1. 前言

Filament中使用了FrameGraph來管理渲染管線，需要準(zhǔn)備兩點(diǎn)：

• 設(shè)備接口抽象：設(shè)備API抽象為Command
• 資源抽象：使用虛擬資源，在實(shí)際用到時(shí)再創(chuàng)建，方便剔除無用資源

下面就圍繞Filament中設(shè)備API抽象為Command代碼部分做一個(gè)解讀：

2. 代碼分析

先貼一段創(chuàng)建頂點(diǎn)緩沖的接口調(diào)用堆棧：

[Inlined] filament::backend::CommandBase::CommandBase(void (*)(filament::backend::Driver &, filament::backend::CommandBase *, int *)) CommandStream.h:63
[Inlined] filament::backend::CommandType<void (filament::backend::Driver::*)(filament::backend::Handle<filament::backend::HwVertexBuffer>, unsigned char, unsigned char, unsigned int, std::__ndk1::array<filament::backend::Attribute, 16u>)>::Command<&filament::backend::Driver::createVertexBufferR(filament::backend::Handle<filament::backend::HwVertexBuffer>, unsigned char, unsigned char, unsigned int, std::__ndk1::array<filament::backend::Attribute, 16u>)>::Command<filament::backend::Handle<filament::backend::HwVertexBuffer>, unsigned char, unsigned char, unsigned int, std::__ndk1::array<filament::backend::Attribute, 16u>>(void (*)(filament::backend::Driver&, filament::backend::CommandBase*, int*), filament::backend::Handle<filament::backend::HwVertexBuffer>&&, unsigned char&&, unsigned char&&, unsigned int&&, std::__ndk1::array<filament::backend::Attribute, 16u>&&) CommandStream.h:154
[Inlined] filament::backend::CommandStream::createVertexBuffer(unsigned char, unsigned char, unsigned int, std::__ndk1::array<…>) DriverAPI.inc:169
filament::FVertexBuffer::FVertexBuffer(filament::FEngine &, const filament::VertexBuffer::Builder &) VertexBuffer.cpp:185
[Inlined] utils::Arena::make<…>(filament::FEngine &, const filament::VertexBuffer::Builder &) Allocator.h:647
[Inlined] filament::FEngine::create<…>(filament::ResourceList<…> &, const filament::FVertexBuffer::Builder &) Engine.cpp:680
filament::FEngine::createVertexBuffer(const filament::VertexBuffer::Builder &) Engine.cpp:690
filament::FEngine::init() Engine.cpp:277
filament::FEngine::create(filament::backend::Backend, filament::backend::Platform *, void *, const filament::Engine::Config *) Engine.cpp:110
[Inlined] FilamentTest::setupFilament() FilamentTest.cpp:98
FilamentTest::init() FilamentTest.cpp:68
boxing::xr::composer::StartBase::instance(ANativeWindow *, int, int) StartBase.h:263
[Inlined] native_OnDrawFrame::$_0::operator()() const JniImpl.cpp:100
[Inlined] std::__ndk1::__invoke<…>(native_OnDrawFrame::$_0 &) type_traits:3874
[Inlined] std::__ndk1::__apply_functor<…>(native_OnDrawFrame::$_0 &, std::__ndk1::tuple<…> &, std::__ndk1::__tuple_indices<…>, std::__ndk1::tuple<…> &&) functional:2853
[Inlined] std::__ndk1::__bind::operator()<…>() functional:2886
[Inlined] std::__ndk1::__invoke<…>(std::__ndk1::__bind<…> &) type_traits:3874
std::__ndk1::__packaged_task_func::operator()() future:1817
[Inlined] std::__ndk1::__packaged_task_function::operator()() const future:1994
std::__ndk1::packaged_task::operator()() future:2214
[Inlined] std::__ndk1::__function::__value_func::operator()() const functional:1884
[Inlined] std::__ndk1::function::operator()() const functional:2556
<lambda>::operator()() const ThreadPool.h:71
[Inlined] decltype(std::__ndk1::forward<boxing::core::ThreadPool::ThreadPool(unsigned int)::'lambda'()>(fp)()) std::__ndk1::__invoke<boxing::core::ThreadPool::ThreadPool(unsigned int)::'lambda'()>(boxing::core::ThreadPool::ThreadPool(unsigned int)::'lambda'()&&) type_traits:3874
[Inlined] std::__ndk1::__thread_execute<…>(std::__ndk1::tuple<…> &, std::__ndk1::__tuple_indices<…>) thread:273
std::__ndk1::__thread_proxy<…>(void *) thread:284
__pthread_start(void*) 0x00000000eab36828
__start_thread 0x00000000eaaed5ce

渲染設(shè)備API定義：

filament\filament\backend\include\private\backend\DriverAPI.inc

DriverAPI.inc中使用大量的宏替換操作，將設(shè)備接口進(jìn)行封裝，或打包，這部分代碼可讀性極差，不過可從其調(diào)用邏輯來進(jìn)行拆解和理解：
先來分析其中一個(gè)接口: createVertexBuffer 創(chuàng)建一個(gè)頂點(diǎn)緩沖

DECL_DRIVER_API_R_N(backend::VertexBufferHandle, createVertexBuffer,uint8_t, bufferCount,uint8_t, attributeCount,uint32_t, vertexCount,backend::AttributeArray, attributes)

