What is the possibility of a control invisible to the naked eye ?, Invisible to the naked eye

What is the possibility of a control invisible to the naked eye ?, Invisible to the naked eye

1. This control is not instantiated at all.
2. No dimension is set.
3. The color of the control is the same as the background color of the parent control (actually displayed, but invisible to the naked eye)
4. Transparency alpha <= 0.01
5. hidden = YES
6. not added to the parent Control
7. Blocked by other controls
8. Incorrect location
9. The parent control has the preceding situation.
10. Special Cases
* The image attribute is not set for UIImageView, or the image name is incorrect.
* No text is set for UILabel, or the text color is the same as the background color of the parent control.
* UITextField does not set text or border style borderStyle
* The total number of pages is not set for UIPageControl and small dots are not displayed.
* The frame of the UIButton imageView and titleLabel is tampered with, or the imageView and titleLabel have no content.
*.....
 
Suggestions for adding a widget (debugging Tips ):
1. It is best to set the background color and size.
2. Do not use the same color as the background color of the parent widget.