nonatomic and readonly property in Objective-C

Question

A few times already I wanted to make a property, which is nonatomic and readonly at the same time.

This has the advantage that I can override the getter and check if an instance has already been created or not. And if not I can simply create it.

At the same time I can protect it from being overwritten.

---

.h

@property (strong, readonly, nonatomic) Foo *bar;

---

.m

- (Foo *)bar {
    if (!_bar) {
        _bar = [[Foo alloc] init];
    }

    return _bar;
}

---

Whenever I do this, the compiler doesn't create an instance variable for me, so _bar doesn't exist.

Why? How can I create a readonly nonatomic property?

Answer 1

Your property declaration is correct. I believe the problem here is that, because your property was declared as readonly , the compiler didn't automatically synthesize an underlying instance variable. The solution in this case is to synthesize one yourself using...

@synthesize bar = _bar;

Answer 2

You could create a private setter:

@interface YourClass()  // In the .m file 

@property (strong, readwrite, nonatomic) Foo *bar;

@end

Then when assigning the variable:

self.bar = [[Foo alloc] init];

EDIT

Mark Adam's answer is also correct.

Answer 3

在实现中添加@synthesize bar = _bar。