javascript – 取消vanilla ECMAScript 6 Promise链

有没有一种方法可以清除JavaScript Promise实例的.thens？

我在QUnit之上编写了一个JavaScript测试框架.该框架通过在Promise中运行每个测试框架来同步运行测试. (抱歉这个代码块的长度.我尽可能地评论它,所以感觉不那么乏味.)

/* Promise extension -- used for easily making an async step with a
       timeout without the Promise knowing anything about the function 
       it's waiting on */
$$.extend(Promise, {
    asyncTimeout: function (timeToLive, errorMessage) {
        var error = new Error(errorMessage || "Operation timed out.");
        var res, // resolve()
            rej, // reject()
            t,   // timeout instance
            rst, // reset timeout function
            p,   // the promise instance
            at;  // the returned asyncTimeout instance

        function createTimeout(reject, tempTtl) {
            return setTimeout(function () {
                // triggers a timeout event on the asyncTimeout object so that,
                // if we want, we can do stuff outside of a .catch() block
                // (may not be needed?)
                $$(at).trigger("timeout");

                reject(error);
            }, tempTtl || timeToLive);
        }

        p = new Promise(function (resolve, reject) {
            if (timeToLive != -1) {
                t = createTimeout(reject);

                // reset function -- allows a one-time timeout different
                //    from the one original specified
                rst = function (tempTtl) {
                    clearTimeout(t);
                    t = createTimeout(reject, tempTtl);
                }
            } else {
                // timeToLive = -1 -- allow this promise to run indefinitely
                // used while debugging
                t = 0;
                rst = function () { return; };
            }

            res = function () {
                clearTimeout(t);
                resolve();
            };

            rej = reject;
        });

        return at = {
            promise: p,
            resolve: res,
            reject: rej,
            reset: rst,
            timeout: t
        };
    }
});

/* framework module members... */

test: function (name, fn, options) {
    var mod = this; // local reference to framework module since promises
                    // run code under the window object

    var defaultOptions = {
        // default max running time is 5 seconds
        timeout: 5000
    }

    options = $$.extend({}, defaultOptions, options);

    // remove timeout when debugging is enabled
    options.timeout = mod.debugging ? -1 : options.timeout;

    // call to QUnit.test()
    test(name, function (assert) {
        // tell QUnit this is an async test so it doesn't run other tests
        // until done() is called
        var done = assert.async();
        return new Promise(function (resolve, reject) {
            console.log("Beginning: " + name);

            var at = Promise.asyncTimeout(options.timeout, "Test timed out.");
            $$(at).one("timeout", function () {
                // assert.fail() is just an extension I made that literally calls
                // assert.ok(false, msg);
                assert.fail("Test timed out");
            });

            // run test function
            var result = fn.call(mod, assert, at.reset);

            // if the test returns a Promise, resolve it before resolving the test promise
            if (result && result.constructor === Promise) {
                // catch unhandled errors thrown by the test so future tests will run
                result.catch(function (error) {
                    var msg = "Unhandled error occurred."
                    if (error) {
                        msg = error.message + "\n" + error.stack;
                    }

                    assert.fail(msg);
                }).then(function () {
                    // resolve the timeout Promise
                    at.resolve();
                    resolve();
                });
            } else {
                // if test does not return a Promise, simply clear the timeout
                // and resolve our test Promise
                at.resolve();
                resolve();
            }
        }).then(function () {
            // tell QUnit that the test is over so that it can clean up and start the next test
            done();
            console.log("Ending: " + name);
        });
    });
}

如果测试超时,我的超时Promise将在测试中断言.fail()以便测试被标记为失败,这一切都很好,但测试继续运行,因为测试Promise(结果)仍在等待解决它.

我需要一个好方法取消我的测试.我可以通过在框架模块this.cancelTest上创建一个字段来完成它,并在测试中经常检查(例如在每次then()迭代的开始)是否取消.但是,理想情况下,我可以使用$$(at).on(“timeout”,/ * here here * /)来清除结果变量中剩余的then()s,以便其余的测试都不会运行.

这样的事情存在吗？

快速更新

我尝试使用Promise.race([result,at.promise]).它没用.

更新2混乱

为了解锁我,我在测试想法中添加了几行mod.cancelTest / polling. (我也删除了事件触发器.)

return new Promise(function (resolve, reject) {
    console.log("Beginning: " + name);

    var at = Promise.asyncTimeout(options.timeout, "Test timed out.");
    at.promise.catch(function () {
        // end the test if it times out
        mod.cancelTest = true;
        assert.fail("Test timed out");
        resolve();
    });

    // ...

}).then(function () {
    // tell QUnit that the test is over so that it can clean up and start the next test
    done();
    console.log("Ending: " + name);
});

我在catch语句中设置了一个断点,它正被击中.我现在困惑的是没有调用then()语句.想法？

更新3

想出最后一件事. fn.call()抛出了一个我没有抓到的错误,所以测试承诺在at.promise.catch()解决之前就拒绝了.

解决方法:

Is there a method for clearing the .thens of a JavaScript Promise instance?

不.至少在ECMAScript 6中没有.默认情况下,Promises(以及他们当时的处理程序)是不可取消的(不幸的是).关于如何以正确的方式做到这一点的es-discuss(例如here)有一些讨论,但无论采取何种方法,它都不会落在ES6中.

目前的观点是子类化将允许使用您自己的实现创建可取消的承诺(不确定它将如何工作).

直到语言委员会找到最好的方法(ES7希望？),你仍然可以使用userland Promise实现,其中许多功能取消.

目前的讨论见于https://github.com/domenic/cancelable-promise和https://github.com/bergus/promise-cancellation草案.