title: How to get constraints is ios8 before layoutSubviews?
body: <p>I have a view that is subclass of UIView,when I add constraints in xib,and now I should get the array constraints, I tried, but it is different between iOS 7 and iOS 8.
  it work for iOS 7,and iOS 8 is be invalid.</p>
  
  <pre><code>-(id)initWithCoder:(NSCoder *)aDecoder{
  
     self = [super initWithCoder:aDecoder];
     if (self) {
  
      NSLog(@"constraints = %@",self.constraints);
      }
      return self;
  }
  </code></pre>