【发布时间】：2018-02-28 16:37:36
【问题描述】：
我的控制器能够暂停、恢复和解决长期运行的承诺。 当我的控制器中止 Promise 时，不知何故我的控制器递归循环被打破了。
前几天了解了Promise，所以可能对reject或者catch不太了解。
首先，一个可以在承诺链的步骤之间检查的状态变量。
var state = "idle"
可以插入一个 shim promise 来检查这个状态变量
// insert this shim before each link of promise chain
function promise_to_check_pause_and_abort(x) {
if (state == "running") {
return Promise.resolve(x) // just keep going
} else if (state == "pause") {
let P = new Promise((resolve, reject) => {
let resume_cb = () => { resolve() }
pause_cb_queue.push(resume_cb)
});
return P;
} else if (state == "abort") {
return Promise.reject("aborted at stage " + x)
} else {
return promise.reject("should not be executing in this invalid state = " + state)
}
}
我的真实应用程序运行许多可能需要 15 分钟以上才能完成的链式 Promise。这不需要证明我的问题。所以这是一个简单的假任务，它返回一个承诺。
function promise_to_do_something_long_running(n) {
if (isNaN(n)) {
n = 0;
}
if (n == 100) {
return Promise.resolve("done")
} else {
let P = new Promise((resolve, reject) => {
setTimeout(() => {
console.log("doing " + n);
resolve();
}, 100)
})
return P
.then(() => { return promise_to_check_pause_and_abort(n) })
.then(() => { return promise_to_do_something_long_running(n + 1) })
}
}
现在是一个简单的文本控制器，用于启动、暂停、恢复和中止上述长期运行的承诺。
// A recursive controller.
// Get response based on current running state.
// Regardless of response, do it again.
function controller() {
if (state == "running") {
rl.question("abort or pause :", (answer) => {
if (answer == "abort") {
state = "abort"
} else if (answer == "pause") {
state = "pause"
};
controller(); // regardless of response or state, get input again
})
} else if (state == "pause") {
rl.question("abort or resume :", (answer) => {
if (answer == "abort") {
state = "abort";
pause_cb_queue.forEach((cb) => { cb() })
pause_cb_queue = []
} else if (answer == "resume") {
state = "running"
pause_cb_queue.forEach((cb) => { cb() })
pause_cb_queue = []
};
controller(); // regardless of response or state, get input again
})
} else if (state == "idle") {
rl.question("start :", (answer) => {
if (answer == "start") {
state = "running";
// controller loop not dependent on resolution of this
// only state is
promise_to_do_something_long_running()
.then((a) => {
console.log("completed task, response :" + a);
state = "idle"
})
.catch((b) => {
console.log("task rejected with response :" + b);
state = "idle"
});
};
controller(); // regardless of response or state, get input again
})
}
}
controller();
一切都很完美，除了当我中止时，控制器递归中断并且不再请求用户输入。怎么能执行的
.catch((b) => {
console.log("task rejected with response :" + b);
state = "idle"
});
导致外部控制器递归停止？
在nodejs中运行的完整代码如下：
'use strict';
const readline = require('readline');
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout
});
var state = "idle"
var pause_cb_queue = []
// insert this shim before each link of promise chain
function promise_to_check_pause_and_abort(x) {
if (state == "running") {
return Promise.resolve(x) // just keep going
} else if (state == "pause") {
let P = new Promise((resolve, reject) => {
let resume_cb = () => { resolve() }
pause_cb_queue.push(resume_cb)
});
return P;
} else if (state == "abort") {
return Promise.reject("aborted at stage " + x)
} else {
return promise.reject("should not be executing in this invalid state = " + state)
}
}
function promise_to_do_something_long_running(n) {
if (isNaN(n)) {
n = 0;
}
if (n == 100) {
return Promise.resolve("done")
} else {
let P = new Promise((resolve, reject) => {
setTimeout(() => {
console.log("doing " + n);
resolve();
}, 100)
})
return P
.then(() => { return promise_to_check_pause_and_abort(n) })
.then(() => { return promise_to_do_something_long_running(n + 1) })
}
}
// A recursive controller.
// Get response based on current running state.
// Regardless of response, do it again.
function controller() {
if (state == "running") {
rl.question("abort or pause :", (answer) => {
if (answer == "abort") {
state = "abort"
} else if (answer == "pause") {
state = "pause"
};
controller(); // regardless of response or state, get input again
})
} else if (state == "pause") {
rl.question("abort or resume :", (answer) => {
if (answer == "abort") {
state = "abort";
pause_cb_queue.forEach((cb) => { cb() })
pause_cb_queue = []
} else if (answer == "resume") {
state = "running"
pause_cb_queue.forEach((cb) => { cb() })
pause_cb_queue = []
};
controller(); // regardless of response or state, get input again
})
} else if (state == "idle") {
rl.question("start :", (answer) => {
if (answer == "start") {
state = "running";
// controller loop not dependent on resolution of this
// only state is
promise_to_do_something_long_running()
.then((a) => {
console.log("completed task, response :" + a);
state = "idle"
})
.catch((b) => {
console.log("task rejected with response :" + b);
state = "idle"
});
};
controller(); // regardless of response or state, get input again
})
}
}
controller();
【问题讨论】：
标签： javascript node.js recursion promise