实现自定义View ,有两种办法：

1、 view上实现

-(void)drawRect:(CGRect)rect；

2、layer上实现

-(void)drawInContext:(CGContextRef)ctx;

方法一：View的drawRect应用

例子：画圆弧

@interface CircleProgressView : UIView
@property (nonatomic, assign) CGFloat progress;
@end

- (void)drawRect:(CGRect)rect {    
    CGFloat radius = self.bounds.size.width / 2;    
    CGFloat lineWidth = 10.0;    
    UIBezierPath * path = [UIBezierPath bezierPathWithArcCenter:CGPointMake(radius, radius) radius:radius - lineWidth / 2 startAngle:0.f endAngle:M_PI * 2 * self.progress clockwise:YES];    
    [[UIColor colorWithRed:0.5 green:0.5 blue:0.9 alpha:1.0] setStroke];    
    [path setLineWidth:lineWidth];   
    [path stroke];
}

-(void)viewDidLoad {    
    [super viewDidLoad];      
    self.circleProgressView = [[CircleProgressView alloc]initWithFrame:CGRectMake(100, 100, 200, 200)];    
    [self.view addSubview:self.circleProgressView];
}
// 控制progress的值到达动画效果
-(void)controlProgressAnimation{
  self.circleProgressView.progress = random();
  [self.circleProgressView setNeedsDisplay];
}

方法二：layer的drawInContext应用

例子：画圆弧

CircleProgressLayer.h 添加 progress属性

@interface CircleProgressLayer : CALayer
@property (nonatomic, assign) CGFloat progress;
@end

1.重载initWithLayer方法

- (instancetype)initWithLayer:(CircleProgressLayer *)layer {    
    NSLog(@"initLayer");    
    if (self = [super initWithLayer:layer]) {        
        self.progress = layer.progress;    
    }    
    return self;
}

2.重载其绘图方法 drawInContext

- (void)drawInContext:(CGContextRef)ctx {    
    CGFloat radius = self.bounds.size.width / 2;    
    CGFloat lineWidth = 10.0;    
    UIBezierPath * path = [UIBezierPath bezierPathWithArcCenter:CGPointMake(radius, radius) radius:radius - lineWidth / 2 startAngle:0.f endAngle:M_PI * 2 * self.progress clockwise:YES];    
    CGContextSetRGBStrokeColor(ctx, 0.5, 0.5, 0.9, 1.0);//笔颜色    
    CGContextSetLineWidth(ctx, 10);//线条宽度    
    CGContextAddPath(ctx, path.CGPath);    
    CGContextStrokePath(ctx);
}

3.progress属性变化时让其自动重绘

+ (BOOL)needsDisplayForKey:(NSString *)key {    
    if ([key isEqualToString:@"progress"]) {        
        return YES;    
    }    
    return [super needsDisplayForKey:key];
}

4.view中使用

-(id)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    if (self) {
        self.circleProgressLayer = [CircleProgressLayer layer];
        self.circleProgressLayer.frame = self.bounds;
        //像素大小比例
        self.circleProgressLayer.contentsScale = [UIScreen mainScreen].scale;
        [self.layer addSublayer:self.circleProgressLayer];
    }
    return self;
}
-(void)setProgress:(CGFloat)progress {
    CABasicAnimation * ani = [CABasicAnimation animationWithKeyPath:@"progress"];
    ani.duration = 5.0 * fabs(progress - _progress);
    ani.toValue = @(progress);
    ani.removedOnCompletion = YES;
    ani.fillMode = kCAFillModeForwards;
    ani.delegate = self;
    [self.circleProgressLayer addAnimation:ani forKey:@"progressAni"];
    _progress = progress;
}
-(void)animationDidStop:(CAAnimation *)anim finished:(BOOL)flag {
    self.circleProgressLayer.progress = self.progress;
}