THPDFDocumentHandlerRegistry Class

THPDFDocumentHandlerRegistry = class
  function RegisterHandler(const Handler: IHPDFDocumentHandler;
    ReplaceExisting: boolean = false): boolean;
  function UnregisterHandler(const HandlerName: string): boolean;
  procedure Clear;
  function GetHandlerCount: Integer;
  function GetHandlerName(Index: Integer): string;
  procedure RegisterStandardHandlers(ReplaceExisting: boolean = false);
  function OpenFromStream(Source: TStream; const SourceName,
    MIMEType: string; const Options: THPDFDocumentHandlerOptions;
    out Document: IHPDFHandledDocument;
    out Info: THPDFDocumentHandlerInfo): boolean;
  function OpenFromFile(const FileName, MIMEType: string;
    const Options: THPDFDocumentHandlerOptions;
    out Document: IHPDFHandledDocument;
    out Info: THPDFDocumentHandlerInfo): boolean;
end;

注册、替换、移除、快照和查找都是线程安全的，最多 32 个 handler，每个 handler 名称占一个不区分大小写的槽位

识别在 registry 锁之外、针对稳定的接口快照运行，得分相同时保持注册顺序

RegisterStandardHandlers opts into XPS, EPUB, and CBZ support without changing PDF-specific loading APIs

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