九：UIView的拖拽

　　做出來效果：屏幕上的一塊UIView，點擊後能自由移動

 1 #import "GreenView.h"
 2 @implementation GreenView
 3 - (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
 4 {
 5     UITouch *touch = [touches anyObject];
 6     CGPoint currentP = [touch locationInView:self];
 7     CGPoint previousP = [touch previousLocationInView:self];
 8     CGFloat offsetX = currentP.x - previousP.x;
 9     CGFloat offsetY = currentP.y - previousP.y;
10     self.transform = CGAffineTransformTranslate(self.transform, offsetX, offsetY);
11 }
12 @end