仅在属性存在时测试它？(Test a property only if it exists?)

 你在哪里获得访问令牌fb添加后通知 
 
 [[NSNotificationCenter defaultCenter] postNotificationName：@“fbDidLogin”object：nil]; 
 
 现在用于初始fb登录的方法是 
 
-(void)fbLogin
{
[[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(fbDidLogin) name:@"fbDidLogin" object:nil];
 // some logic for intial fb login page
 }
 
 另外在fbDidLogin方法中添加这个 
 
- (void)fbDidLogin 
{   
[[NSNotificationCenter defaultCenter]removeObserver:self name:@"fbDidLogin" object:nil]; 
[self.switch setOn:YES animated:YES];    
}
 
 这里fbDIdLogin将在接收到令牌时立即被调用 
 
 如果访问令牌为零，则可以对fb登录失败执行类似的过程。 

where u get access token for fb add post notification
 
[[NSNotificationCenter defaultCenter] postNotificationName:@"fbDidLogin" object:nil]; 
 
Now the method for intial fb login is 
 
-(void)fbLogin
{
[[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(fbDidLogin) name:@"fbDidLogin" object:nil];
 // some logic for intial fb login page
 }
 
Also in fbDidLogin method add this 
 
- (void)fbDidLogin 
{   
[[NSNotificationCenter defaultCenter]removeObserver:self name:@"fbDidLogin" object:nil]; 
[self.switch setOn:YES animated:YES];    
}
 
Here fbDIdLogin will be called as soon as access token is recieved
 
Similar process can be done for fb login failed if access token is nil .