copy 和 strong（或retain）的区别

http://stackoverflow.com/questions/18526909/whether-i-should-use-propertynonatomic-copy-or-propertynonatomic-strong-fo

'copy' will cause the setter for that property to create a copy of the object, and is otherwise identical to strong. You would use this to make sure that if someone sets your property to a mutable string, then mutates the string, you still have the original value. If the string isn't mutable, Cocoa will silently optimize out the copy operation, which is nice :)

'strong' will keep the property's value alive until it's set to something else. If you want incoming mutable strings to change out from under you (not impossible, but not all that common, a thing to want), then strong would be the right thing to do. Generally strong is more useful for objects that represent something more complex than a simple "value" (i.e. not NSString, NSNumber, NSValue, etc...).

'assign' is the default (and indeed only) possible setting for an integer. Integers can't be retained or copied like objects.

For attributes whose type is an immutable value class that conforms to the NSCopying protocol, you almost always should specify copy in your @property declaration. Specifying retain is something you almost never want in such a situation.In non ARC strong will work like retain

Here's why you want to do that:

NSMutableString *someName = [NSMutableString stringWithString:@"Chris"];
Person *p = [[[Person alloc] init] autorelease];
p.name = someName;
[someName setString:@"Debajit"];

The current value of the Person.name property will be different depending on whether the property is declared retain or copy — it will be @"Debajit" if the property is marked retain, but @"Chris" if the property is marked copy.

Since in almost all cases you want to prevent mutating an object's attributes behind its back, you should mark the properties representing them copy. (And if you write the setter yourself instead of using @synthesize you should remember to actually use copy instead of retain in it.)