将view剪切为各种形状的view（类似微信聊天图片展示）。

UIImage *normal = [UIImage imageNamed:@"chatto_bg_normal"];// 这个是最终形状的图片。

UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)]; //这个要被裁剪的view

UIImageView *imageViewMask = [[UIImageView alloc] initWithImage:normal];

imageViewMask.frame = CGRectInset(imageView.frame, 0.0f, 0.0f);

imageView.layer.mask = imageViewMask.layer;

[self.view addSubview:imageView];