如何在V8 JavaScript中进行异步调用

Question

我一直在尝试在V8中进行异步调用，但没有成功。我想要运行的示例JavaScript代码是：

function test ()
{
    logMessage ('asynchronous call made!');
}
saveFunc(test);

saveFunc函数的作用是保存test函数以便在脚本运行后，C++代码调用它时使用。但每次我尝试执行保存的函数时，程序崩溃了。我做错了什么？

以下是完整示例代码。谢谢提前帮助。 示例代码：

// Copyright 2015 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include <iostream>
#include <string>

#include "libplatform/libplatform.h"
#include "v8.h"

using namespace v8;

Local<Context> context;
v8::Local<v8::Function> savedFunc;

class ArrayBufferAllocator : public v8::ArrayBuffer::Allocator {
public:
    virtual void* Allocate(size_t length) {
        void* data = AllocateUninitialized(length);
        return data == NULL ? data : memset(data, 0, length);
    }
    virtual void* AllocateUninitialized(size_t length) { return malloc(length); }
    virtual void Free(void* data, size_t) { free(data); }
};

std::string parseV8Value(v8::Local<v8::Value> str)
{
    if (str.IsEmpty() == true)
        return ("");

    v8::String::Utf8Value newStr(str);

    return (*newStr);
}

void logMessage(const v8::FunctionCallbackInfo<v8::Value> &args)
{
    std::string applicationSource = parseV8Value(args[0]);
    std::cout << applicationSource << "\n";
}

void getInput(const v8::FunctionCallbackInfo<v8::Value> &args)
{
    v8::Local<v8::Function> func = v8::Local<v8::Function>::Cast (args[0]);
    std::string input = "";

    std::cin >> input;

    v8::Local<v8::Value> *args2 = new v8::Local<v8::Value>[1];
    args2[0] = v8::String::NewFromUtf8 (args.GetIsolate (), input.c_str ());

    func->Call (context->Global (), 1, args2);

    delete []args2;
    args2 = NULL;
}

void saveFunc(const v8::FunctionCallbackInfo<v8::Value> &args)
{
    savedFunc = v8::Local<v8::Function>::Cast(args[0]);
}

int main(int argc, char* argv[]) {
    // Initialize V8.
    V8::InitializeICU();
    V8::InitializeExternalStartupData(argv[0]);
    Platform* platform = platform::CreateDefaultPlatform();
    V8::InitializePlatform(platform);
    V8::Initialize();

    // Create a new Isolate and make it the current one.
    ArrayBufferAllocator allocator;
    Isolate::CreateParams create_params;
    create_params.array_buffer_allocator = &allocator;
    Isolate* isolate = Isolate::New(create_params);
    {
        Isolate::Scope isolate_scope(isolate);

        // Create a stack-allocated handle scope.
        HandleScope handle_scope(isolate);

        // Create a new context.
        v8::Local<v8::ObjectTemplate> global = v8::ObjectTemplate::New(isolate);
        global->Set(v8::String::NewFromUtf8(isolate, "logMessage"),
            v8::FunctionTemplate::New(isolate, logMessage));
        global->Set(v8::String::NewFromUtf8(isolate, "getInput"),
            v8::FunctionTemplate::New(isolate, getInput));
        global->Set(v8::String::NewFromUtf8(isolate, "saveFunc"),
            v8::FunctionTemplate::New(isolate, saveFunc));
        context = Context::New(isolate, NULL, global);

        // Enter the context for compiling and running the hello world script.
        Context::Scope context_scope(context);

        // Create a string containing the JavaScript source code.
        Local<String> source =
            String::NewFromUtf8(isolate, 
            "function test (){ logMessage ('asynchronous call made!'); }saveFunc(test);",,
                NewStringType::kNormal).ToLocalChecked();

        // Compile the source code.
        Local<Script> script = Script::Compile(context, source).ToLocalChecked();

        // Run the script to get the result.
        Local<Value> result = script->Run(context).ToLocalChecked();

        v8::Local<v8::Value> *args = new v8::Local<v8::Value>[0];
        savedFunc->Call(context->Global(), 0, args);

        delete []args;
        args = NULL;
    }

    // Dispose the isolate and tear down V8.
    isolate->Dispose();
    V8::Dispose();
    V8::ShutdownPlatform();
    delete platform;
    return 0;
}

- Someone13

---

我不确定我理解你想做什么。saveFunc() 是否应该和 process.nextTick() 做相同的事情？ - Patrick Roberts

你必须理解本地句柄和持久句柄之间的区别：https://developers.google.com/v8/embed#handles-and-garbage-collection - AnatolyS

