如何在iphone屏幕上繪製連續線

Question

我只是在學習核心圖形。 所以我創建了一個新的項目，包括一個UIView類。 我現在已經在我UIView實現文件

-(id) initWithCoder:(NSCoder*)sourceCoder 
{ 
    if((self = [super initWithCoder:sourceCoder])) 
    { 
     //place any other initialization here 
    } 
    return self; 
} 

- (void)drawRect:(CGRect)rect { 

    CGContextRef context = UIGraphicsGetCurrentContext(); 

    CGContextSetLineWidth(context,4); 
    CGContextSetStrokeColorWithColor(context, myColor.CGColor); 


    CGContextMoveToPoint(context,startPoint.x , startPoint.y); 
    CGContextAddLineToPoint(context, endPoint.x, endPoint.y); 
    CGContextStrokePath(context); 

} 
- (void) touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event 
{ 
    UITouch* touchPoint = [touches anyObject]; 
    startPoint = [touchPoint locationInView:self]; 
    endPoint = [touchPoint locationInView:self]; 

    [self setNeedsDisplay]; 
} 

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event 
{ 
    UITouch* touch = [touches anyObject]; 
    endPoint=[touch locationInView:self]; 
    [self setNeedsDisplay]; 
} 

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event 
{ 
    UITouch* touch = [touches anyObject]; 
    endPoint = [touch locationInView:self]; 
    [self setNeedsDisplay]; 
} 

下面的代碼，但它只是在創建訂單，如果我再次嘗試繪製，然後前行丟失。

但我想上一行不應該丟失，我該怎麼做？

Answer 1

將視圖的clearsContextBeforeDrawing property設置爲NO。

Answer 2

設置一個UIImageView爲背景，在那裏保存上下文（系）：「如何使用的drawRect在現有的視圖中繪製」

CGContextStrokePath(UIGraphicsGetCurrentContext()); 
<your imageView>.image = UIGraphicsGetImageFromCurrentImageContext(); 
UIGraphicsEndImageContext();