手动截图

    if(UIGraphicsBeginImageContextWithOptions != NULL)

    {

        UIGraphicsBeginImageContextWithOptions(_view.frame.size, NO, 0.0);

    } else {

        UIGraphicsBeginImageContext(_view.frame.size);

    }

    [_view.layerrenderInContext:UIGraphicsGetCurrentContext()];

    UIImage *cropImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

cropImage就是截下来的UIImage，将其写入本地