ES6中的Generator的引入，极大程度上改变了JavaScript程序员对迭代器的看法，并为解决callback hell提供了新方法。
迭代器模式是很常用的设计模式，但是实现起来，很多东西是程序化的；当迭代规则比较复杂时，维护迭代器内的状态，是比较麻烦的。 于是有了generator，何为generator？
Generators: a better way to build Iterators.
借助 yield 关键字,可以更优雅的实现fibonacci数列。
function* fibonacci() {
let a = 0, b = 1;
while(true) {
yield a;
[a, b] = [b, a + b];
}
}yield可以暂停运行流程，那么便为改变执行流程提供了可能。这和Python的coroutine类似。
Generator之所以可用来控制代码流程，就是通过yield来将两个或者多个Generator的执行路径互相切换。这种切换是语句级别的，而不是函数调用级别的。其本质是CPS变换。
yield之后，实际上本次调用就结束了，控制权实际上已经转到了外部调用了generator的next方法的函数，调用的过程中伴随着状态的改变。那么如果外部函数不继续调用next方法，那么yield所在函数就相当于停在yield那里了。所以把异步的东西做完，要函数继续执行，只要在合适的地方再次调用generator 的next就行，就好像函数在暂停后，继续执行。
Generator function 和 yield 关键字处理是在 parser.cc, 我们看到 AST 解析函数： Parser::ParseEagerFunctionBody()
3928 ZoneList<Statement*>* Parser::ParseEagerFunctionBody(
3929 const AstRawString* function_name, int pos, Variable* fvar,
3930 Token::Value fvar_init_op, FunctionKind kind, bool* ok) {
3931 .....
3954 // For generators, allocate and yield an iterator on function entry.
3955 if (IsGeneratorFunction(kind)) {
3956 ZoneList<Expression*>* arguments =
3957 new(zone()) ZoneList<Expression*>(0, zone());
3958 CallRuntime* allocation = factory()->NewCallRuntime(
3959 ast_value_factory()->empty_string(),
3960 Runtime::FunctionForId(Runtime::kCreateJSGeneratorObject), arguments,
3961 pos);
3962 VariableProxy* init_proxy = factory()->NewVariableProxy(
3963 function_state_->generator_object_variable());
3964 Assignment* assignment = factory()->NewAssignment(
3965 Token::INIT_VAR, init_proxy, allocation, RelocInfo::kNoPosition);
3966 VariableProxy* get_proxy = factory()->NewVariableProxy(
3967 function_state_->generator_object_variable());
3968 Yield* yield = factory()->NewYield(
3969 get_proxy, assignment, Yield::kInitial, RelocInfo::kNoPosition);
3970 body->Add(factory()->NewExpressionStatement(
3971 yield, RelocInfo::kNoPosition), zone());
3972 }
3973
3974 ParseStatementList(body, Token::RBRACE, false, NULL, CHECK_OK);
3975
3976 if (IsGeneratorFunction(kind)) {
3977 VariableProxy* get_proxy = factory()->NewVariableProxy(
3978 function_state_->generator_object_variable());
3979 Expression* undefined =
3980 factory()->NewUndefinedLiteral(RelocInfo::kNoPosition);
3981 Yield* yield = factory()->NewYield(get_proxy, undefined, Yield::kFinal,
3982 RelocInfo::kNoPosition);
3983 body->Add(factory()->NewExpressionStatement(
3984 yield, RelocInfo::kNoPosition), zone());
3985 }
3986 ...
L3955 判断是否是Generator function。 ParseStatementList 解析 function 函数体。
注意，Generator function 也是一种 function, 在 V8中，同样用 JSFunction 表示。
在两个 if 函数体中，创建了 Yield::kInitial和 Yield::kFinal 两个Yield AST 节点。
Yield 状态分为:
enum Kind {
kInitial, // The initial yield that returns the unboxed generator object.
kSuspend, // A normal yield: { value: EXPRESSION, done: false }
kDelegating, // A yield*.
kFinal // A return: { value: EXPRESSION, done: true }
};机器码生成(x64平台)主要集中在 runtime-generator.cc, full-codegen-x64.cc。
runtime-generator.cc 提供了 Create, Suspend, Resume, Close等 stub 代码段，
给 full-codegen 内联使用，生成汇编代码。
我们先来看到 RUNTIME_FUNCTION(Runtime_CreateJSGeneratorObject),
14 RUNTIME_FUNCTION(Runtime_CreateJSGeneratorObject) {
15 HandleScope scope(isolate);
16 DCHECK(args.length() == 0);
17
18 JavaScriptFrameIterator it(isolate);
19 JavaScriptFrame* frame = it.frame();
20 Handle<JSFunction> function(frame->function());
21 RUNTIME_ASSERT(function->shared()->is_generator());
22
23 Handle<JSGeneratorObject> generator;
24 if (frame->IsConstructor()) {
25 generator = handle(JSGeneratorObject::cast(frame->receiver()));
26 } else {
27 generator = isolate->factory()->NewJSGeneratorObject(function);
28 }
29 generator->set_function(*function);
30 generator->set_context(Context::cast(frame->context()));
31 generator->set_receiver(frame->receiver());
32 generator->set_continuation(0);
33 generator->set_operand_stack(isolate->heap()->empty_fixed_array());
34 generator->set_stack_handler_index(-1);
35
36 return *generator;
37 }函数根据当前的 Frame, 创建一个 JSGeneratorObject对象来储存 JSFunction, Context ，pc 指针，
设置操作数栈为空。
yield 后，实际上就是保存当前的执行环境，L74保存当前的操作数栈，并保存到JSGeneratorObject对象中。
40 RUNTIME_FUNCTION(Runtime_SuspendJSGeneratorObject) {
41 HandleScope handle_scope(isolate);
42 DCHECK(args.length() == 1);
43 CONVERT_ARG_HANDLE_CHECKED(JSGeneratorObject, generator_object, 0);
44
45 JavaScriptFrameIterator stack_iterator(isolate);
46 JavaScriptFrame* frame = stack_iterator.frame();
47 RUNTIME_ASSERT(frame->function()->shared()->is_generator());
48 DCHECK_EQ(frame->function(), generator_object->function());
49
50 // The caller should have saved the context and continuation already.
