Lazarus Reading XML- with TXMLDocument and TXPathVariable

也就是使用XPath的方式，具体语法规则查看http://www.w3school.com.cn/xpath/xpath_syntax.asp，说明得相当详细。这里列举例子是说明在Lazarus/FPC下具体应用于实现，以及注意事项。首先可以构建一个“ReadXPath”的函数方便调用。毕竟每次使用EvaluateXPathExpression后还有些任务要处理……。

function ReadXPath(const aNode: TDOMNode; const aPath: string): TDOMNode;
var
  rv: TXPathVariable;
  tl: TFPList;
begin
  Result := nil;
  if Assigned(aNode) then
  begin
    rv := EvaluateXPathExpression(aPath, aNode);
    if Assigned(rv) then
    begin
      tl := rv.AsNodeSet;
      if Assigned(tl) then
      begin
        if tl.Count > 0 then
        begin
          Result := TDOMNode(tl[0]);
        end;
      end;
    end;
  end;
end;

具体使用了，要记住返回的其实是“元素”，就算强制约定了“属性”——[@Attrib],所以要读取任何值，都要按“扫描到元素”的方式来处理。

function ReadCFG: boolean;
var
.....
  vConfigXml: string = '';
  HistoryPath: string = '';
  TracePath: string = '';
  vXP: TDOMNode;
.....
begin
  Result := False;
  ReadXMLFile(xmlCfg, vConfigXml);
  vXP := ReadXPath(xmlCfg, '/Config/HistoryPath[@value]');
  if Assigned(vXP) then
  begin
    if vXP.HasAttributes then
      HistoryPath := vXP.Attributes.Item[0].NodeValue;
  end;
  vXP := ReadXPath(xmlCfg, '/Config/TracePath[@value]');
  if Assigned(vXP) then
  begin
    if vXP.HasAttributes then
      TracePath := vXP.Attributes.Item[0].NodeValue;
  end;

  if (HistoryPath <> '') and (TracePath <> '') then
      .....

end;