Add new TSourceFileInfo.FileTypeFromFilterIdx method

New method to get the file type associated with a given index within a
filter string.

To implement this many changes were made to the internals of
TSourceFileInfo, the main one of which was that the filter string is
now generated every time the FileTypeInfo property is updated instead of
the filter string being built on request.

Author: delphidabbler

Src/USourceFileInfo.pas

@@ -90,10 +90,24 @@   TSourceFileInfo = class(TObject)
   strict private
     var
       ///  <summary>Stores information about the different source code output
-      //   types required by save source dialog boxes.</summary>
+      ///  types required by save source dialog boxes.</summary>
       fFileTypeInfo: TDictionary<TSourceFileType,TSourceFileTypeInfo>;
-      //   <summary>Value of DefaultFileName property.</summary>
+      ///  <summary>Maps a one-based index of a file filter within the current
+      ///  filter string to the corresponding <c>TSourceFileType</c> that was
+      ///  used to create the filter string entry.</summary>
+      fFilterIdxToFileTypeMap: TDictionary<Integer,TSourceFileType>;
+      ///  <summary>Value of DefaultFileName property.</summary>
       fDefaultFileName: string;
+      ///  <summary>Filter string for use in open / save dialog boxes from
+      ///  descriptions and file extensions of each supported file type.
+      ///  </summary>
+      fFilterString: string;
+    ///  <summary>Generates a new filter string and filter index to file type
+    ///  map from the current state of the <c>FileTypeInfo</c> property.
+    ///  </summary>
+    ///  <remarks>This method MUST be called every time the <c>FileTypeInfo</c>
+    ///  property is updated.</remarks>
+    procedure GenerateFilterInfo;
     ///  <summary>Read accessor for FileTypeInfo property.</summary>
     ///  <exception>Raises <c>EListError</c> if <c>FileType</c> is not contained
     ///  in the property.</exception>
@@ -110,9 +124,14 @@   TSourceFileInfo = class(TObject)
     constructor Create;
     destructor Destroy; override;
 
-    ///  <summary>Builds filter string for use in open / save dialog boxes from
+    ///  <summary>Returns filter string for use in open / save dialog boxes from
     ///  descriptions and file extensions of each supported file type.</summary>
     function FilterString: string;
+
+    ///  <summary>Returns the file type associated with a file filter at the
+    ///  given one-based index within the current filter string.</summary>
+    function FileTypeFromFilterIdx(const Idx: Integer): TSourceFileType;
+
     ///  <summary>Information about each supported file type that is of use to
     ///  save source dialog boxes.</summary>
     ///  <exception>A <c>EListError</c> exception is raised if no information
@@ -144,30 +163,48 @@ constructor TSourceFileInfo.Create;
 begin
   inherited Create;
   fFileTypeInfo := TDictionary<TSourceFileType,TSourceFileTypeInfo>.Create;
+  fFilterIdxToFileTypeMap := TDictionary<Integer,TSourceFileType>.Create;
 end;
 
 destructor TSourceFileInfo.Destroy;
 begin
+  fFilterIdxToFileTypeMap.Free;
   fFileTypeInfo.Free;
   inherited;
 end;
 
+function TSourceFileInfo.FileTypeFromFilterIdx(
+  const Idx: Integer): TSourceFileType;
+begin
+  Result := fFilterIdxToFileTypeMap[Idx];
+end;
+
 function TSourceFileInfo.FilterString: string;
+begin
+  Result := fFilterString;
+end;
+
+procedure TSourceFileInfo.GenerateFilterInfo;
 const
   cFilterFmt = '%0:s (*%1:s)|*%1:s';  // format string for creating file filter
 var
   FT: TSourceFileType;  // loops thru all source file types
+  FilterIdx: Integer;   // current index in filter string
 begin
-  Result := '';
+  fFilterIdxToFileTypeMap.Clear;
+  FilterIdx := 1;     // filter index is one based
+  fFilterString := '';
   for FT := Low(TSourceFileType) to High(TSourceFileType) do
   begin
     if not fFileTypeInfo.ContainsKey(FT) then
       Continue;
-    if Result <> '' then
-      Result := Result + '|';
-    Result := Result + Format(
+    if fFilterString <> '' then
+      fFilterString := fFilterString + '|';
+    fFilterString := fFilterString + Format(
       cFilterFmt, [fFileTypeInfo[FT].DisplayName, fFileTypeInfo[FT].Extension]
     );
+    fFilterIdxToFileTypeMap.Add(FilterIdx, FT);
+    Inc(FilterIdx);
   end;
 end;
 
@@ -203,6 +240,7 @@ procedure TSourceFileInfo.SetFileTypeInfo(const FileType: TSourceFileType;
     fFileTypeInfo[FileType] := Info
   else
     fFileTypeInfo.Add(FileType, Info);
+  GenerateFilterInfo;
 end;
 
 { TSourceFileTypeInfo }