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#ifndef nsIStyleRule_h___
#define nsIStyleRule_h___

#include <stdio.h>

#include "nsISupports.h"

class nsIStyleSheet;
class nsPresContext;
class nsIContent;
struct nsRuleData;

// IID for the nsIStyleRule interface {40ae5c90-ad6a-11d1-8031-006008159b5a}
#define NS_ISTYLE_RULE_IID     \
{0x40ae5c90, 0xad6a, 0x11d1, {0x80, 0x31, 0x00, 0x60, 0x08, 0x15, 0x9b, 0x5a}}

/**
 * An object implementing |nsIStyleRule| (henceforth, a rule) represents
 * immutable stylistic information that either applies or does not apply
 * to a given element.  It belongs to an object or group of objects that
 * implement |nsIStyleSheet| and |nsIStyleRuleProcessor| (henceforth, a
 * sheet).
 *
 * A rule becomes relevant to the computation of style data when
 * |nsIStyleRuleProcessor::RulesMatching| creates a rule node that
 * points to the rule.  (A rule node, |nsRuleNode|, is a node in the
 * rule tree, which is a lexicographic tree indexed by rules.  The path
 * from the root of the rule tree to the |nsRuleNode| for a given
 * |nsStyleContext| contains exactly the rules that match the element
 * that the style context is for, in priority (weight, origin,
 * specificity) order.)
 *
 * The computation of style data uses the rule tree, which calls
 * |nsIStyleRule::MapRuleInfoInto| below.
 *
 * It is worth emphasizing that the data represented by a rule
 * implementation are immutable.  When the data need to be changed, a
 * new rule object must be created.  Failing to do this will lead to
 * bugs in the handling of dynamic style changes, since the rule tree
 * caches the results of |MapRuleInfoInto|.
 *
 * |nsIStyleRule| objects are owned by |nsRuleNode| objects (in addition
 * to typically being owned by their sheet), which are in turn garbage
 * collected (with the garbage collection roots being style contexts).
 */


class nsIStyleRule : public nsISupports {
public:
  NS_DEFINE_STATIC_IID_ACCESSOR(NS_ISTYLE_RULE_IID)

  /**
   * |nsIStyleRule::MapRuleInfoInto| is a request to copy all stylistic
   * data represented by the rule that:
   *   + are relevant for |aRuleData->mSID| (the style struct ID)
   *   + are not already filled into the data struct
   * into the appropriate data struct in |aRuleData|.  It is important
   * that only empty data are filled in, since the rule tree is walked
   * from highest priority rule to least, so that the walk can stop if
   * all needed data are found.  Thus overwriting non-empty data will
   * break CSS cascading rules.
   */
  NS_IMETHOD MapRuleInfoInto(nsRuleData* aRuleData)=0;

#ifdef DEBUG
  NS_IMETHOD List(FILE* out = stdout, PRInt32 aIndent = 0) const = 0;
#endif
};


#endif /* nsIStyleRule_h___ */