UITextField的PlaceHolder居中显示,可以直接访问到UITextField的placeholderLabel进行操作：
UILabel * placeholderLabel = [self.searchTextField valueForKey:@"_placeholderLabel"];
placeholderLabel.textAlignment = NSTextAlignmentCenter;
placeholderLabel.textColor = [UIColor redColor];
placeholderLabel.font = [UIFont systemFontOfSize:16];
