There is a difference of function from FDStackView to UIStackView. “- (void)insertArrangedSubview:(UIView *)view atIndex:(NSUInteger)stackIndex” /

In UIStackView ：
/*
Adds the view as a subview of the container if it isn't already.
Updates the stack index (but not the subview index) of the
arranged subview if it's already in the arrangedSubviews list.
*/
- (void)insertArrangedSubview:(UIView *)view atIndex:(NSUInteger)stackIndex;

But FDStackView can not update it.

So that I have to treate the function differently.

Start by locating FDStackView's implementation of insertArrangedSubview:atIndex: and compare its behavior with the UIStackView contract quoted in the issue. Verify the case where an existing arranged subview is inserted at a different stack index; the work is done when that index is updated consistently.

Written by the indexing model from the issue text.