Acquire Basic Programming Skills:

1.The Root Class and Objective-C Objects.

2.Create Objects.

3.Thinking in Terms of Objects.

4.Manage the Object Graph to Avoid Memory Leaks.

5.Manage Object Mutability.

6.Create and Use Value Objects.

7.Create and Use Collections.

8.Verify Object Capabilities at Runtime.

9.Compare Objects.

10.Copy Objects.

NSObject 

NSURL *aURL = [[NSURL alloc] initFileURLWithPath:NSTemporaryDirectory() isDir:YES];

NSString *myString = [NSString stringWithFormat:@"Customer:%@",self.record.customerName];

Reference:

    Strong(default)

    Weak(use __weak or @property(weak)

@property(weak) IBOutlet NSString *theName

 Immutable classes:
   NSData/NSDate/NSNumber/NSValue

• -(BOOL) isKindOfClass: classObj 用来判断是否是某个类或其子类的实例


• -(BOOL) isMemberOfClass: classObj 用来判断是否是某个类的实例


• -(BOOL) respondsToSelector: selector 用来判断是否有以某个名字命名的方法(被封装在一个selector的对象里传递)


• +(BOOL) instancesRespondToSelector: selector 用来判断实例是否有以某个名字命名的方法. 和上面一个不同之处在于, 前面这个方法可以用在实例和类上，而此方法只能用在类上.


• -(id) performSelector: selector

1. A weak reference implies that the referring object does not own the referenced object. The lifetime of an object is determined by how many strong references there are to it. An object is not freed as long as there is a strong reference to it.