Add Swappy & Pre-Transformed Swapchain

- Adds Swappy for Android for stable frame pacing
- Implements pre-transformed Swapchain so that Godot's compositor is in
charge of rotating the screen instead of Android's compositor
(performance optimization for phones that don't have HW rotator)

============================

The work was performed by collaboration of TheForge and Google. I am
merely splitting it up into smaller PRs and cleaning it up.

Changes from original PR:

- Removed "display/window/frame_pacing/android/target_frame_rate" option
to use Engine::get_max_fps instead.
- Target framerate can be changed at runtime using Engine::set_max_fps.
- Swappy is enabled by default.
- Added documentation.
- enable_auto_swap setting is replaced with swappy_mode.

3 files changed, 739 insertions, 0 deletions

diff --git a/thirdparty/swappy-frame-pacing/common/gamesdk_common.h b/thirdparty/swappy-frame-pacing/common/gamesdk_common.h
new file mode 100644
index 0000000000..d29ac01af3
--- /dev/null
+++ b/thirdparty/swappy-frame-pacing/common/gamesdk_common.h
@@ -0,0 +1,41 @@
+/*
+ * Copyright 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/*
+ * This is the main interface to the Android Performance Tuner library, also
+ * known as Tuning Fork.
+ *
+ * It is part of the Android Games SDK and produces best results when integrated
+ * with the Swappy Frame Pacing Library.
+ *
+ * See the documentation at
+ * https://developer.android.com/games/sdk/performance-tuner/custom-engine for
+ * more information on using this library in a native Android game.
+ *
+ */
+
+#pragma once
+
+// There are separate versions for each GameSDK component that use this format:
+#define ANDROID_GAMESDK_PACKED_VERSION(MAJOR, MINOR, BUGFIX) \
+    ((MAJOR << 16) | (MINOR << 8) | (BUGFIX))
+// Accessors
+#define ANDROID_GAMESDK_MAJOR_VERSION(PACKED) ((PACKED) >> 16)
+#define ANDROID_GAMESDK_MINOR_VERSION(PACKED) (((PACKED) >> 8) & 0xff)
+#define ANDROID_GAMESDK_BUGFIX_VERSION(PACKED) ((PACKED) & 0xff)
+
+#define AGDK_STRING_VERSION(MAJOR, MINOR, BUGFIX, GIT) \
+#MAJOR "." #MINOR "." #BUGFIX "." #GIT
diff --git a/thirdparty/swappy-frame-pacing/swappyVk.h b/thirdparty/swappy-frame-pacing/swappyVk.h
new file mode 100644
index 0000000000..020683cbc4
--- /dev/null
+++ b/thirdparty/swappy-frame-pacing/swappyVk.h
@@ -0,0 +1,420 @@
+/*
+ * Copyright 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * @defgroup swappyVk Swappy for Vulkan
+ * Vulkan part of Swappy.
+ * @{
+ */
+
+#pragma once
+
+#include "jni.h"
+#include "swappy_common.h"
+
+#ifndef VK_NO_PROTOTYPES
+#define VK_NO_PROTOTYPES 1
+#endif
+#include <vulkan/vulkan.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Determine any Vulkan device extensions that must be enabled for a new
+ * VkDevice.
+ *
+ * Swappy-for-Vulkan (SwappyVk) benefits from certain Vulkan device extensions
+ * (e.g. VK_GOOGLE_display_timing).  Before the application calls
+ * vkCreateDevice, SwappyVk needs to look at the list of available extensions
+ * (returned by vkEnumerateDeviceExtensionProperties) and potentially identify
+ * one or more extensions that the application must add to:
+ *
+ * - VkDeviceCreateInfo::enabledExtensionCount
+ * - VkDeviceCreateInfo::ppEnabledExtensionNames
+ *
+ * before the application calls vkCreateDevice.  For each VkPhysicalDevice that
+ * the application will call vkCreateDevice for, the application must call this
+ * function, and then must add the identified extension(s) to the list that are
+ * enabled for the VkDevice.  Similar to many Vulkan functions, this function
+ * can be called twice, once to identify the number of required extensions, and
+ * again with application-allocated memory that the function can write into.
+ *
+ * @param[in]    physicalDevice          - The VkPhysicalDevice associated with
+ * the available extensions.