這里不是真的創(chuàng)建，而要看這個(gè)宏接口在哪里使用，我們主要看看這兩個(gè)地方：

  CommandStream.h  //命令流Driver.h   //設(shè)備接口

這兩個(gè)文件中都對(duì)DriverAPI.inc進(jìn)行了include，但是意義完全不一樣，先看DECL_DRIVER_API_R_N：

#define DECL_DRIVER_API_R_N(R, N, ...) \DECL_DRIVER_API_RETURN(R, N, PAIR_ARGS_N(ARG, ##__VA_ARGS__), PAIR_ARGS_N(PARAM, ##__VA_ARGS__))

關(guān)鍵在DECL_DRIVER_API_RETURN這個(gè)宏，在CommandStream.h和Driver.h頭文件中include文件DriverAPI.inc 之前分別定義了自己的DECL_DRIVER_API_RETURN宏，看看CommandStream.h中：

#define DECL_DRIVER_API(methodName, paramsDecl, params)                                         \inline void methodName(paramsDecl) {                                                        \DEBUG_COMMAND_BEGIN(methodName, false, params);                                         \using Cmd = COMMAND_TYPE(methodName);                                                   \void* const p = allocateCommand(CommandBase::align(sizeof(Cmd)));                       \new(p) Cmd(mDispatcher.methodName##_, APPLY(std::move, params));                        \DEBUG_COMMAND_END(methodName, false);                                                   \}#define DECL_DRIVER_API_SYNCHRONOUS(RetType, methodName, paramsDecl, params)                    \inline RetType methodName(paramsDecl) {                                                     \DEBUG_COMMAND_BEGIN(methodName, true, params);                                          \AutoExecute callOnExit([=](){                                                           \DEBUG_COMMAND_END(methodName, true);                                                \});                                                                                     \return apply(&Driver::methodName, mDriver, std::forward_as_tuple(params));              \}#define DECL_DRIVER_API_RETURN(RetType, methodName, paramsDecl, params)                         \inline RetType methodName(paramsDecl) {                                                     \DEBUG_COMMAND_BEGIN(methodName, false, params);                                         \RetType result = mDriver.methodName##S();                                               \using Cmd = COMMAND_TYPE(methodName##R);                                                \void* const p = allocateCommand(CommandBase::align(sizeof(Cmd)));                       \new(p) Cmd(mDispatcher.methodName##_, RetType(result), APPLY(std::move, params));       \DEBUG_COMMAND_END(methodName, false);                                                   \return result;                                                                          \}

上面三個(gè)宏的作用基本是一樣的，都將要調(diào)用的函數(shù)和參數(shù)封裝為了Command，不同之處在于DECL_DRIVER_API是command無返回值的，DECL_DRIVER_API_SYNCHRONOUS是封裝為command后同步執(zhí)行的，DECL_DRIVER_API_RETURN是需要返回值的
主要看看DECL_DRIVER_API_RETURN：

RetType result = mDriver.methodName##S();    

將方法名后面拼接了S，調(diào)用拿到返回類型
看看拼接S后的實(shí)現(xiàn)：

Handle<HwVertexBuffer> OpenGLDriver::createVertexBufferS() noexcept {return initHandle<GLVertexBuffer>();
}

initHandle()這句在filament內(nèi)存池HandleArena上創(chuàng)建了一個(gè)GLVertexBuffer對(duì)象，然后根據(jù)內(nèi)存地址創(chuàng)建了對(duì)象的唯一handeID
再看下面這句：
using Cmd = COMMAND_TYPE(methodName##R);
方法名后面拼接了R，然后獲取了command的類型，沒有執(zhí)行方法，看看拼接R后的實(shí)現(xiàn)：

void OpenGLDriver::createVertexBufferR(Handle<HwVertexBuffer> vbh,uint8_t bufferCount,uint8_t attributeCount,uint32_t elementCount,AttributeArray attributes) {DEBUG_MARKER()construct<GLVertexBuffer>(vbh, bufferCount, attributeCount, elementCount, attributes);
}

內(nèi)存池HandleArena上創(chuàng)建了一個(gè)GLVertexBuffer對(duì)象
再看下面一句

void* const p = allocateCommand(CommandBase::align(sizeof(Cmd)));   
new(p) Cmd(mDispatcher.methodName##_, RetType(result), APPLY(std::move, params));   

在CommandStream內(nèi)部的環(huán)形緩沖上申請(qǐng)了一塊Command對(duì)象的內(nèi)存p，然后在內(nèi)存p上new了對(duì)象Command
看看CommandBase* execute執(zhí)行函數(shù)的實(shí)現(xiàn)：

inline CommandBase* execute(Driver& driver) {// returning the next command by output parameter allows the compiler to perform the// tail-call optimization in the function called by mExecute, however that comes at// a cost here (writing and reading the stack at each iteration), in the end it's// probably better to pay the cost at just one location.intptr_t next;mExecute(driver, this, &next);return reinterpret_cast<CommandBase*>(reinterpret_cast<intptr_t>(this) + next);
}

mExecute就是上面new(p) Cmd(mDispatcher.methodName##_, RetType(result), APPLY(std::move, params)); 后的函數(shù)和參數(shù)的封裝體，然后拿到了下一個(gè)圓形緩沖中下一個(gè)command的地址偏移量next，返回下一個(gè)command地址
CommandStream中執(zhí)行command，執(zhí)行完然后獲取下一個(gè)執(zhí)行。。。

mDriver.execute([this, buffer]() {Driver& UTILS_RESTRICT driver = mDriver;CommandBase* UTILS_RESTRICT base = static_cast<CommandBase*>(buffer);while (UTILS_LIKELY(base)) {base = base->execute(driver);}
});