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// Copyright (c) 2012 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.
// This module provides a way to monitor a file or directory for changes.
#include "base/base_export.h"
#include "base/callback.h"
#include "base/files/file_path.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/single_thread_task_runner.h"
namespace base {
// This class lets you register interest in changes on a FilePath.
// The callback will get called whenever the file or directory referenced by the
// FilePath is changed, including created or deleted. Due to limitations in the
// underlying OS APIs, FilePathWatcher has slightly different semantics on OS X
// than on Windows or Linux. FilePathWatcher on Linux and Windows will detect
// modifications to files in a watched directory. FilePathWatcher on Mac will
// detect the creation and deletion of files in a watched directory, but will
// not detect modifications to those files. See for
// details.
class BASE_EXPORT FilePathWatcher {
// Callback type for Watch(). |path| points to the file that was updated,
// and |error| is true if the platform specific code detected an error. In
// that case, the callback won't be invoked again.
typedef base::Callback<void(const FilePath& path, bool error)> Callback;
// Used internally to encapsulate different members on different platforms.
class PlatformDelegate : public base::RefCountedThreadSafe<PlatformDelegate> {
// Start watching for the given |path| and notify |delegate| about changes.
virtual bool Watch(const FilePath& path,
bool recursive,
const Callback& callback) WARN_UNUSED_RESULT = 0;
// Stop watching. This is called from FilePathWatcher's dtor in order to
// allow to shut down properly while the object is still alive.
// It can be called from any thread.
virtual void Cancel() = 0;
friend class base::RefCountedThreadSafe<PlatformDelegate>;
friend class FilePathWatcher;
virtual ~PlatformDelegate();
// Stop watching. This is only called on the thread of the appropriate
// message loop. Since it can also be called more than once, it should
// check |is_cancelled()| to avoid duplicate work.
virtual void CancelOnMessageLoopThread() = 0;
scoped_refptr<base::SingleThreadTaskRunner> task_runner() const {
return task_runner_;
void set_task_runner(scoped_refptr<base::SingleThreadTaskRunner> runner) {
task_runner_ = std::move(runner);
// Must be called before the PlatformDelegate is deleted.
void set_cancelled() {
cancelled_ = true;
bool is_cancelled() const {
return cancelled_;
scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
bool cancelled_;
virtual ~FilePathWatcher();
// A callback that always cleans up the PlatformDelegate, either when executed
// or when deleted without having been executed at all, as can happen during
// shutdown.
static void CancelWatch(const scoped_refptr<PlatformDelegate>& delegate);
// Returns true if the platform and OS version support recursive watches.
static bool RecursiveWatchAvailable();
// Invokes |callback| whenever updates to |path| are detected. This should be
// called at most once, and from a MessageLoop of TYPE_IO. Set |recursive| to
// true, to watch |path| and its children. The callback will be invoked on
// the same loop. Returns true on success.
// Recursive watch is not supported on all platforms and file systems.
// Watch() will return false in the case of failure.
bool Watch(const FilePath& path, bool recursive, const Callback& callback);
scoped_refptr<PlatformDelegate> impl_;
} // namespace base