+ * @param[in]    availableExtensionCount - This is the returned value of
+ *                    pPropertyCount from vkEnumerateDeviceExtensionProperties.
+ * @param[in]    pAvailableExtensions    - This is the returned value of
+ *                    pProperties from vkEnumerateDeviceExtensionProperties.
+ * @param[inout] pRequiredExtensionCount - If pRequiredExtensions is nullptr,
+ * the function sets this to the number of extensions that are required.  If
+ * pRequiredExtensions is non-nullptr, this is the number of required extensions
+ * that the function should write into pRequiredExtensions.
+ * @param[inout] pRequiredExtensions - If non-nullptr, this is
+ * application-allocated memory into which the function will write the names of
+ *                    required extensions.  It is a pointer to an array of
+ *                    char* strings (i.e. the same as
+ *                    VkDeviceCreateInfo::ppEnabledExtensionNames).
+ */
+void SwappyVk_determineDeviceExtensions(
+    VkPhysicalDevice physicalDevice, uint32_t availableExtensionCount,
+    VkExtensionProperties* pAvailableExtensions,
+    uint32_t* pRequiredExtensionCount, char** pRequiredExtensions);
+
+/**
+ * @brief Tell Swappy the queueFamilyIndex used to create a specific VkQueue
+ *
+ * Swappy needs to know the queueFamilyIndex used for creating a specific
+ * VkQueue so it can use it when presenting.
+ *
+ * @param[in]  device            - The VkDevice associated with the queue
+ * @param[in]  queue             - A device queue.
+ * @param[in]  queueFamilyIndex  - The queue family index used to create the
+ * VkQueue.
+ *
+ */
+void SwappyVk_setQueueFamilyIndex(VkDevice device, VkQueue queue,
+                                  uint32_t queueFamilyIndex);
+
+// TBD: For now, SwappyVk assumes only one VkSwapchainKHR per VkDevice, and that
+// applications don't re-create swapchains.  Is this long-term sufficient?
+
+/**
+ * Internal init function. Do not call directly.
+ * See SwappyVk_initAndGetRefreshCycleDuration instead.
+ * @private
+ */
+bool SwappyVk_initAndGetRefreshCycleDuration_internal(
+    JNIEnv* env, jobject jactivity, VkPhysicalDevice physicalDevice,
+    VkDevice device, VkSwapchainKHR swapchain, uint64_t* pRefreshDuration);
+
+/**
+ * @brief Initialize SwappyVk for a given device and swapchain, and obtain the
+ * approximate time duration between vertical-blanking periods.
+ *
+ * Uses JNI to query AppVsyncOffset and PresentationDeadline.
+ *
+ * If your application presents to more than one swapchain at a time, you must
+ * call this for each swapchain before calling swappyVkSetSwapInterval() for it.
+ *
+ * The duration between vertical-blanking periods (an interval) is expressed as
+ * the approximate number of nanoseconds between vertical-blanking periods of
+ * the swapchain’s physical display.
+ *
+ * If the application converts this number to a fraction (e.g. 16,666,666 nsec
+ * to 0.016666666) and divides one by that fraction, it will be the approximate
+ * refresh rate of the display (e.g. 16,666,666 nanoseconds corresponds to a
+ * 60Hz display, 11,111,111 nsec corresponds to a 90Hz display).
+ *
+ * @param[in]  env - JNIEnv that is assumed to be from AttachCurrentThread
+ * function
+ * @param[in]  jactivity - NativeActivity object handle, used for JNI
+ * @param[in]  physicalDevice   - The VkPhysicalDevice associated with the
+ * swapchain
+ * @param[in]  device    - The VkDevice associated with the swapchain
+ * @param[in]  swapchain - The VkSwapchainKHR the application wants Swappy to
+ * swap
+ * @param[out] pRefreshDuration - The returned refresh cycle duration
+ *
+ * @return bool            - true if the value returned by pRefreshDuration is
+ * valid, otherwise false if an error.
+ */
+bool SwappyVk_initAndGetRefreshCycleDuration(JNIEnv* env, jobject jactivity,
+                                             VkPhysicalDevice physicalDevice,
+                                             VkDevice device,
+                                             VkSwapchainKHR swapchain,
+                                             uint64_t* pRefreshDuration);
+
+/**
+ * @brief Tell Swappy which ANativeWindow to use when calling to ANativeWindow_*
+ * API.
