主要的原理包括：

继承UIView ，重载drawrect和重载触摸事件

待实现的功能还有，路径数组保存等。

用可变数据保存path路径

画曲线是通过二次贝塞尔曲线实现的

这里可以得到画图的UIImage对象

 UIGraphicsBeginImageContext(self.bounds.size);
    [self.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *result=UIGraphicsGetImageFromCurrentImageContext();
   
    UIGraphicsEndImageContext();
    return  result;

#import "testdrow.h"

@implementation testdrow
 UIColor *pick_color;
int choose;
UIBezierPath *mpath ;
UIBezierPath *eraser_path;
NSTimer *mytime;
UIImage *tempdraw;
CGContextRef CONTEXT;
-(void) settime:(NSTimer *)test_time{
   if(choose==1)
       choose=0;
   else if(choose==0)
        choose=1;
    [self setNeedsDisplay];
}
- (IBAction)test_draw:(id)sender {
    mytime=[NSTimer scheduledTimerWithTimeInterval:0.2 target:self selector:@selector(settime:) userInfo:nil repeats:YES];
}

- (IBAction)eraser:(id)sender {
   //pick_color=[UIColor alloc];
   // [mpath closePath];
    pick_color=[UIColor blackColor];
    choose=-1;
}
 
- (id)initWithFrame:(CGRect)frame
{
    mpath=[[UIBezierPath bezierPath] init];
    eraser_path=[[UIBezierPath bezierPath]init];
   //pick_color=[[UIColor alloc]init];
  pick_color=[UIColor new];
    pick_color=[UIColor greenColor];
    choose=1;
    [self setBackgroundColor:[UIColor blackColor]];
    self.backgroundColor=[UIColor blackColor];
    self = [super initWithFrame:frame];
    if (self) {
        // Initialization code
        //self init
    
    }
    
    
    return self;
    
    
}

/*
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    // Drawing code
}
*/
//int x,y;
//int firstx,firsty;


/*
- (void) drawRect:(CGRect)rect{
    CGContextRef CONTEXT=UIGraphicsGetCurrentContext();
    CGFloat temp=25.4;
    CGContextSetStrokeColorWithColor(CONTEXT,[UIColor redColor].CGColor);
    CGContextSetLineWidth(CONTEXT,5.8);
    CGContextMoveToPoint(CONTEXT,firstx,firsty);
    CGContextAddLineToPoint(CONTEXT, x, y);
    //CGContextAddLines(CONTEXT, //, <#size_t count#>)
       CGContextStrokePath(CONTEXT);
    printf("drawRect\n");
}
 */
-(void) drawRect1:(CGRect)rect{
    switch(choose){
        case 1:
        case -1:
            [tempdraw drawInRect:rect];
            break;
        case 0:
            //CONTEXT=UIGraphicsGetCurrentContext();
            //[self drawRect2:rect];
            
            break;
    }
   
    
}
- (void)drawRect:(CGRect)rect
{
   // pick_color=[UIColor alloc];
   // UIGraphicsBeginImageContext(self.frame.size);
    
    mpath.lineWidth = 15.0;
    
    mpath.lineCapStyle = kCGLineJoinRound; //线条拐角
    mpath.lineJoinStyle = kCGLineCapRound;
  CGContextRef CONTEXT=UIGraphicsGetCurrentContext();
  //  UIGraphicsBeginImageContext(SELF.CGSizeMake);
CGContextSetBlendMode(CONTEXT, kCGBlendModeNormal);
    CGContextSetShadowWithColor(CONTEXT,CGSizeMake(1, 1),18.f,  [pick_color CGColor]);
    
    UIColor *color =pick_color;
    //[UIColor greenColor];
    [color set]; //设置线条颜色
    UIColor *back_color=[UIColor blackColor];
    //UIBezierPath* apath =mpath;
    //[UIBezierPath bezierPath];
    //终点处理
    
    switch(choose){
        case 1:[mpath stroke];
            break;
        case -1:
            [eraser_path stroke];
            break;
        case 0:
            [back_color set];
          
            break;
    }
        //Draws line 根据坐标点连线
}
    
static CGPoint midpoint(CGPoint p0, CGPoint p1) {
    return (CGPoint) {
        (p0.x + p1.x) / 2.0,
        (p0.y + p1.y) / 2.0
    };
}

 -  (UIImage *) getimage{
   
    UIGraphicsBeginImageContext(self.bounds.size);
    [self.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *result=UIGraphicsGetImageFromCurrentImageContext();
   
    UIGraphicsEndImageContext();
    return  result;
   
}


- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{
       UITouch *touch=[touches anyObject];
    CGPoint point=[touch locationInView:self];
    CGPoint mpoint=midpoint(lastpoint, point);
    switch(choose){
        case 1:[mpath addQuadCurveToPoint:mpoint controlPoint:lastpoint];
            break;
        case -1:
            [eraser_path addQuadCurveToPoint:mpoint controlPoint:lastpoint];
            break;
    }
    
    lastpoint=point;
    int tempx=point.x;
    int tempy=point.y;
    NSLog(@"touches MOVE%d %d \n",tempx,tempy);
    

    UIGraphicsBeginImageContext(self.frame.size);
    
    [self.layer renderInContext:UIGraphicsGetCurrentContext()];
    CONTEXT=UIGraphicsGetCurrentContext();
  [tempdraw drawInRect:self.frame];
    //[self drawRect2:self.frame];
    UIImage *result=UIGraphicsGetImageFromCurrentImageContext();
    
    UIGraphicsEndImageContext();
    tempdraw=result;
    [self setNeedsDisplay];
    
}

- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    //[super touchesBegan:<#touches#> withEvent:<#event#>];
  //  touches uitouch
   // mpath=[[UIBezierPath bezierPath] init];
    if(choose==0){
        [mytime invalidate];
        choose=1;
    }
    CGPoint point=[[touches anyObject] locationInView:self];
    firstx=point.x;
    firsty=point.y;
    switch(choose){
        case 1:    [mpath moveToPoint:point];
            break;
        case -1:
            [eraser_path moveToPoint:point];
    }
    lastpoint=point;
    //NSLog(@"%d %d \n",x,y);
    
  [self setNeedsDisplay];
    
}
-(void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{
   // [mpath closePath];
}
 
@end