对齐UISlider拇指图像 - objective-c

Question

I've customized my UISlider but my thumb image looks strange, I mean it's position not aligned by center:

But should be like this:

And here is code:

UIImage *leftTrack = [[UIImage imageNamed:@"blueTrack.png"] stretchableImageWithLeftCapWidth:3 topCapHeight:0];
UIImage *rightTrack = [[UIImage imageNamed:@"whiteTrack.png"] stretchableImageWithLeftCapWidth:3 topCapHeight:0];
[slider setThumbImage:[UIImage imageNamed:@"thumbButton.png"] forState:UIControlStateNormal];
[slider setMinimumTrackImage:leftTrack forState:UIControlStateNormal];
[slider setMaximumTrackImage:rightTrack forState:UIControlStateNormal];

Where could be the problem?

EDIT: I found solution of this problem. Thumb image background was transparent, and I've not seen that thumb image was not aligned by center:

Answer 1

There are two ways:

Firs one is to use

- (CGRect)trackRectForBounds:(CGRect)bounds;

or

- (CGRect)thumbRectForBounds:(CGRect)bounds trackRect:(CGRect)rect value:(float)value;

methods of UISlider when subclassing to lay out thumb the way you want

Second way is to adjust your artwork: