iOS loadNibNamed confusion, what is best practice?

问题:

I'm familiar with most of the process of creating an XIB for my own UIView subclass, but not everything is working properly for me - it's mostly to do with the IBOutlets linking up. I can get them to work in what seems like a roundabout way.

My setup is this:

• I have MyClass.h and MyClass.m. They have IBOutlets for a UIView (called view) and a UILabel (called myLabel). I added the 'view' property because some examples online seemed to suggest that you need this, and it actually solved an issue where I was getting a crash because it couldn't find the view property, I guess not even in the UIView parent class.
• I have an XIB file called MyClass.xib, and its File's Owner custom class is MyClass, which prefilled correctly after my .h and .m for that class existed.

My init method is where I'm having issues.

I tried to use the NSBundle mainBundle's 'loadNibNamed' method and set the owner to 'self', hoping that I'd be creating an instance of the view and it'd automatically get its outlets matched to the ones in my class (I know how to do this and I'm careful with it). I then thought I'd want to make 'self' equal to the subview at index 0 in that nib, rather than doing

self = [super init];

or anything like that.

I sense that I'm doing things wrong here, but examples online have had similar things going on in the init method, but they assign that subview 0 to the view property and add it as a child - but is that not then a total of two MyClass instances? One essentially unlinked to IBOutlets, containing the child MyClass instantiated via loadNibNamed? Or at best, is it not a MyClass instance with an extra intermediary UIView containing all the IBOutlets I originally wanted as direct children of MyClass? That poses a slight annoyance when it comes to doing things like instanceOfMyClass.frame.size.width, as it returns 0, when the child UIView that's been introduced returns the real frame size I was looking for.

Is the thing I'm doing wrong that I'm messing with loadNibNamed inside an init method? Should I be doing something more like this?

MyClass *instance = [[MyClass alloc] init];
[[NSBundle mainBundle] loadNibNamed:@"MyClass" owner:instance options:nil];  

Or like this?

MyClass *instance = [[[NSBundle mainBundle] loadNibNamed:@"MyClass" owner:nil options:nil] objectAtIndex:0]; 

Thanks in advance for any assitance.

回答1:

The second option is the correct one. The most defensive code you could do is like this:

+ (id)loadNibNamed:(NSString *)nibName ofClass:(Class)objClass {
    if (nibName && objClass) {
        NSArray *objects = [[NSBundle mainBundle] loadNibNamed:nibName 
                                                         owner:nil 
                                                       options:nil];            
        for (id currentObject in objects ){
            if ([currentObject isKindOfClass:objClass])
                return currentObject;
        }
    }

    return nil;
}

And call like this:

MyClass *myClassInstance = [Utility loadNibNamed:@"the_nib_name" 
                                         ofClass:[MyClass class]]; 
// In my case, the code is in a Utility class, you should 
// put it wherever it fits best

I'm assuming your MyClass is a subclass of UIView? If that's the case, then you need to make sure that the UIView of your .xib is actually of MyClass class. That is defined on the third Tab on the right-part in the interface builder, after you select the view

回答2:

All you need to do is create the subview via loadNibNamed, set the frame, and add it to the subview. For example, I'm adding three subviews using my MyView class, which is a UIView subclass whose interface is defined in a NIB, MyView.xib:

So, I define initWithFrame for my UIView subclass:

- (id)initWithFrame:(CGRect)frame
{
    NSLog(@"%s", __FUNCTION__);

    self = [super initWithFrame:frame];
    if (self)
    {
        NSArray *nibContents = 
          [[NSBundle mainBundle] loadNibNamed:@"MyView" 
                                        owner:self 
                                      options:nil];
        [self addSubview:nibContents[0]];
    }
    return self;
}

So, for example, in my UIViewController, I can load a couple of these subclassed UIView objects like so:

for (NSInteger i = 0; i < 3; i++)
{
    CGRect frame = CGRectMake(0.0, i * 100.0 + 75.0, 320.0, 100.0);
    MyView *myView = [[MyView alloc] initWithFrame:frame];
    [self.view addSubview:myView];

    // if you want, do something with it: 
    // Here I'm initializing a text field and label

    myView.textField.text = [NSString stringWithFormat:@"MyView textfield #%d",
                              i + 1];
    myView.label.text = [NSString stringWithFormat:@"MyView label #%d", 
                          i + 1];
}

I originally advised the use controllers, and I'll keep that answer below for historical reference.

---

Original answer:

I don't see any references to view controllers here. Usually you'd have a subclass of UIViewController, which you would then instantiate with

MyClassViewController *controller = 
  [[MyClassViewController alloc] initWithNibName:@"MyClass" 
                                          bundle:nil];

// then you can do stuff like
//
// [self presentViewController:controller animated:YES completion:nil];

The NIB file, MyClass.xib, could specify that the base class for the UIView, if you want, where you have all of the view related code (e.g. assuming that MyClass was a subclass of UIView).

回答3:

Here's one method that I use:

1. Create a subclass for UIView, this will be called MyClass
2. Create a view xib file. Open in interface builder, click File's Owner and in the Identity Inspector, change the class to that of your parent view controller, e.g. ParentViewController.
3. Click the view already in the list of objects and change it's class in Identity Inspector to MyClass.
4. Any outlets/actions that you declare in MyClass will be connected by click-dragging from View (not File's Owner). If you want to connect them to variables from ParentViewController then click-drag from File's Owner.
5. Now in your ParentViewController you need to declare an instance variable for MyClass.

ParentViewController.h add the following:

@class MyClass

@interface ParentViewController : UIViewController {
    MyClass *myClass;
}

@property (strong, nonatomic) MyClass *myClass;

Synthesize this in your implementation and add the following in your viewDidLoad method:

- (void)viewDidLoad {
    [super viewDidLoad];

    [[NSBundle mainBundle] loadNibNamed:@"MyClass" owner:self options:nil];
    self.myClass.frame = CGRectMake(X,Y,W,H); //put your values in.
    [self.view addSubview:self.myClass];
}