According to official iOS Documentation about UIView, Frame and Bounds. We could make a cheatsheet below:

• frame describes the view’s location and size in its superview’s coordinate system.
• bounds describes the view’s location and size in its own coordinate system. With origin (0, 0) and same size as frame by default.

There is also another saying on internet - bounds is where the subviews are allowed to draw with respect to itself. But it is true only if clipsToBounds is set to TRUE.

In fact, frame, bounds and also center, they are interlocked. Setting one property changes the others in the mean time. Changing the size portion of bounds grows or shrinks the view relative to its center point. Changing the size also changes the size of the rectangle in the frame property to match.

Example

Below is an example to show how bounds effect a view's superview and subview

image.png

// 父 View
let superview: UIView = UIView(frame: CGRect(x: 100, y: 100, width: 200, height: 200))
superview.backgroundColor = .yellow
        
// 子 View
let childview: UIView = UIView(frame: CGRect(x: 50, y: 50, width: 100, height: 100))
childview.backgroundColor = .red
        
// 子 View 的 子 View
let childchildview: UIView = UIView(frame: CGRect(x: 0, y: 0, width: 50, height: 50))
childchildview.backgroundColor = .green
        
self.view.addSubview(superview)
superview.addSubview(childview)
childview.addSubview(childchildview)
        
// 打印信息
print("superview")
print(superview.frame)  // (100.0, 100.0, 200.0, 200.0)
print(superview.bounds) // (0.0, 0.0, 200.0, 200.0)
print("childview")
print(childview.frame)  // (50.0, 50.0, 100.0, 100.0)
print(childview.bounds) // (0.0, 0.0, 100.0, 100.0)
print("childchildview")
print(childchildview.frame)  // (0.0, 0.0, 50.0, 50.0)
print(childchildview.bounds) // (0.0, 0.0, 50.0, 50.0)

If now we change the origin point of childview's bounds:

childview.bounds = CGRect(x: 100, y: 100, width: 100, height: 100)

So what would happen here is the coordinate system of childview changes. It has no bussiness with its superview but only effects its subview (the "childchildview"). Let's draw below the NEW coordinate system of childview:

new_coordinate_system.png

• The new coordinate system of childview moves to upper left relative to its old position as origin point of bounds changes from (0, 0) to (100, 100)
• childview's frame does not change with the respect of its superview, still at (50, 50) in superview's coordinate system.
• Because the origin point portion of frame of childchildview is at (0, 0), so with respect of its superview - childview, the green rectangle moves also to the new position according to this new coordinate system.

Next, let's set clipsToBounds to true:

childview.clipsToBounds = true

So now, the childchildview is not visible anymore. From now on, we could say bounds is where the subviews are allowed to draw with respect to itself with clipsToBounds set to true.