Subview of UILabel disappears when the text was changed to Unicode string - ios

Question

I am seeing a strange behavior of UILabel and its subview on iOS development.
I want to have a overlay view on the UILabel and want to change only the text of the label.
It works well when the text is digits or alphabets.
However when I changed the text to a Unicode string, the subview disappears.
To reproduce the problem, I created a simple project which just have a label and a button.
The label has a "A" as the initial text.
When the button was tapped, the following method will be called and change the text of UILabel cyclically.
- (void)pushButton:(id)sender
{
if ([[_label text] isEqualToString:#"A"]) {
[_label setText:#"B"];
// add a subview on the label
UIView *view = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 20, 20)];
[view setBackgroundColor:[UIColor blueColor]];
[_label addSubview:view];
} else if ([[_label text] isEqualToString:#"B"]) {
[_label setText:#"C"];
} else if ([[_label text] isEqualToString:#"C"]) {
[_label setText:#"D"];
} else if ([[_label text] isEqualToString:#"D"]) {
[_label setText:#"\u2605"];
// after this, the subview disappears
} else {
[_label setText:#"A"];
// after this, the subview appears again
}
}
When I tap the button at the first time, the text is changed to "B" and the subview appears.
Second time and third time, the text is changed to "C" and "D" and the subview is still there.
Howerver the fourth time, the text is changed to "★" and the subview disappears.
Fifth time, the text is changed to "A" and the subview appears again.
In this code, I don't remove the subview and the new subview is created in every cycle.
However in any time the text was changed to "★", all these subviews disappear.
How can I keep the subview being appeared on the UILabel?

Answer 1

UILabel is not intended to have subviews. The solution is to make your blue rectangle a subview of your label's superview. You can position it so that it appears in front of your UILabel.
Thus, where you have this code:
UIView *view = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 20, 20)];
[view setBackgroundColor:[UIColor blueColor]];
[_label addSubview:view];
You would instead say this:
UIView *sup = [_label superview];
UIView *view = [UIView new];
[view setBackgroundColor:[UIColor blueColor]];
CGRect r = CGRectMake(0,0,20,20);
r = [sup convertRect:r fromView:_label];
view.frame = r;
[sup addSubview: view];