从屏幕截取一块区域，将其赋给imageView

UIGraphicsBeginImageContext(self.bounds.size);

[self.layerrenderInContext:UIGraphicsGetCurrentContext()];

UIImage *cardImage = UIGraphicsGetImageFromCurrentImageContext();

if (self.card == nil) {

self.card = [[UIImageView alloc] initWithImage:cardImage];

}

else

{

self.card.image = cardImage;

}

}

self.card.center = [touch locationInView:self];

[self addSubview:self.card];

UIGraphicsEndImageContext();

self.layer.contents = nil;