StateQueue.h revision dc998c809e084b617990b281e2ed5271830cc2e0 
1/*
2 * Copyright (C) 2012 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#ifndef ANDROID_AUDIO_STATE_QUEUE_H
18#define ANDROID_AUDIO_STATE_QUEUE_H
19
20namespace android {
21
22// manages a FIFO queue of states
23template<typename T> class StateQueue {
24
25public:
26            StateQueue();
27    virtual ~StateQueue();
28
29    // Observer APIs
30
31    // Poll for a state change.  Returns a pointer to a read-only state,
32    // or NULL if the state has not been initialized yet.
33    // If a new state has not pushed by mutator since the previous poll,
34    // then the returned pointer will be unchanged.
35    // The previous state pointer is guaranteed to still be valid;
36    // this allows the observer to diff the previous and new states.
37    const T* poll();
38
39    // Mutator APIs
40
41    // Begin a mutation.  Returns a pointer to a read/write state, except the
42    // first time it is called the state is write-only and _must_ be initialized.
43    // Mutations cannot be nested.
44    // If the state is dirty and has not been pushed onto the state queue yet, then
45    // this new mutation will be squashed together with the previous one.
46    T*      begin();
47
48    // End the current mutation and indicate whether caller modified the state.
49    // If didModify is true, then the state is marked dirty (in need of pushing).
50    // There is no rollback option because modifications are done in place.
51    // Does not automatically push the new state onto the state queue.
52    void    end(bool didModify = true);
53
54    // Push a new state, if any, out to the observer via the state queue.
55    // For BLOCK_NEVER, returns:
56    //      true if not dirty, or dirty and pushed successfully
57    //      false if dirty and not pushed because that would block; remains dirty
58    // For BLOCK_UNTIL_PUSHED and BLOCK_UNTIL_ACKED, always returns true.
59    // No-op if there are no pending modifications (not dirty), except
60    //      for BLOCK_UNTIL_ACKED it will wait until a prior push has been acknowledged.
61    // Must not be called in the middle of a mutation.
62    enum block_t {
63        BLOCK_NEVER,        // do not block
64        BLOCK_UNTIL_PUSHED, // block until there's a slot available for the push
65        BLOCK_UNTIL_ACKED,  // also block until the push is acknowledged by the observer
66    };
67    bool    push(block_t block = BLOCK_NEVER);
68
69    // Return whether the current state is dirty (modified and not pushed).
70    bool    isDirty() const { return mIsDirty; }
71
72private:
73    static const unsigned kN = 4;       // values != 4 are not supported by this code
74    T                 mStates[kN];      // written by mutator, read by observer
75
76    // "volatile" is meaningless with SMP, but here it indicates that we're using atomic ops
77    volatile const T* mNext; // written by mutator to advance next, read by observer
78    volatile const T* mAck;  // written by observer to acknowledge advance of next, read by mutator
79
80    // only used by observer
81    const T*          mCurrent;         // most recent value returned by poll()
82
83    // only used by mutator
84    T*                mMutating;        // where updates by mutator are done in place
85    const T*          mExpecting;       // what the mutator expects mAck to be set to
86    bool              mInMutation;      // whether we're currently in the middle of a mutation
87    bool              mIsDirty;         // whether mutating state has been modified since last push
88    bool              mIsInitialized;   // whether mutating state has been initialized yet
89
90};  // class StateQueue
91
92}   // namespace android
93
94#endif  // ANDROID_AUDIO_STATE_QUEUE_H
95