[原][学习笔记][2011.11.11]drawset不擦除原有图画

    在touchEnded里面将这次touch操作所经过的点存到一个可变长数组里面, 然后把这个数组存到一个dictionary里面. 在drawset每次画之前先遍历这个dictionary里面的数组(每组点), 把这些点画出来, 然后再画当前touch操作.

    代码如下: 

//类定义如下
@interface KeepDraw: UIView{
   NSMutableArray * currentPoints;
   NSMutableDictionary * savedPoints;
   int dictPos;//dictPos用来给savedPoints设定key. 其实也可以用savedPoints.count来设定, 但是比较ambiguous.
}

方法如下: 

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{
	NSMutableArray * tmp = [[NSMutableArray alloc] init];
	for(int i = 0; i < currentPoints.count - 1; i ++)//把每一组值都存到一个临时数组里面, 然后再交给savedPoints
		[tmp addObjet: [currentPoints objectAtIndex: i]];
	[savedPoint setObject: tmp forKey: [NSString stringWithFormat: @"%d", dictPos]];
	dictPos ++;
	[currentPoints removeAllObjects];//记得要removecurrentPoints, 要不然每次存到savedPoints里面的东西都会重复之前存的.
}

- (void)touchesCancelled:(NSSet *)touches withEven: (UIEvent*) event{
	[self touchesEnded: touches withEvent: event];
}

- (void) drawRect: (CGRect)rect{
	if(currentPoints.count == 0)	return;
	CGContextRef context = UIGraphicsGetCurrentContext();
	CGContextSetLineWidth(context, 4);
	CGContextSetFillColorWithColor(context, [[UIColor redColor] CGColor]);

	for(int i = 0; i < dictPos; i ++){   //就是这里了, 从savedPoints里面把每一组值都取出来, 然后画出来.
		NSArray * tmp = [savedPoints objectForKey: [NSString stringWithFormat: @"%d", i]];
		for(int j = 0; j < tmp.count - 1; j ++)
			drawGraphics(j);
	}
	for(int i = 0; i < currentPoints.count - 1; i ++)
		drawGraphics(i);
}

drawGraphics 的定义如下:  

void drawGraphics(int i){
			CGPoint x1 = [[tmp objectAtIndex: j] CGPointValue];
			CGPoint x2 = [[tmp objectAtIndex: j + 1] CGPointValue];
			CGContextMoveToPoint(context, x1.x, x1.y);
			CGContextAddLineToPoint(context, x2.x, x2.y);
			CGContextStrokPath(context);
}