NLog详解(五) - Condition Expression

Sample

<!-- during normal execution only log Info messages -->     <defaultFilter>level >= LogLevel.Info</defaultFilter>         <!-- if there is at least one error, log everything from trace level -->     <when exists="level >= LogLevel.Error" filter="level >= LogLevel.Trace" />

实现将字符串转换成表达式进行逻辑判断，然后做出相应的动作。这个过程的实现使用Condition Expression。

从简单的例子开始：

public void BooleanOperatorTest() {  AssertEvaluationResult(false, "false or false");  AssertEvaluationResult(true, "false or true");  AssertEvaluationResult(true, "true or false");  AssertEvaluationResult(true, "true or true");  AssertEvaluationResult(false, "false and false");  AssertEvaluationResult(false, "false and true");  AssertEvaluationResult(false, "true and false");  AssertEvaluationResult(true, "true and true");  AssertEvaluationResult(false, "not true");  AssertEvaluationResult(true, "not false");  AssertEvaluationResult(false, "not not false");  AssertEvaluationResult(true, "not not true"); } 

输入的是字符串 false or false ，我们希望的结果是得到 false ，大概的过程是这样的:

CondtionParser

 /// <summary> /// Condition parser. Turns a string representation of condition expression /// into an expression tree. /// </summary> public class ConditionParser

第一步就是初始化这个Parser

var parser = new ConditionParser(new SimpleStringReader(expressionText), configurationItemFactories);

顺带初始化了tokenizer

    private ConditionParser(SimpleStringReader stringReader, ConfigurationItemFactory configurationItemFactory)     {         this.configurationItemFactory = configurationItemFactory;         this.tokenizer = new ConditionTokenizer(stringReader);     }

构造函数里直接取到了第一个token false

    public ConditionTokenizer(SimpleStringReader stringReader)     {         this.stringReader = stringReader;         this.TokenType = ConditionTokenType.BeginningOfInput;         this.GetNextToken();     }

获取的过程也比较的简单，一个接一个的读入 char， 遇到非指定的字符即停止

private void ParseKeyword(char ch) {  int i;  this.TokenType = ConditionTokenType.Keyword;  StringBuilder sb = new StringBuilder();  sb.Append((char)ch);  this.ReadChar();  while ((i = this.PeekChar()) != -1)  {   if ((char)i == '_' || (char)i == '-' || char.IsLetterOrDigit((char)i))   {    sb.Append((char)this.ReadChar());   }   else   {    break;   }  }  this.TokenValue = sb.ToString(); } 

这里有一系列比较复杂的表达式树生成的过程：

private ConditionExpression ParseBooleanOr() {  ConditionExpression expression = this.ParseBooleanAnd();  while (this.tokenizer.IsKeyword("or") || this.tokenizer.IsToken(ConditionTokenType.Or))  {   this.tokenizer.GetNextToken();   expression = new ConditionOrExpression(expression, this.ParseBooleanAnd());  }  return expression; } 

最后拿到的表达式树是这样的:

左右的表达式为 ConditionLiteralExpression 。

     private ConditionExpression ParseLiteralExpression() {     //......     if (this.tokenizer.TokenType == ConditionTokenType.Keyword)     {  string keyword = this.tokenizer.EatKeyword();  if (0 == string.Compare(keyword, "level", StringComparison.OrdinalIgnoreCase))  {      return new ConditionLevelExpression();  }  if (0 == string.Compare(keyword, "logger", StringComparison.OrdinalIgnoreCase))  {      return new ConditionLoggerNameExpression();  }  if (0 == string.Compare(keyword, "message", StringComparison.OrdinalIgnoreCase))  {      return new ConditionMessageExpression();  }  if (0 == string.Compare(keyword, "loglevel", StringComparison.OrdinalIgnoreCase))  {      this.tokenizer.Expect(ConditionTokenType.Dot);      return new ConditionLiteralExpression(LogLevel.FromString(this.tokenizer.EatKeyword()));  }  if (0 == string.Compare(keyword, "true", StringComparison.OrdinalIgnoreCase))  {      return new ConditionLiteralExpression(true);  } 

再来个例子:

public void ConditionMethodsTest() {  AssertEvaluationResult(true, "starts-with('foobar','foo')");  AssertEvaluationResult(false, "starts-with('foobar','bar')");  AssertEvaluationResult(true, "ends-with('foobar','bar')");  AssertEvaluationResult(false, "ends-with('foobar','foo')");  AssertEvaluationResult(0, "length('')");  AssertEvaluationResult(4, "length('${level}')");  AssertEvaluationResult(false, "equals(1, 2)");  AssertEvaluationResult(true, "equals(3.14, 3.14)");  AssertEvaluationResult(true, "contains('foobar','ooba')");  AssertEvaluationResult(false, "contains('foobar','oobe')");  AssertEvaluationResult(false, "contains('','foo')");  AssertEvaluationResult(true, "contains('foo','')"); } 

这里出现了一些条件方法 starts-with , ends-with , equals , contains ,可以通过这些操作，当log的message包含特定的字符串的时候才写日志

[ConditionMethod("starts-with")] 是以属性反射的方式在初始化的时候加载的。

在parse的过程中，多一步判断是否是ConditionMethods并创建实例

private ConditionMethodExpression ParsePredicate(string functionName) {  try  {   var methodInfo = this.configurationItemFactory.ConditionMethods.CreateInstance(functionName);   return new ConditionMethodExpression(functionName, methodInfo, par);  }  catch (Exception exception)  {   if (exception.MustBeRethrown())   {    throw;   }   throw new ConditionParseException("Cannot resolve function '" + functionName + "'", exception);  } } 

那么怎么知道该字符串是否是ConditionMethods呢？这里还是简单从左到右一个个的吃字符。

public string EatKeyword() {  if (this.TokenType != ConditionTokenType.Keyword)  {   throw new ConditionParseException("Identifier expected");  }  string s = (string)this.TokenValue;  this.GetNextToken();  return s; } 

当出现特殊的char时候做出判断。

  private static ConditionTokenType[] BuildCharIndexToTokenType() {  CharToTokenType[] charToTokenType =  {   new CharToTokenType('(', ConditionTokenType.LeftParen),   new CharToTokenType(')', ConditionTokenType.RightParen),   new CharToTokenType('.', ConditionTokenType.Dot),   new CharToTokenType(',', ConditionTokenType.Comma),   new CharToTokenType('!', ConditionTokenType.Not),   new CharToTokenType('-', ConditionTokenType.Minus),  };