Refactor PrinterDiscoverer and PrinterDetector to use a common interface.

chrome/browser/chromeos/printing/combining_printer_detector.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_CHROMEOS_PRINTING_COMBINING_PRINTER_DETECTOR_H_
+#define CHROME_BROWSER_CHROMEOS_PRINTING_COMBINING_PRINTER_DETECTOR_H_
+
+#include <memory>
+
+#include "chrome/browser/chromeos/printing/printer_detector.h"
+
+namespace chromeos {
+
+// CombiningPrinterDetector implements PrinterDetector and exports the combined
+// results of one or more underlying PrinterDetectors.
+class CHROMEOS_EXPORT CombiningPrinterDetector : public PrinterDetector {
+ public:
+  // Static factory
+  static std::unique_ptr<CombiningPrinterDetector> Create();
+
+  ~CombiningPrinterDetector() override = default;
+
+  // Add an underlying detector to be used.  Ownership is not taken.  All
+  // detectors should be added on the same sequence.  Calling AddDetector after
+  // observers have been added is disallowed.
+  virtual void AddDetector(PrinterDetector* detector) = 0;
+
+  // Same as AddDetector, but ownership of the detector is taken.
+  virtual void AddOwnedDetector(std::unique_ptr<PrinterDetector> detector) = 0;
+};
+
+}  // namespace chromeos
+
+#endif  // CHROME_BROWSER_CHROMEOS_PRINTING_COMBINING_PRINTER_DETECTOR_H_