More About @dynamic

The opposite of @synthesize is @dynamic. When you specify @dynamic for a property, the compiler expects you to supply your own implementation of the accessor methods for that property. If you don’t, you’ll get a warning at compile time, and then an error at runtime when you try to use the property.

Why would you want to write the accessor methods when the compiler can do the work for you? The compiler only synthesizes relatively simple accessors that just set or return the property. Sometimes, you may need to do more. You might need to take some other actions when a property is set. For example, you might want to filter the input to make sure the property being set stays within an acceptable range.