全屏截图:
UIWindow *screenWindow = [[UIApplication sharedApplication] keyWindow];
UIGraphicsBeginImageContext(screenWindow.frame.size);
[screenWindow.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext(); //viewImage为当前屏幕的截图
UIImageWriteToSavedPhotosAlbum(viewImage, nil, nil, nil);//保存到相册