Making view resize to its parent when added with addSubview

that's all you need

childView.frame = parentView.bounds

If you aren’t using Auto Layout, have you tried setting the child view’s autoresize mask? Try this:

myChildeView.autoresizingMask = (UIViewAutoresizingFlexibleWidth |
                                 UIViewAutoresizingFlexibleHeight);

Also, you may need to call

myParentView.autoresizesSubviews = YES;

to get the parent view to resize its subviews automatically when its frame changes.

If you’re still seeing the child view drawing outside of the parent view’s frame, there’s a good chance that the parent view is not clipping its contents. To fix that, call

myParentView.clipsToBounds = YES;

Tested in Xcode 9.4, Swift 4 Another way to solve this issue is , You can add

override func layoutSubviews() {
        self.frame = (self.superview?.bounds)!
    }

in subview class.

Just copy the parent view's frame to the child-view then add it. After that autoresizing will work. Actually you should only copy the size CGRectMake(0, 0, parentView.frame.size.width, parentView.frame.size.height)

childView.frame = CGRectMake(0, 0, parentView.frame.size.width, parentView.frame.size.height);
[parentView addSubview:childView];