UIButton 的field.clearButtonMode改变叉号按钮的大小

    UIButton *button = [textField valueForKey:@"_clearButton"];
    [button setImage:[UIImage imageNamed:@"icon_blueclear"] forState:UIControlStateNormal];

    field.clearButtonMode = UITextFieldViewModeWhileEditing;

- (BOOL)textFieldShouldClear:(UITextField *)textField
{
    //返回一个BOOL值指明是否允许根据用户请求清除内容
    //可以设置在特定条件下才允许清除内容
    _searchTableView.hidden= YES;
    [self.view endEditing:YES];
    [self.searchTF resignFirstResponder];
    return YES;
}