title: How doesn't UILabel implementation intrinsicContentSize?
body: <p>As we known, UILabel has intrinsicContentSize, my puzzle is when we pin a label to it's superview with leading and trailing offset 10 for example, and set the numberOfLines to 0, assume the superview is a viewcontroller's view that has a width 320, and how label knows it's width is 300.
  Another, when there are mutiple constraint that the label is compressed, and how the label know it's compressed and recalculate it's size? </p>