Question

I have an Objective-c class “MyClass”. In MyClass.m I have a class extension that declares a CGFloat property:

@interface MyClass ()
@property (nonatomic) CGFloat myFloat;
@end

@implementation MyClass
@synthesize myFloat;
//...
@end

What changes (if anything) when the property is declared using the readonly keyword?

@interface MyClass ()
@property (nonatomic, readonly) CGFloat myFloat;
@end

@implementation MyClass
@synthesize myFloat;
//...
@end

Perhaps in the first case I can say self.myFloat = 123.0; and CGFloat f = self.myFloat; inside MyClass? Then in the second case the readonly keyword prevents the assignment self.myFloat = 123.0; but allows the read CGFloat f = self.myFloat;

Answer 1

The option readonly means that only the getter method is being declared for this property. Thus, without a setter, it can’t be modified via myObject.myFloat=0.5f;

If you don’t declare it readonly, it’s read write by default.

Declaring your property via () extension does not modify the access mode but it modifies the scope; it will be a “private” property.