此代码正常工作:
let promise;
try {
promise = parent();
//but I want: await parent();
await continueStack();
} catch (e) {
console.log('Caught error in endpoint');
}
eventBus.on('stop::all', () => {
console.warn('Caught stop::all event');
promise.cancel();
});
它取消了程序其他部分发出的事件的承诺。我的问题是我不能拥有await parent()
,因为那时尚未执行监听事件。如果我改成这样:
let promise;
eventBus.on('stop::all', () => {
console.warn('Caught stop::all event');
promise.cancel();
});
try {
promise = await parent();
await continueStack();
} catch (e) {
console.log('Caught error in endpoint');
}
那我有错误
TypeError:无法读取未定义的属性“取消”
,它将崩溃,并且continueStack()
从未执行。我该如何调和这些?
我已经用上述场景创建了整个示例:
https://gist.github.com/BorysTyminski/147dc2e0c44c8d64386253d563ff6db4
要运行它,您需要获取两个文件,安装package.json依赖项并运行cURL / Postman请求GET到localhost:5000 / test
答案 0 :(得分:1)
您可以通过将promise
变量设置为parent
,然后等待它来完成此操作:
const EventEmitter = require('events');
const Promise = require('bluebird');
Promise.config({
cancellation: true,
})
const parent = new Promise(resolve => {
setTimeout( () => {
resolve();
}, 100);
});
const continueStack = new Promise(resolve => {
setTimeout( () => {
resolve();
}, 100);
});
let promise;
const emitter = new EventEmitter();
emitter.on('stop', function() {
if(promise && promise.cancel){
promise.cancel();
console.log('promise cancelled');
} else {
console.log('somehow promise was not properly set at this point');
}
});
setTimeout(_ => {
emitter.emit('stop');
}, 80);
async function operation() {
try {
promise = parent;
await promise;
console.log('after promise. This log should never appear.');
await continueStack;
console.log('after wait 2. This log should never appear.');
} catch(e) {
console.log('Even if the promise is cancelled, this is not called.');
}
}
operation();
(使用该节点运行该代码段只会记录“已取消承诺”):在执行异步函数内的await promise
之后什么都没有。)
一些注意事项:
cancellable
设置为true。 await
不像睡觉。它不会阻止javascript线程,事件监听器或任何事件发射器。它只是确保await
之后的代码将等待已承诺的等待。但这仅在异步函数的上下文中。
取消Promise不会触发捕获处理程序。这让我感到困惑,但我想这就是它的工作原理。