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      1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 //
      5 // CancelableCallback is a wrapper around base::Callback that allows
      6 // cancellation of a callback. CancelableCallback takes a reference on the
      7 // wrapped callback until this object is destroyed or Reset()/Cancel() are
      8 // called.
      9 //
     10 // NOTE:
     11 //
     12 // Calling CancelableCallback::Cancel() brings the object back to its natural,
     13 // default-constructed state, i.e., CancelableCallback::callback() will return
     14 // a null callback.
     15 //
     16 // THREAD-SAFETY:
     17 //
     18 // CancelableCallback objects must be created on, posted to, cancelled on, and
     19 // destroyed on the same thread.
     20 //
     21 //
     22 // EXAMPLE USAGE:
     23 //
     24 // In the following example, the test is verifying that RunIntensiveTest()
     25 // Quit()s the message loop within 4 seconds. The cancelable callback is posted
     26 // to the message loop, the intensive test runs, the message loop is run,
     27 // then the callback is cancelled.
     28 //
     29 // void TimeoutCallback(const std::string& timeout_message) {
     30 //   FAIL() << timeout_message;
     31 //   MessageLoop::current()->QuitWhenIdle();
     32 // }
     33 //
     34 // CancelableClosure timeout(base::Bind(&TimeoutCallback, "Test timed out."));
     35 // MessageLoop::current()->PostDelayedTask(FROM_HERE, timeout.callback(),
     36 //                                         4000)  // 4 seconds to run.
     37 // RunIntensiveTest();
     38 // MessageLoop::current()->Run();
     39 // timeout.Cancel();  // Hopefully this is hit before the timeout callback runs.
     40 //
     41 
     42 #ifndef BASE_CANCELABLE_CALLBACK_H_
     43 #define BASE_CANCELABLE_CALLBACK_H_
     44 
     45 #include <utility>
     46 
     47 #include "base/base_export.h"
     48 #include "base/bind.h"
     49 #include "base/callback.h"
     50 #include "base/callback_internal.h"
     51 #include "base/compiler_specific.h"
     52 #include "base/logging.h"
     53 #include "base/macros.h"
     54 #include "base/memory/weak_ptr.h"
     55 
     56 namespace base {
     57 
     58 template <typename Sig>
     59 class CancelableCallback;
     60 
     61 template <typename... A>
     62 class CancelableCallback<void(A...)> {
     63  public:
     64   CancelableCallback() : weak_factory_(this) {}
     65 
     66   // |callback| must not be null.
     67   explicit CancelableCallback(const base::Callback<void(A...)>& callback)
     68       : callback_(callback), weak_factory_(this) {
     69     DCHECK(!callback.is_null());
     70     InitializeForwarder();
     71   }
     72 
     73   ~CancelableCallback() {}
     74 
     75   // Cancels and drops the reference to the wrapped callback.
     76   void Cancel() {
     77     weak_factory_.InvalidateWeakPtrs();
     78     forwarder_.Reset();
     79     callback_.Reset();
     80   }
     81 
     82   // Returns true if the wrapped callback has been cancelled.
     83   bool IsCancelled() const {
     84     return callback_.is_null();
     85   }
     86 
     87   // Sets |callback| as the closure that may be cancelled. |callback| may not
     88   // be null. Outstanding and any previously wrapped callbacks are cancelled.
     89   void Reset(const base::Callback<void(A...)>& callback) {
     90     DCHECK(!callback.is_null());
     91 
     92     // Outstanding tasks (e.g., posted to a message loop) must not be called.
     93     Cancel();
     94 
     95     // |forwarder_| is no longer valid after Cancel(), so re-bind.
     96     InitializeForwarder();
     97 
     98     callback_ = callback;
     99   }
    100 
    101   // Returns a callback that can be disabled by calling Cancel().
    102   const base::Callback<void(A...)>& callback() const {
    103     return forwarder_;
    104   }
    105 
    106  private:
    107   void Forward(A... args) const {
    108     callback_.Run(std::forward<A>(args)...);
    109   }
    110 
    111   // Helper method to bind |forwarder_| using a weak pointer from
    112   // |weak_factory_|.
    113   void InitializeForwarder() {
    114     forwarder_ = base::Bind(&CancelableCallback<void(A...)>::Forward,
    115                             weak_factory_.GetWeakPtr());
    116   }
    117 
    118   // The wrapper closure.
    119   base::Callback<void(A...)> forwarder_;
    120 
    121   // The stored closure that may be cancelled.
    122   base::Callback<void(A...)> callback_;
    123 
    124   // Used to ensure Forward() is not run when this object is destroyed.
    125   base::WeakPtrFactory<CancelableCallback<void(A...)>> weak_factory_;
    126 
    127   DISALLOW_COPY_AND_ASSIGN(CancelableCallback);
    128 };
    129 
    130 typedef CancelableCallback<void(void)> CancelableClosure;
    131 
    132 }  // namespace base
    133 
    134 #endif  // BASE_CANCELABLE_CALLBACK_H_
    135