+ * @param[in]  device    - The VkDevice associated with the swapchain
+ * @param[in]  swapchain - The VkSwapchainKHR the application wants Swappy to
+ * swap
+ * @param[in]  window    - The ANativeWindow that was used to create the
+ * VkSwapchainKHR
+ */
+void SwappyVk_setWindow(VkDevice device, VkSwapchainKHR swapchain,
+                        ANativeWindow* window);
+
+/**
+ * @brief Tell Swappy the duration of that each presented image should be
+ * visible.
+ *
+ * If your application presents to more than one swapchain at a time, you must
+ * call this for each swapchain before presenting to it.
+ *
+ * @param[in]  device    - The VkDevice associated with the swapchain
+ * @param[in]  swapchain - The VkSwapchainKHR the application wants Swappy to
+ * swap
+ * @param[in]  swap_ns   - The duration of that each presented image should be
+ *                    visible in nanoseconds
+ */
+void SwappyVk_setSwapIntervalNS(VkDevice device, VkSwapchainKHR swapchain,
+                                uint64_t swap_ns);
+
+/**
+ * @brief Tell Swappy to present one or more images to corresponding swapchains.
+ *
+ * Swappy will call vkQueuePresentKHR for your application.  Swappy may insert a
+ * struct to the pNext-chain of VkPresentInfoKHR, or it may insert other Vulkan
+ * commands in order to attempt to honor the desired swap interval.
+ *
+ * @note If your application presents to more than one swapchain at a time, and
+ * if you use a different swap interval for each swapchain, Swappy will attempt
+ * to honor the swap interval for each swapchain (being more successful on
+ * devices that support an underlying presentation-timing extension, such as
+ * VK_GOOGLE_display_timing).
+ *
+ * @param[in]  queue     - The VkQueue associated with the device and swapchain
+ * @param[in]  pPresentInfo - A pointer to the VkPresentInfoKHR containing the
+ *                    information about what image(s) to present on which
+ *                    swapchain(s).
+ */
+VkResult SwappyVk_queuePresent(VkQueue queue,
+                               const VkPresentInfoKHR* pPresentInfo);
+
+/**
+ * @brief Destroy the SwappyVk instance associated with a swapchain.
+ *
+ * This API is expected to be called before calling vkDestroySwapchainKHR()
+ * so Swappy can cleanup its internal state.
+ *
+ * @param[in]  device    - The VkDevice associated with SwappyVk
+ * @param[in]  swapchain - The VkSwapchainKHR the application wants Swappy to
+ * destroy
+ */
+void SwappyVk_destroySwapchain(VkDevice device, VkSwapchainKHR swapchain);
+
+/**
+ * @brief Destroy any swapchains associated with the device and clean up the
+ * device's resources
+ *
+ * This function should be called after SwappyVk_destroySwapchain if you no
+ * longer need the device.
+ *
+ * @param[in]  device     - The VkDevice associated with SwappyVk
+ */
+void SwappyVk_destroyDevice(VkDevice device);
+
+/**
+ * @brief Enables Auto-Swap-Interval feature for all instances.
+ *
+ * By default this feature is enabled. Changing it is completely
+ * optional for fine-tuning swappy behaviour.
+ *
+ * @param[in]  enabled - True means enable, false means disable
+ */
+void SwappyVk_setAutoSwapInterval(bool enabled);
+
+/**
+ * @brief Enables Auto-Pipeline-Mode feature for all instances.
+ *
+ * By default this feature is enabled. Changing it is completely
+ * optional for fine-tuning swappy behaviour.
+ *
+ * @param[in]  enabled - True means enable, false means disable
+ */
+void SwappyVk_setAutoPipelineMode(bool enabled);
+
+/**
+ * @brief Sets the maximal swap duration for all instances.
+ *
+ * Sets the maximal duration for Auto-Swap-Interval in milliseconds.
+ * If SwappyVk is operating in Auto-Swap-Interval and the frame duration is
+ * longer than the provided duration, SwappyVk will not do any pacing and just
+ * submit the frame as soon as possible.
+ *
+ * @param[in]  max_swap_ns - maximal swap duration in milliseconds.
