async await 语法实现

原理

async本质上是将串行写法的promise转换为并行（同步）写法，经过自动执行器，将其转换为串行便可实现相同效果
将await替换为yield，转换为generate函数，而后建立其迭代器，经过next,使得函数进入下一步yield
定义自动执行器：判断next返回的done属性,为true时，结束递归
不为true时，递归执行执行器根据返回value的类型使用promise.then(... next(res)) 或者 next(value)，最终实现并行 转 串行
复制代码

实现

async function asyncDetail(){
    	// do something...
    	console.log('aaa');
    	await a();  
    
    	// do something...
    	console.log('bbb');
    	await b();
    
    	// do something...
    	console.log('ccc');
    	await c();
    }
    
    步骤一：
    function* asyncDetailGenerate(){
    	// do something...
    	console.log('aaa');
    	const a = yield test();  
    	console.log('a');
    	
    	// do something...
    	console.log('bbb');
    	const b = yield test();  
    	console.log('b');
    	
    	const d = yield 'dddd';
    	console.log(d);
    	
    	// do something...
    	console.log('ccc');
    	const c = yield test();  
    	console.log('c');
    }
    步骤二：
    const runner = asyncDetailGenerate();
    步骤三：定制执行器
    const autoRunner = (runner,{value,done})=>{
    	if(done){
            return value;
    	}else{
          if(value instanceof Promise){
            value.then(res=>{
                const nextResult = runner.next(res);
                autoRunner(runner,nextResult);
    	    })
          }else{
            const nextResult = runner.next(value);
            autoRunner(runner,nextResult);
          }
    		
    	}
    }
    
    步骤四：autoRunner(runner,runner.next());
复制代码