模拟创建类变量，static变量加类方法，单例

@interface Model
+ (int) value;
+ (void) setValue:(int)val;
@end
 
@implementation Model
static int value;
+ (int) value
{ @synchronized(self) { return value; } }
+ (void) setValue:(int)val
{ @synchronized(self) { value = val; } }
@end

// Foo.h
@interface Foo {
}
 
+(NSDictionary*) dictionary;
 
// Foo.m
+(NSDictionary*) dictionary
{
  static NSDictionary* fooDict = nil;
 
  static dispatch_once_t oncePredicate;
 
  dispatch_once(&oncePredicate, ^{
        // create dict
    });
 
  return fooDict;
}