51 DCHECK_EQ(generator_object->context(), Context::cast(frame->context()));
52 DCHECK_LT(0, generator_object->continuation());
53
54 // We expect there to be at least two values on the operand stack: the return
55 // value of the yield expression, and the argument to this runtime call.
56 // Neither of those should be saved.
57 int operands_count = frame->ComputeOperandsCount();
58 DCHECK_GE(operands_count, 2);
59 operands_count -= 2;
60
61 if (operands_count == 0) {
62 // Although it's semantically harmless to call this function with an
63 // operands_count of zero, it is also unnecessary.
64 DCHECK_EQ(generator_object->operand_stack(),
65 isolate->heap()->empty_fixed_array());
66 DCHECK_EQ(generator_object->stack_handler_index(), -1);
67 // If there are no operands on the stack, there shouldn't be a handler
68 // active either.
69 DCHECK(!frame->HasHandler());
70 } else {
71 int stack_handler_index = -1;
72 Handle<FixedArray> operand_stack =
73 isolate->factory()->NewFixedArray(operands_count);
74 frame->SaveOperandStack(*operand_stack, &stack_handler_index);
75 generator_object->set_operand_stack(*operand_stack);
76 generator_object->set_stack_handler_index(stack_handler_index);
77 }
78
79 return isolate->heap()->undefined_value();
80 }
Resume 对应于外部的 next，要恢复执行，首先我们得知道需要执行的 pc 指针偏移，机器代码存储在
JSFunction 的 Code 对象中, L105 拿到 pc 首地址， L106从 JSGeneratorObject对象
取出偏移 offset 。
L108 设置当前 Frame 的 pc 偏移。L118 恢复操作数栈, L126-L130根据恢复的 mode, 返回 value。
90 RUNTIME_FUNCTION(Runtime_ResumeJSGeneratorObject) {
91 SealHandleScope shs(isolate);
92 DCHECK(args.length() == 3);
93 CONVERT_ARG_CHECKED(JSGeneratorObject, generator_object, 0);
94 CONVERT_ARG_CHECKED(Object, value, 1);
95 CONVERT_SMI_ARG_CHECKED(resume_mode_int, 2);
96 JavaScriptFrameIterator stack_iterator(isolate);
97 JavaScriptFrame* frame = stack_iterator.frame();
98
99 DCHECK_EQ(frame->function(), generator_object->function());
100 DCHECK(frame->function()->is_compiled());
101
102 STATIC_ASSERT(JSGeneratorObject::kGeneratorExecuting < 0);
103 STATIC_ASSERT(JSGeneratorObject::kGeneratorClosed == 0);
104
105 Address pc = generator_object->function()->code()->instruction_start();
106 int offset = generator_object->continuation();
107 DCHECK(offset > 0);
108 frame->set_pc(pc + offset);
109 ...
113 generator_object->set_continuation(JSGeneratorObject::kGeneratorExecuting);
114
115 FixedArray* operand_stack = generator_object->operand_stack();
116 int operands_count = operand_stack->length();
117 if (operands_count != 0) {
118 frame->RestoreOperandStack(operand_stack,
119 generator_object->stack_handler_index());
120 generator_object->set_operand_stack(isolate->heap()->empty_fixed_array());
121 generator_object->set_stack_handler_index(-1);
122 }
123
124 JSGeneratorObject::ResumeMode resume_mode =
125 static_cast<JSGeneratorObject::ResumeMode>(resume_mode_int);
126 switch (resume_mode) {
127 case JSGeneratorObject::NEXT:
128 return value;
129 case JSGeneratorObject::THROW:
130 return isolate->Throw(value);
131 }
132 ...
133 }这边我们关注下 args 参数， args[0]是JSGeneratorObject 对象generator_object， args[1]是Object 对象
value, 也就是 next 的返回值，args[2]是表示 resume 模式的值。
对应的我们看到 FullCodeGenerator::EmitGeneratorResume() 中的这几行代码：
2296 __ Push(rbx);
2297 __ Push(result_register());
2298 __ Push(Smi::FromInt(resume_mode));
2299 __ CallRuntime(Runtime::kResumeJSGeneratorObject, 3);L2297从 result 寄存器中取出 value, L2299调用 RUNTIME_FUNCTION(Runtime_ResumeJSGeneratorObject)。
这样，从 yield value 到 g.next() 取出 value, 相信大家有了一个大概的认知了。
我们看到node.js依托 v8层面实现了协程，有兴趣的同学可以关心下 fibjs, 它是用 C库实现了协程，遇到异步调用就 "yield" 放弃 CPU， 交由协程调度，也解决了 callback hell 的问题。 本质思想上两种方案没本质区别：
- Generator是利用yield特殊关键字来暂停执行，而fibers是利用Fiber.yield()暂停
- Generator是利用函数返回的Generator句柄来控制函数的继续执行，而fibers是在异步回调中利用Fiber.current.run()继续执行。