遮擋效果圖：

效果圖一

效果圖二

原理：放兩張視圖，一張視圖控制灰色星星，另一張控制黃色星星。
黃色星星覆蓋灰色星星，再通過值來改變黃色星星的視圖frame的寬度。

代碼：

#import <UIKit/UIKit.h>
@interface starView : UIView {
    
    UIView *_yellowView;
    UIView *_grayView;    
}
@property(nonatomic,assign) float rating;
@end

#import "starView.h"

@implementation starView

//1.創(chuàng)建視圖

- (void)awakeFromNib {
    
    UIImage *yellowImage = [UIImage imageNamed:@"yellow"];
    UIImage *grayImage = [UIImage imageNamed:@"gray"];
    
    _yellowView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, yellowImage.size.width* 5,yellowImage.size.height)];
    _grayView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, grayImage.size.width * 5, grayImage.size.height)];
    
    //通過圖片來創(chuàng)建背景
    _yellowView.backgroundColor = [UIColor colorWithPatternImage:yellowImage];
    _grayView.backgroundColor = [UIColor colorWithPatternImage:grayImage];
    
    [self addSubview:_grayView];
    [self addSubview:_yellowView];
    
    //方法比例
    CGFloat scale = self.frame.size.height / yellowImage.size.height;
    _yellowView.transform = CGAffineTransformMakeScale(scale, scale);
    _grayView.transform = CGAffineTransformMakeScale(scale, scale);
    //坐標歸零
    CGRect frame = self.frame;
    frame.origin = CGPointZero;
    _yellowView.frame = frame;
    _grayView.frame = frame;
    
    
}

//2.修改值
- (void)setRating:(float)rating {
    
    _rating = rating;
    
    float s = rating / 10;
    
    CGRect frame = _yellowView.frame;
    
    frame.size.width = self.frame.size.width * s;
    
    _yellowView.frame =frame;
    
}

@end