+ */
+void SwappyVk_setMaxAutoSwapIntervalNS(uint64_t max_swap_ns);
+
+/**
+ * @brief The fence timeout parameter can be set for devices with faulty
+ * drivers. Its default value is 50,000,000.
+ */
+void SwappyVk_setFenceTimeoutNS(uint64_t fence_timeout_ns);
+
+/**
+ * @brief Get the fence timeout parameter, for devices with faulty
+ * drivers. Its default value is 50,000,000.
+ */
+uint64_t SwappyVk_getFenceTimeoutNS();
+
+/**
+ * @brief Inject callback functions to be called each frame.
+ *
+ * @param[in]  tracer - Collection of callback functions
+ */
+void SwappyVk_injectTracer(const SwappyTracer* tracer);
+
+/**
+ * @brief Remove callbacks that were previously added using
+ * SwappyVk_injectTracer.
+ *
+ * Only removes callbacks that were previously added using
+ * SwappyVK_injectTracer. If SwappyVK_injectTracker was not called with the
+ * tracer, then there is no effect.
+ *
+ * @param[in]  tracer - Collection of callback functions
+ */
+void SwappyVk_uninjectTracer(const SwappyTracer* tracer);
+
+/**
+ * @brief A structure enabling you to provide your own Vulkan function wrappers
+ * by calling ::SwappyVk_setFunctionProvider.
+ *
+ * Usage of this functionality is optional.
+ */
+typedef struct SwappyVkFunctionProvider {
+    /**
+     * @brief Callback to initialize the function provider.
+     *
+     * This function is called by Swappy before any functions are requested.
+     * E.g. so you can call dlopen on the Vulkan library.
+     */
+    bool (*init)();
+
+    /**
+     * @brief Callback to get the address of a function.
+     *
+     * This function is called by Swappy to get the address of a Vulkan
+     * function.
+     * @param name The null-terminated name of the function.
+     */
+    void* (*getProcAddr)(const char* name);
+
+    /**
+     * @brief Callback to close any resources owned by the function provider.
+     *
+     * This function is called by Swappy when no more functions will be
+     * requested, e.g. so you can call dlclose on the Vulkan library.
+     */
+    void (*close)();
+} SwappyVkFunctionProvider;
+
+/**
+ * @brief Set the Vulkan function provider.
+ *
+ * This enables you to provide an object that will be used to look up Vulkan
+ * functions, e.g. to hook usage of these functions.
+ *
+ * To use this functionality, you *must* call this function before any others.
+ *
+ * Usage of this function is entirely optional. If you do not use it, the Vulkan
+ * functions required by Swappy will be dynamically loaded from libvulkan.so.
+ *
+ * @param[in] provider - provider object
+ */
+void SwappyVk_setFunctionProvider(
+    const SwappyVkFunctionProvider* pSwappyVkFunctionProvider);
+
+/**
+ * @brief Get the swap interval value, in nanoseconds, for a given swapchain.
+ *
+ * @param[in] swapchain - the swapchain to query
+ */
+uint64_t SwappyVk_getSwapIntervalNS(VkSwapchainKHR swapchain);
+
+/**
+ * @brief Get the supported refresh periods of this device. Call once with
+ * out_refreshrates set to nullptr to get the number of supported refresh
+ * periods, then call again passing that number as allocated_entries and
+ * an array of size equal to allocated_entries that will be filled with the
+ * refresh periods.
+ */
+int SwappyVk_getSupportedRefreshPeriodsNS(uint64_t* out_refreshrates,
+                                          int allocated_entries,
+                                          VkSwapchainKHR swapchain);
+/**
+ * @brief Check if Swappy is enabled for the specified swapchain.
+ *
+ * @return false if SwappyVk_initAndGetRefreshCycleDuration was not
+ * called for the specified swapchain, true otherwise.
+ */
+bool SwappyVk_isEnabled(VkSwapchainKHR swapchain, bool* isEnabled);
+
+/**
+ * @brief Toggle statistics collection on/off
+ *
+ * By default, stats collection is off and there is no overhead related to
+ * stats. An app can turn on stats collection by calling
+ * `SwappyVk_enableStats(swapchain, true)`. Then, the app is expected to call
+ * ::SwappyVk_recordFrameStart for each frame before starting to do any CPU
+ * related work. Stats will be logged to logcat with a 'FrameStatistics' tag. An
+ * app can get the stats by calling ::SwappyVk_getStats.
