title: property value issue in iOS Singleton
body: <p>In below code, I try to add new object into myArray.However, after calling doSome method, the value of myArray is still nil, why?</p>
  
  <pre><code>+ (void) doSome {
    NSMutableArray *xx = [[self sharedInstance] myArray];
    if (xx == nil) {
      xx = [NSMutableArray array];
    }
    [xx addObject:@"dfdsf"];
  </code></pre>
  
  <p>}</p>