恭喜你，你找到了大多数人都不懂的JavaScript基础之一——并发和异步调用并不是来自JavaScript本身，而是来自像Chromium或Node这样的主机环境包装它——例如，Node使用LibUV驱动它。 - Benjamin Gruenbaum

Answer 1

好的，所以在保存函数时，您必须使用Persistent handle。此外，当您访问context->Global时，确保仍然在HandleScope范围内。以下是已更正的代码：

// Copyright 2015 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include <iostream>
#include <string>

#include "libplatform/libplatform.h"
#include "v8.h"

using namespace v8;

Isolate* isolate = NULL;
Local<Context> context;
v8::Persistent<v8::Function> *savedFunc = NULL;

class ArrayBufferAllocator : public v8::ArrayBuffer::Allocator {
public:
    virtual void* Allocate(size_t length) {
        void* data = AllocateUninitialized(length);
        return data == NULL ? data : memset(data, 0, length);
    }
    virtual void* AllocateUninitialized(size_t length) { return malloc(length); }
    virtual void Free(void* data, size_t) { free(data); }
};

std::string parseV8Value(v8::Local<v8::Value> str)
{
    if (str.IsEmpty() == true)
        return ("");

    v8::String::Utf8Value newStr(str);

    return (*newStr);
}

void logMessage(const v8::FunctionCallbackInfo<v8::Value> &args)
{
    std::string applicationSource = parseV8Value(args[0]);
    std::cout << applicationSource << "\n";
}

void getInput(const v8::FunctionCallbackInfo<v8::Value> &args)
{
    v8::Local<v8::Function> func = v8::Local<v8::Function>::Cast (args[0]);
    std::string input = "";

    std::cin >> input;

    v8::Local<v8::Value> *args2 = new v8::Local<v8::Value>[1];
    args2[0] = v8::String::NewFromUtf8 (args.GetIsolate (), input.c_str ());

    func->Call (context->Global (), 1, args2);

    delete []args2;
    args2 = NULL;
}

void saveFunc(const v8::FunctionCallbackInfo<v8::Value> &args)
{
    v8::Local<v8::Function> func = v8::Local<v8::Function>::Cast(args[0]);
    savedFunc = new v8::Persistent<v8::Function>();
    savedFunc->Reset(isolate, func);
}

int main(int argc, char* argv[]) {
    // Initialize V8.
    V8::InitializeICU();
    V8::InitializeExternalStartupData(argv[0]);
    Platform* platform = platform::CreateDefaultPlatform();
    V8::InitializePlatform(platform);
    V8::Initialize();

    // Create a new Isolate and make it the current one.
    ArrayBufferAllocator allocator;
    Isolate::CreateParams create_params;
    create_params.array_buffer_allocator = &allocator;
    isolate = Isolate::New(create_params);
    {
        Isolate::Scope isolate_scope(isolate);

        // Create a stack-allocated handle scope.
        HandleScope handle_scope(isolate);

        // Create a new context.
        v8::Local<v8::ObjectTemplate> global = v8::ObjectTemplate::New(isolate);
        global->Set(v8::String::NewFromUtf8(isolate, "logMessage"),
            v8::FunctionTemplate::New(isolate, logMessage));
        global->Set(v8::String::NewFromUtf8(isolate, "getInput"),
            v8::FunctionTemplate::New(isolate, getInput));
        global->Set(v8::String::NewFromUtf8(isolate, "saveFunc"),
            v8::FunctionTemplate::New(isolate, saveFunc));
        context = Context::New(isolate, NULL, global);

        // Enter the context for compiling and running the hello world script.
        Context::Scope context_scope(context);

        // The "asynchronous" javascript call to make
        Local<String> source =
            String::NewFromUtf8(isolate, 
    "function test (){ logMessage ('asynchronous call made!'); }saveFunc(test);",
                NewStringType::kNormal).ToLocalChecked();

        // Compile the source code.
        Local<Script> script = Script::Compile(context, source).ToLocalChecked();

        // Run the script to get the result.
        Local<Value> result = script->Run(context).ToLocalChecked();

        v8::Local<v8::Value> *args = new v8::Local<v8::Value>[0];
        v8::Local<v8::Value> recv = context->Global();
        v8::Local<v8::Function> func = savedFunc->Get(isolate);
        func->Call (recv, 0, args);

        delete savedFunc;
        savedFunc = NULL;

        delete []args;
        args = NULL;
    }

    // Dispose the isolate and tear down V8.
    isolate->Dispose();
    V8::Dispose();
    V8::ShutdownPlatform();
    delete platform;
    return 0;
}

- Someone13