Take a look at the image bellow. The content view is marked using ‘#’ where as the scrollable area (example: screen) ismarked using ‘-‘:

contentOffset

As you can clearly see, the content that we want to display in the scroll viewdoesn’t entirely fit in the scrollable area so we need to scroll in order for usto view the content.
The Apple documentation describes contentOffset
as follows:var contentOffset: CGPoint
“The point at which the origin of the content view is offset from the origin of the scroll view.”
In other words it defines the point in the content view that is visible at the top left of the scroll view bounds.We can use this property to scroll programmatically .
So for example:

就是在content里找到相应offset的点，然后和scrollview的0点相对
contentOffset = CGPoint(x:0.0, y:0.0)
 
 ________________ <------ content view's y = 0
|  ############  |
|  #          #  |
|  #          #  |
|  #          #  |
|  #          #  |
 --# ---------#--  
   #          #  
   #          #  
   #          #
   #          #
   ############

contentOffset = CGPoint(x: 0.0, y: 42.0)  
   ############
   #    42    #
 __#__________#__  <------ content view's y = 42
|  #          #  |
|  #          #  |
|  #          #  |
|  #          #  |
|  #          #  |
 --#----------#--
   #          #
   ############   

contentOffset = CGPoint(x: 0.0, y: -66.0)  
 ________________  <------ content view's y = -66
|                |
|      +66       |
|  ############  | 
|  #          #  |
|  #          #  |
 --# ---------#--  
   #          #  
   #          #  
   #          #
   #          #
   #          #
   #          #
   ############ 

contentInset

The Apple documentation describes contentInset
as follows:var contentInset: UIEdgeInsets
“The distance that the content view is inset from the enclosing scroll view.”
In other words it is the distance that the content view is inset from the enclosing scroll view (padding).So, if you want to add more scrollable space at the bottom or at the top, you can use the property contentInset.topand contentInset.bottom, so you can add some extra space without changing the content size.It is important to note that through the contentInset we add more scrollable space. So an contentInset.top = 10will add an extra space of 10 at the top. One way to think of it is “how much to shift the content down the scroll viewwhen we’re scrolled all the way to the top”. When we scroll down this padding also scrolls along with the rest of the content.

contentInset = UIEdgeInsetsMake(66.0, 0.0, 0.0, 0.0)
// Scrolled all the way to the top
 ________________  
|                |
|       66       |
|  ############  | 
|  #          #  |
|  #          #  |
 --# ---------#--  
   #          #  
   #          #  
   #          #
   #          #
   #          #
   #          #
   ############ 


contentInset = UIEdgeInsetsMake(0.0, 0.0, 66.0, 0.0)
// Scrolled all the way to the bottom
   ############   
   #          #  
   #          #  
   #          # 
   #          #  
   #          #  
 __#__________#__
|  #          #  |
|  #          #  |
|  #          #  |
|  ############  |
|       66       |
 ----------------