找到searchbar的searchField属性通过KVC修改属性值
UITextField *searchField = [searchBar valueForKey:@"_searchField"];
if (searchField) {
[searchField setValue:[UIColor blackColor] forKeyPath:@"_placeholderLabel.textColor"];
[searchField setValue:[UIFont systemFontOfSize:13] forKeyPath:@"_placeholderLabel.font"];
}