+ *
+ * SwappyVk_initAndGetRefreshCycleDuration must have been called successfully
+ * before for this swapchain, otherwise there is no effect in this call. Frame
+ * stats are only available if the platform supports VK_GOOGLE_display_timing
+ * extension.
+ *
+ * @param[in]  swapchain - The swapchain for which frame stat collection is
+ *                           configured.
+ * @param      enabled   - Whether to enable/disable frame stat collection.
+ */
+void SwappyVk_enableStats(VkSwapchainKHR swapchain, bool enabled);
+
+/**
+ * @brief Should be called if stats have been enabled with SwappyVk_enableStats.
+ *
+ * When stats collection is enabled with SwappyVk_enableStats, the app is
+ * expected to call this function for each frame before starting to do any CPU
+ * related work. It is assumed that this function will be called after a
+ * successful call to vkAcquireNextImageKHR. See ::SwappyVk_enableStats for more
+ * conditions.
+ *
+ * @param[in]  queue     - The VkQueue associated with the device and swapchain
+ * @param[in]  swapchain - The swapchain where the frame is presented to.
+ * @param[in]  image     - The image in swapchain that corresponds to the frame.
+
+ * @see SwappyVk_enableStats.
+ */
+void SwappyVk_recordFrameStart(VkQueue queue, VkSwapchainKHR swapchain, uint32_t image);
+
+/**
+ * @brief Returns the stats collected, if statistics collection was toggled on.
+ *
+ * Given that this API uses VkSwapchainKHR and the potential for this call to be
+ * done on different threads, all calls to ::SwappyVk_getStats
+ * must be externally synchronized with other SwappyVk calls. Unsynchronized
+ * calls may lead to undefined behavior. See ::SwappyVk_enableStats for more
+ * conditions.
+ *
+ * @param[in]  swapchain   - The swapchain for which stats are being queried.
+ * @param      swappyStats - Pointer to a SwappyStats that will be populated with
+ *                             the collected stats. Cannot be NULL.
+ * @see SwappyStats
+ */
+void SwappyVk_getStats(VkSwapchainKHR swapchain, SwappyStats *swappyStats);
+
+/**
+ * @brief Clears the frame statistics collected so far.
+ *
+ * All the frame statistics collected are reset to 0, frame statistics are
+ * collected normally after this call. See ::SwappyVk_enableStats for more
+ * conditions.
+ *
+ * @param[in]  swapchain   - The swapchain for which stats are being cleared.
+ */
+void SwappyVk_clearStats(VkSwapchainKHR swapchain);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+/** @} */
diff --git a/thirdparty/swappy-frame-pacing/swappy_common.h b/thirdparty/swappy-frame-pacing/swappy_common.h
new file mode 100644
index 0000000000..b711ca910f
--- /dev/null
+++ b/thirdparty/swappy-frame-pacing/swappy_common.h
@@ -0,0 +1,278 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * @defgroup swappy_common Swappy common tools
+ * Tools to be used with Swappy for OpenGL or Swappy for Vulkan.
+ * @{
+ */
+
+#pragma once
+
+#include <android/native_window.h>
+#include <stdint.h>
+
+#include "common/gamesdk_common.h"
+
+/** @brief Swap interval for 60fps, in nanoseconds. */
+#define SWAPPY_SWAP_60FPS (16666667L)
+
+/** @brief Swap interval for 30fps, in nanoseconds. */
+#define SWAPPY_SWAP_30FPS (33333333L)
+
+/** @brief Swap interval for 20fps, in nanoseconds. */
+#define SWAPPY_SWAP_20FPS (50000000L)
+
+/**
+ * The longest duration, in refresh periods, represented by the statistics.
+ * @see SwappyStats
+ */
+#define MAX_FRAME_BUCKETS 6
+
+/** @cond INTERNAL */
+
+#define SWAPPY_SYSTEM_PROP_KEY_DISABLE "swappy.disable"
+
+// Internal macros to track Swappy version, do not use directly.
