uitableview截取整张表（必须预加载所有数据）

UIGraphicsBeginImageContext(self.tabView.bounds.size);

UIGraphicsBeginImageContextWithOptions(self.tabView.bounds.size, YES, self.tabView.layer.contentsScale);

    [self.tabView.layer renderInContext:UIGraphicsGetCurrentContext()];

UIImage *img = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

UIImageWriteToSavedPhotosAlbum(img, nil, nil, nil);

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask, YES);

NSString *filePath = [[paths objectAtIndex:0] stringByAppendingPathComponent:[NSString stringWithFormat:@"sms.jpg"]];   // 保存文件的名称

BOOL result = [UIImagePNGRepresentation(img)writeToFile: filePath    atomically:YES];

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask, YES);

NSString *filePath = [[paths objectAtIndex:0] stringByAppendingPathComponent:[NSString stringWithFormat:@"sms.jpg"]];   // 保存文件的名称

UIImage *imgs = [[UIImage alloc] initWithContentsOfFile:filePath];