一、前情概要

系统提供UISlider，但在开发过程中经常需要自定义，本次需求内容是实现一个拥有渐变色的滑动条，且渐变色随着手指touch的位置不同改变区域，类似如下

可以使用CAGradientLayer实现渐变效果，但是发现手指滑动的快时会有不跟手的情况。我们可以重写左侧有渐变色的UIView的drawRect: 方法来绘制渐变色

二、具体实现

左侧的渐变色UIView
HLProgressView.h

@interface HLProgressView : UIView
@property (nonatomic, assign) CGSize viewSize;
@end

HLProgressView.m

@implementation HLProgressView

- (instancetype)initWithFrame:(CGRect)frame{
    self = [super initWithFrame:frame];
    if (self) {
    }
    return self;
}

- (void)setViewSize:(CGSize)viewSize {
    _viewSize = viewSize;
    self.frame = CGRectMake(0, 0, viewSize.width, viewSize.height);
    // setNeedsDisplay会自动调用drawRect方法
    [self setNeedsDisplay];
}
    
- (void)drawRect:(CGRect)rect {
    CGSize size = self.bounds.size;
    // 获取图形上下文对象CGContextRef
    CGContextRef context = UIGraphicsGetCurrentContext();
    // 创建一个颜色空间
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    // 设置的颜色 每四个元素表示一种颜色，值的范围0~1，分别表示R、G、B、透明度 
    CGFloat colors[] = {
        55.0/255.0, 180.0/255.0, 255.0/255.0, 1.0,
        55.0/255.0, 80.0/255.0, 255.0/255.0, 1.0
    };
    // 渐变的位置信息范围0~1 0表示开始的位置 1表示结束的位置
    CGFloat gradientLocations[] = {0, 1};
    // 渐变的个数
    size_t locationCount = 2;
    // 创建渐变
    CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, colors, gradientLocations, locationCount);
    // 指定渐变的开始位置和结束位置 这里设置完效果是整块区域的水平方向的渐变
    CGPoint gradientStartPoint = CGPointMake(0, size.height/2);
    CGPoint gradientEndPoint = CGPointMake(size.width, size.height/2);
    // 将渐变画到上下文中，最后一个参数表示发散的方式
    CGContextDrawLinearGradient(context, gradient, gradientStartPoint, gradientEndPoint, kCGGradientDrawsBeforeStartLocation);
    // 释放内存
    CGGradientRelease(gradient);
    CGColorSpaceRelease(colorSpace);
}

@end

滑动条
UICustomSlider.h

@interface UICustomSlider : UIView
@end

UICustomSlider.m

@interface UICustomSlider ()
@property (nonatomic, strong) HLProgressView *progressView;
@end

@implementation UICustomSlider

- (instancetype)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    if (self) {
        self.backgroundColor = [UIColor grayColor];
        self.clipsToBounds = YES; //不显示超过父视图的内容
        self.layer.cornerRadius = 8;
        self.progressView = [[HLProgressView alloc] initWithFrame:CGRectMake(0, 0, 140, 44)];
        [self addSubview:self.progressView];
    }
    return self;
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    CGPoint point = [touches.anyObject locationInView:self];
    self.progressView.viewSize = CGSizeMake(point.x, self.bounds.size.height);
}

- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    CGPoint point = [touches.anyObject locationInView:self];
    self.progressView.viewSize = CGSizeMake(point.x, self.bounds.size.height);
}

- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    CGPoint point = [touches.anyObject locationInView:self];
    self.progressView.viewSize = CGSizeMake(point.x, self.bounds.size.height);
}

@end

调用滑动条
ViewController.m

#import "GradientSlider.h"
@interface ViewController ()
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    UICustomSlider *customSlider = [[UICustomSlider alloc] initWithFrame:CGRectMake(20, 100, 280, 44)];
    [self.view addSubview:customSlider];
}