+#define SWAPPY_MAJOR_VERSION 2
+#define SWAPPY_MINOR_VERSION 0
+#define SWAPPY_BUGFIX_VERSION 0
+#define SWAPPY_PACKED_VERSION                                                  \
+    ANDROID_GAMESDK_PACKED_VERSION(SWAPPY_MAJOR_VERSION, SWAPPY_MINOR_VERSION, \
+                                   SWAPPY_BUGFIX_VERSION)
+
+// Internal macros to generate a symbol to track Swappy version, do not use
+// directly.
+#define SWAPPY_VERSION_CONCAT_NX(PREFIX, MAJOR, MINOR, BUGFIX, GITCOMMIT) \
+    PREFIX##_##MAJOR##_##MINOR##_##BUGFIX##_##GITCOMMIT
+#define SWAPPY_VERSION_CONCAT(PREFIX, MAJOR, MINOR, BUGFIX, GITCOMMIT) \
+    SWAPPY_VERSION_CONCAT_NX(PREFIX, MAJOR, MINOR, BUGFIX, GITCOMMIT)
+#define SWAPPY_VERSION_SYMBOL                                          \
+    SWAPPY_VERSION_CONCAT(Swappy_version, SWAPPY_MAJOR_VERSION,        \
+                          SWAPPY_MINOR_VERSION, SWAPPY_BUGFIX_VERSION, \
+                          AGDK_GIT_COMMIT)
+
+// Define this to 1 to enable all logging from Swappy, by default it is
+// disabled in a release build and enabled in a debug build.
+#ifndef ENABLE_SWAPPY_LOGGING
+#define ENABLE_SWAPPY_LOGGING 0
+#endif
+/** @endcond */
+
+/** @brief Id of a thread returned by an external thread manager. */
+typedef uint64_t SwappyThreadId;
+
+/**
+ * @brief A structure enabling you to set how Swappy starts and joins threads by
+ * calling
+ * ::Swappy_setThreadFunctions.
+ *
+ * Usage of this functionality is optional.
+ */
+typedef struct SwappyThreadFunctions {
+    /** @brief Thread start callback.
+     *
+     * This function is called by Swappy to start thread_func on a new thread.
+     * @param user_data A value to be passed the thread function.
+     * If the thread was started, this function should set the thread_id and
+     * return 0. If the thread was not started, this function should return a
+     * non-zero value.
+     */
+    int (*start)(SwappyThreadId* thread_id, void* (*thread_func)(void*),
+                 void* user_data);
+
+    /** @brief Thread join callback.
+     *
+     * This function is called by Swappy to join the thread with given id.
+     */
+    void (*join)(SwappyThreadId thread_id);
+
+    /** @brief Thread joinable callback.
+     *
+     * This function is called by Swappy to discover whether the thread with the
+     * given id is joinable.
+     */
+    bool (*joinable)(SwappyThreadId thread_id);
+} SwappyThreadFunctions;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Return the version of the Swappy library at runtime.
+ */
+uint32_t Swappy_version();
+
+/**
+ * @brief Call this before any other functions in order to use a custom thread
+ * manager.
+ *
+ * Usage of this function is entirely optional. Swappy uses std::thread by
+ * default.
+ *
+ */
+void Swappy_setThreadFunctions(const SwappyThreadFunctions* thread_functions);
+
+/**
+ * @brief Return the full version of the Swappy library at runtime, e.g.
+ * "1.9.0_8a85ab7c46"
+ */
+const char* Swappy_versionString();
+
+/**
+ * @brief Swappy frame statistics, collected if toggled on with
+ * ::SwappyGL_enableStats or ::SwappyVk_enableStats.
+ */
+typedef struct SwappyStats {
+    /** @brief Total frames swapped by swappy */
+    uint64_t totalFrames;
+
+    /** @brief Histogram of the number of screen refreshes a frame waited in the
+     * compositor queue after rendering was completed.
+     *
+     * For example:
+     *     if a frame waited 2 refresh periods in the compositor queue after
+     * rendering was done, the frame will be counted in idleFrames[2]
+     */
+    uint64_t idleFrames[MAX_FRAME_BUCKETS];
+
+    /** @brief Histogram of the number of screen refreshes passed between the
+     * requested presentation time and the actual present time.
