声明一个iVar，属性

Question

I have just inherited an iOS project and just have a query about the way the ivars and properties are assigned.

In this project ivars are named similar to the property but begin with an underscore. Then, when the property is being synthesized it is done like this

@synthesize property = _ivar;

I find this a little odd. I have just worked on one iOS project before but when I was doing both the ivar and property had the same name. I would then simply synthesize the property and everything was hunky dory.

Why did this guy do things differently? What am I missing?

Answer 1

Somewhere along the line it became conventional to use the _var form as protection against accidentally referencing the variable when the developer should have been referencing self.property . It is now established to the point that, with the latest tools, you can drop the @synthesize lines entirely and @synthesize x = _x; is simulated automatically.

Answer 2

You not missing anything, it a choice of style. When using the _ivar way you can't easily make a mistake with the ivar or the property. Also as stated by Mike Weller this is what Apple recommends.

If also help if you are using delegate where they pass the send with the same name as your ivar. For example:

If you declare a property tableView and do the same for the ivar then the compiler will complain about redeclaring the variable tableView in a method like:

- (void) tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
   // Dot some thing with the table object
}

Because both the class scope and the method scope hava the variable name tableView and you can't really tell which one is to be used.

In the latest versions of Xcode you do not even have to declare the ivar's any more, if you header file looks like this:

@interface MyTableViewController : UIViewController <UITableViewDelegate> 
    @property (nonatomic, weak) IBOutlet UITableView *tableView;
@end

Then you don't even have to @synthesize the property any more, this is done for you automatically and a ivar with the name _tableView is also create. But accessing the variable via the property with the self.tableView syntax is preferred.

There is an exception on this, if you where to reintroduce a property with which already exists in the super calls you must call @synthesize . It will again create the '_ivar' for you.

An other exception is that if you are using primitive types the compiler will not create the _ivar for you. You don't need to create them both in the header you can just do @synthesize myInt = _myInt and the ivar is created for you.

Answer 3

this practice forces you to call and set your iVars using property. Incase where you want to use iVar,u call it by _iVar, otherwise there will be no chance of calling iVar directly without using property by mistake. It will show error and you will get reminded that oh i need to put self.

Answer 4

In the latest version of XCode you need not to synthesize, it does for you. But again with an underscore.

However you can override it by declaring it @synthesize property=property; or @synthesize property; and even you will be aware that you can give any other name @synthesize property=myproperty;

However initially it was just a matter of style, like using variable name and action names eg numberArray, nameTextField, cancelButton, but now it is seldom used and people are asked to name numbers, name, cancel respectively.

But I would suggest to follow apple guidelines to use _property.