [ViewController saveData] method invocation clearly suggests that you're trying to call a " class method ", and you undoubtedly want to call your "instance method" (because it's your instance of your view controller that has the data you want to save). To achieve that, you have two basic choices:

1. You can have your app delegate call the saveData method in your view controller.

   In various comments, you mention that you've "tried instantiating the VC every way and in every place I can imagine." Don't. Midhun's example was a conceptual one, illustrating the difference between a class and instance method. But, while you want to call the instance method, you want to call this for your existing instance of your view controller, definitely not instantiating a new view controller.

   So, you may ask, how do you get a reference to the existing instance of your view controller? What you'll want to do is to (a) create a property in your app delegate to hold the reference to the view controller with the saveData method; (b) have that view controller set that property of the app delegate. So, first, create a property in your app delegate's .h file to reference the view controller:

    @property (weak, nonatomic) ViewController *viewController; 

   Obviously, don't forget the #import "ViewController.h" line early in the .h file.

   Second, have the viewDidLoad method of the view controller update the app delegate's viewController property:

    - (void)viewDidLoad { [super viewDidLoad]; AppDelegate *delegate = [[UIApplication sharedApplication] delegate]; delegate.viewController = self; } 

   Again, don't forget to #import "AppDelegate.h" at the top of your ViewController.m file.

   Having done that, then app delegate's applicationDidEnterBackground can now reference this property that you set in viewDidLoad

    - (void)applicationDidEnterBackground:(UIApplication *)application { [self.viewController saveData]; } 

   Frankly, if you did this technique, I might suggest further refinements, notably employing a delegate-protocol pattern, but I'm going to defer that conversation until after you have mastered the above technique.

2. Even easier than the above is to eliminate this app delegate applicationDidEnterBackground code altogether, and just have your view controller, itself, respond to the system notification that is associated with the app entering background. And, needless to say, you put this code right in the view controller itself.

    - (void)viewDidLoad { [super viewDidLoad]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(saveData) name:UIApplicationDidEnterBackgroundNotification object:nil]; } - (void)dealloc { [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidEnterBackgroundNotification object:nil]; } - (void)saveData { // save your data here } 

   As you can see, I register to observe UIApplicationDidEnterBackgroundNotification in viewDidLoad , but also make sure to remove my observer in dealloc . I also made sure that my @selector method name exactly matched my method name (eg, in my example, no parameters, and hence no colon).