Quartz 2D 练习1触摸简单绘图

总结：

1、单点触摸时，获取坐标：[[touches anyObject] locationInView:self];

2、需要更新图形显示内容时，调用 [self setNeedsDisplay];

3、在绘图事件函数中使用 UIGraphicsGetCurrentContext() 得到画布

4、画线使用 StrokePath

5、绘图前清除旧的显示内容可使用背景色填充视图范围（本例中可以不要）

6、绘图程序一般要支持屏幕自动旋转，在视图控制器的shouldAutorotateToInterfaceOrientation函数中返回YES

TouchesView.h：

#import <UIKit/UIKit.h>

@interface TouchesView : UIView {
    CGPoint _startPos;
    CGPoint _lastPos;
    BOOL    _touched;
}

@end

TouchesView.m：

#import "TouchesView.h"

@implementation TouchesView

- (void)dealloc
{
    [super dealloc];
}

- (BOOL)isMultipleTouchEnabled
{
    return NO;
}

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    
    CGContextSetFillColorWithColor(context, self.backgroundColor.CGColor);
    CGContextFillRect(context, self.bounds);
    
    if (_touched) {
        if (CGPointEqualToPoint(_startPos, _lastPos)) {
            CGContextStrokeEllipseInRect(context, 
                CGRectMake(_startPos.x - 40, _startPos.y - 40, 80, 80));
        } else {
            CGContextBeginPath(context);
            CGContextMoveToPoint(context, _startPos.x, _startPos.y);
            CGContextAddLineToPoint(context, _lastPos.x, _lastPos.y);
            CGContextStrokePath(context);
        }
    }
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    _startPos = [[touches anyObject] locationInView:self];
    _lastPos = _startPos;
    _touched = YES;
    [self setNeedsDisplay];
    
    NSLog(@"touchesBegan: %f,%f", _startPos.x, _startPos.y);
}

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

    NSLog(@"touchesMoved: %f,%f", _lastPos.x, _lastPos.y);
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    _touched = NO;
    [self setNeedsDisplay];
}

@end