ppapi/api/ppb_message_loop.idl

/* 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.
 */

/**
 * Defines the PPB_MessageLoop interface.
 */
label Chrome {
  M25 = 1.0
};

/**
 * A message loop allows PPAPI calls to be issued on a thread. You may not
 * issue any API calls on a thread without creating a message loop. It also
 * allows you to post work to the message loop for a thread.
 *
 * To process work posted to the message loop, as well as completion callbacks
 * for asynchronous operations, you must run the message loop via Run().
 *
 * Note the system manages the lifetime of the instance (and all associated
 * resources). If the instance is deleted from the page, background threads may
 * suddenly see their PP_Resource handles become invalid. In this case, calls
 * will fail with PP_ERROR_BADRESOURCE. If you need to access data associated
 * with your instance, you will probably want to create some kind of threadsafe
 * proxy object that can handle asynchronous destruction of the instance object.
 *
 * Typical usage:
 *   On the main thread:
 *    - Create the thread yourself (using pthreads).
 *    - Create the message loop resource.
 *    - Pass the message loop resource to your thread's main function.
 *    - Call PostWork() on the message loop to run functions on the thread.
 *
 *   From the background thread's main function:
 *    - Call AttachToCurrentThread() with the message loop resource.
 *    - Call Run() with the message loop resource.
 *
 *   Your callbacks should look like this:
 *   @code
 *   void DoMyWork(void* user_data, int32_t status) {
 *     if (status != PP_OK) {
 *       Cleanup();  // e.g. free user_data.
 *       return;
 *     }
 *     ... do your work...
 *   }
 *   @endcode
 * For a C++ example, see ppapi/utility/threading/simple_thread.h
 *
 * (You can also create the message loop resource on the background thread,
 * but then the main thread will have no reference to it should you want to
 * call PostWork()).
 *
 *
 * THREAD HANDLING
 *
 * The main thread has an implicitly created message loop. The main thread is
 * the thread where PPP_InitializeModule and PPP_Instance functions are called.
 * You can retrieve a reference to this message loop by calling
 * GetForMainThread() or, if your code is on the main thread, GetCurrent() will
 * also work.
 *
 * Some special threads created by the system can not have message loops. In
 * particular, the background thread created for audio processing has this
 * requirement because it's intended to be highly responsive to keep up with
 * the realtime requirements of audio processing. You can not make PPAPI calls
 * from these threads.
 *
 * Once you associate a message loop with a thread, you don't have to keep a
 * reference to it. The system will hold a reference to the message loop for as
 * long as the thread is running. The current message loop can be retrieved
 * using the GetCurrent() function.
 *
 * It is legal to create threads in your plugin without message loops, but
 * PPAPI calls will fail unless explicitly noted in the documentation.
 *
 * You can create a message loop object on a thread and never actually run the
 * message loop. This will allow you to call blocking PPAPI calls (via
 * PP_BlockUntilComplete()). If you make any asynchronous calls, the callbacks
 * from those calls will be queued in the message loop and never run. The same
 * thing will happen if work is scheduled after the message loop exits and
 * the message loop is not run again.
 *
 *
 * DESTRUCTION AND ERROR HANDLING
 *
 * Often, your application will associate memory with completion callbacks. For
 * example, the C++ CompletionCallbackFactory has a small amount of
 * heap-allocated memory for each callback. This memory will be leaked if the
 * callback is never run. To avoid this memory leak, you need to be careful
 * about error handling and shutdown.
 *
 * There are a number of cases where posted callbacks will never be run:
 *
 *  - You tear down the thread (via pthreads) without "destroying" the message
 *    loop (via PostQuit with should_destroy = PP_TRUE). In this case, any
 *    tasks in the message queue will be lost.
 *
 *  - You create a message loop, post callbacks to it, and never run it.
 *
 *  - You quit the message loop via PostQuit with should_destroy set to
 *    PP_FALSE. In this case, the system will assume the message loop will be
 *    run again later and keep your tasks.
 *
 * To do proper shutdown, call PostQuit with should_destroy = PP_TRUE. This
 * will prohibit future work from being posted, and will allow the message loop
 * to run until all pending tasks are run.
 *
 * If you post a callback to a message loop that's been destroyed, or to an
 * invalid message loop, PostWork will return an error and will not run the
 * callback. This is true even for callbacks with the "required" flag set,
 * since the system may not even know what thread to issue the error callback
 * on.
 *
 * Therefore, you should check for errors from PostWork and destroy any
 * associated memory to avoid leaks. If you're using the C++
 * CompletionCallbackFactory, use the following pattern:
 * @code
 * pp::CompletionCallback callback = factory_.NewOptionalCallback(...);
 * int32_t result = message_loop.PostWork(callback);
 * if (result != PP_OK)
 *   callback.Run(result);
 * @endcode
 * This will run the callback with an error value, and assumes that the
 * implementation of your callback checks the "result" argument and returns
 * immediately on error.
 */
interface PPB_MessageLoop {
  /**
   * Creates a message loop resource.
   *
   * This may be called from any thread. After your thread starts but before
   * issuing any other PPAPI calls on it, you must associate it with a message
   * loop by calling AttachToCurrentThread.
   */
  PP_Resource Create(PP_Instance instance);