+     *
+     * For example:
+     *     if a frame was presented 2 refresh periods after the requested
+     * timestamp swappy set, the frame will be counted in lateFrames[2]
+     */
+    uint64_t lateFrames[MAX_FRAME_BUCKETS];
+
+    /** @brief Histogram of the number of screen refreshes passed between two
+     * consecutive frames
+     *
+     * For example:
+     *     if frame N was presented 2 refresh periods after frame N-1
+     *     frame N will be counted in offsetFromPreviousFrame[2]
+     */
+    uint64_t offsetFromPreviousFrame[MAX_FRAME_BUCKETS];
+
+    /** @brief Histogram of the number of screen refreshes passed between the
+     * call to Swappy_recordFrameStart and the actual present time.
+     *
+     * For example:
+     *     if a frame was presented 2 refresh periods after the call to
+     * `Swappy_recordFrameStart` the frame will be counted in latencyFrames[2]
+     */
+    uint64_t latencyFrames[MAX_FRAME_BUCKETS];
+} SwappyStats;
+
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+/**
+ * Pointer to a function that can be attached to SwappyTracer::preWait
+ * @param userData Pointer to arbitrary data, see SwappyTracer::userData.
+ */
+typedef void (*SwappyPreWaitCallback)(void*);
+
+/**
+ * Pointer to a function that can be attached to SwappyTracer::postWait.
+ * @param userData Pointer to arbitrary data, see SwappyTracer::userData.
+ * @param cpu_time_ns Time for CPU processing of this frame in nanoseconds.
+ * @param gpu_time_ns Time for GPU processing of previous frame in nanoseconds.
+ */
+typedef void (*SwappyPostWaitCallback)(void*, int64_t cpu_time_ns,
+                                       int64_t gpu_time_ns);
+
+/**
+ * Pointer to a function that can be attached to SwappyTracer::preSwapBuffers.
+ * @param userData Pointer to arbitrary data, see SwappyTracer::userData.
+ */
+typedef void (*SwappyPreSwapBuffersCallback)(void*);
+
+/**
+ * Pointer to a function that can be attached to SwappyTracer::postSwapBuffers.
+ * @param userData Pointer to arbitrary data, see SwappyTracer::userData.
+ * @param desiredPresentationTimeMillis The target time, in milliseconds, at
+ * which the frame would be presented on screen.
+ */
+typedef void (*SwappyPostSwapBuffersCallback)(
+    void*, int64_t desiredPresentationTimeMillis);
+
+/**
+ * Pointer to a function that can be attached to SwappyTracer::startFrame.
+ * @param userData Pointer to arbitrary data, see SwappyTracer::userData.
+ * @param desiredPresentationTimeMillis The time, in milliseconds, at which the
+ * frame is scheduled to be presented.
+ */
+typedef void (*SwappyStartFrameCallback)(void*, int currentFrame,
+                                         int64_t desiredPresentationTimeMillis);
+
+/**
+ * Pointer to a function that can be attached to
+ * SwappyTracer::swapIntervalChanged. Call ::SwappyGL_getSwapIntervalNS or
+ * ::SwappyVk_getSwapIntervalNS to get the latest swapInterval.
+ * @param userData Pointer to arbitrary data, see SwappyTracer::userData.
+ */
+typedef void (*SwappySwapIntervalChangedCallback)(void*);
+
+/**
+ * @brief Collection of callbacks to be called each frame to trace execution.
+ *
+ * Injection of these is optional.
+ */
+typedef struct SwappyTracer {
+    /**
+     * Callback called before waiting to queue the frame to the composer.
+     */
+    SwappyPreWaitCallback preWait;
+
+    /**
+     * Callback called after wait to queue the frame to the composer is done.
+     */
+    SwappyPostWaitCallback postWait;
+
+    /**
+     * Callback called before calling the function to queue the frame to the
+     * composer.
+     */
+    SwappyPreSwapBuffersCallback preSwapBuffers;
+
+    /**
+     * Callback called after calling the function to queue the frame to the
+     * composer.
+     */
+    SwappyPostSwapBuffersCallback postSwapBuffers;
+
+    /**
+     * Callback called at the start of a frame.
+     */
+    SwappyStartFrameCallback startFrame;
+
+    /**
+     * Pointer to some arbitrary data that will be passed as the first argument
+     * of callbacks.
+     */
+    void* userData;
+
+    /**
+     * Callback called when the swap interval was changed.
+     */
+    SwappySwapIntervalChangedCallback swapIntervalChanged;
+} SwappyTracer;
+
+/** @} */