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.

首先定位 FDStackView 对 insertArrangedSubview:atIndex: 的实现，并将其行为与 issue 中引用的 UIStackView 契约进行比较。验证现有 arranged subview 被插入到不同 stack 索引的情况；当该索引得到一致更新时，工作即完成。

由索引模型根据 Issue 内容生成。