  /**
   * Returns a resource identifying the message loop for the main thread. The
   * main thread always has a message loop created by the system.
   */
  PP_Resource GetForMainThread();

  /**
   * Returns a reference to the PPB_MessageLoop object attached to the current
   * thread. If there is no attached message loop, the return value will be 0.
   */
  PP_Resource GetCurrent();

  /**
   * Sets the given message loop resource as being the associated message loop
   * for the currently running thread.
   *
   * You must call this function exactly once on a thread before making any
   * PPAPI calls. A message loop can only be attached to one thread, and the
   * message loop can not be changed later. The message loop will be attached
   * as long as the thread is running or until you quit with should_destroy
   * set to PP_TRUE.
   *
   * If this function fails, attempting to run the message loop will fail.
   * Note that you can still post work to the message loop: it will get queued
   * up should the message loop eventually be successfully attached and run.
   *
   * @return
   *   - PP_OK: The message loop was successfully attached to the thread and is
   *     ready to use.
   *   - PP_ERROR_BADRESOURCE: The given message loop resource is invalid.
   *   - PP_ERROR_INPROGRESS: The current thread already has a message loop
   *     attached. This will always be the case for the main thread, which has
   *     an implicit system-created message loop attached.
   *   - PP_ERROR_WRONG_THREAD: The current thread type can not have a message
   *     loop attached to it. See the interface level discussion about these
   *     special threads, which include realtime audio threads.
   */
  int32_t AttachToCurrentThread([in] PP_Resource message_loop);

  /**
   * Runs the thread message loop. Running the message loop is required for you
   * to get issued completion callbacks on the thread.
   *
   * The message loop identified by the argument must have been previously
   * successfully attached to the current thread.
   *
   * You may not run nested run loops. Since the main thread has an
   * implicit message loop that the system runs, you may not call Run on the
   * main thread.
   *
   * @return
   *   - PP_OK: The message loop was successfully run. Note that on
   *     success, the message loop will only exit when you call PostQuit().
   *   - PP_ERROR_BADRESOURCE: The given message loop resource is invalid.
   *   - PP_ERROR_WRONG_THREAD: You are attempting to run a message loop that
   *     has not been successfully attached to the current thread. Call
   *     AttachToCurrentThread().
   *   - PP_ERROR_INPROGRESS: You are attempting to call Run in a nested
   *     fashion (Run is already on the stack). This will occur if you attempt
   *     to call run on the main thread's message loop (see above).
   */
  int32_t Run([in] PP_Resource message_loop);

  /**
   * Schedules work to run on the given message loop. This may be called from
   * any thread. Posted work will be executed in the order it was posted when
   * the message loop is Run().
   *
   * @param message_loop The message loop resource.
   *
   * @param callback The completion callback to execute from the message loop.
   *
   * @param delay_ms The number of milliseconds to delay execution of the given
   * completion callback. Passing 0 means it will get queued normally and
   * executed in order.
   *
   *
   * The completion callback will be called with PP_OK as the "result" parameter
   * if it is run normally. It is good practice to check for PP_OK and return
   * early otherwise.
   *
   * The "required" flag on the completion callback is ignored. If there is an
   * error posting your callback, the error will be returned from PostWork and
   * the callback will never be run (because there is no appropriate place to
   * run your callback with an error without causing unexpected threading
   * problems). If you associate memory with the completion callback (for
   * example, you're using the C++ CompletionCallbackFactory), you will need to
   * free this or manually run the callback. See "Destruction and error
   * handling" above.
   *
   *
   * You can call this function before the message loop has started and the
   * work will get queued until the message loop is run. You can also post
   * work after the message loop has exited as long as should_destroy was
   * PP_FALSE. It will be queued until the next invocation of Run().
   *
   * @return
   *   - PP_OK: The work was posted to the message loop's queue. As described
   *     above, this does not mean that the work has been or will be executed
   *     (if you never run the message loop after posting).
   *   - PP_ERROR_BADRESOURCE: The given message loop resource is invalid.
   *   - PP_ERROR_BADARGUMENT: The function pointer for the completion callback
   *     is null (this will be the case if you pass PP_BlockUntilComplete()).
   *   - PP_ERROR_FAILED: The message loop has been destroyed.
   */
  int32_t PostWork([in] PP_Resource message_loop,
                   [in] PP_CompletionCallback callback,
                   [in] int64_t delay_ms);

  /**
   * Posts a quit message to the given message loop's work queue. Work posted
   * before that point will be processed before quitting.
   *
   * This may be called on the message loop registered for the current thread,
   * or it may be called on the message loop registered for another thread. It
   * is an error to attempt to PostQuit() the main thread loop.
   *
   * @param should_destroy Marks the message loop as being in a destroyed state
   * and prevents further posting of messages.
   *
   * If you quit a message loop without setting should_destroy, it will still
   * be attached to the thread and you can still run it again by calling Run()
   * again. If you destroy it, it will be detached from the current thread.
   *
   * @return
   *   - PP_OK: The request to quit was successfully posted.
   *   - PP_ERROR_BADRESOURCE: The message loop was invalid.
   *   - PP_ERROR_WRONG_THREAD: You are attempting to quit the main thread.
   *     The main thread's message loop is managed by the system and can't be
   *     quit.
   */
  int32_t PostQuit([in] PP_Resource message_loop, PP_Bool should_destroy);
};