kriskowal/q 学习笔记

q/promise

promise 实现

// demo
 Q.Promise(function(resolve, reject, notify) {}
  .then()



执行流程

Promise - > then - > NextTick(放入task队列) -- > flush(下一个循环， 执行队列) --> 执行 defer.promiseDispatch (resolve等放入message) -- > 执行 defer.become -- > NextTick(放入task队列)
-- > flush(下一个循环， 执行队列) -- > 执行 promise.promiseDispatch(执行resolve等方法) -- > 完成

// 解析
function promise(resolver) {
    if (typeof resolver !== "function") {
        throw new TypeError("resolver must be a function.");
    }
    var deferred = defer(); // 延迟执行对象
    try {
        resolver(deferred.resolve, deferred.reject, deferred.notify); // (resolve,reject) = > {}
    } catch (reason) {
        deferred.reject(reason);
    }
    return deferred.promise;
}

function defer() {
    // if "messages" is an "Array", that indicates that the promise has not yet
    // been resolved.  If it is "undefined", it has been resolved.  Each
    // element of the messages array is itself an array of complete arguments to
    // forward to the resolved promise.  We coerce the resolution value to a
    // promise using the `resolve` function because it handles both fully
    // non-thenable values and other thenables gracefully.
    var messages = [], progressListeners = [], resolvedPromise;

    var deferred = object_create(defer.prototype); // 创建 defer
    var promise = object_create(Promise.prototype); // 创建 promise，作为 defer 的 promise 返回

    promise.promiseDispatch = function (resolve, op, operands) {
        var args = array_slice(arguments);
        if (messages) {
            messages.push(args); // 添加 args
            if (op === "when" && operands[1]) { // progress operand
                progressListeners.push(operands[1]);
            }
        } else {
            Q.nextTick(function () {
                resolvedPromise.promiseDispatch.apply(resolvedPromise, args);
            });
        }
    };

    // XXX deprecated
    promise.valueOf = function () {
        if (messages) {
            return promise;
        }
        var nearerValue = nearer(resolvedPromise);
        if (isPromise(nearerValue)) {
            resolvedPromise = nearerValue; // shorten chain
        }
        return nearerValue;
    };

    promise.inspect = function () {
        if (!resolvedPromise) {
            return { state: "pending" };
        }
        return resolvedPromise.inspect();
    };

    if (Q.longStackSupport && hasStacks) {
        try {
            throw new Error();
        } catch (e) {
            // NOTE: don't try to use `Error.captureStackTrace` or transfer the
            // accessor around; that causes memory leaks as per GH-111. Just
            // reify the stack trace as a string ASAP.
            //
            // At the same time, cut off the first line; it's always just
            // "[object Promise]
", as per the `toString`.
            promise.stack = e.stack.substring(e.stack.indexOf("
") + 1);
            promise.stackCounter = longStackCounter++;
        }
    }

    // NOTE: we do the checks for `resolvedPromise` in each method, instead of
    // consolidating them into `become`, since otherwise we'd create new
    // promises with the lines `become(whatever(value))`. See e.g. GH-252.

    function become(newPromise) {
        resolvedPromise = newPromise;

        if (Q.longStackSupport && hasStacks) {
            // Only hold a reference to the new promise if long stacks
            // are enabled to reduce memory usage
            promise.source = newPromise;
        }

        array_reduce(messages, function (undefined, message) {
            Q.nextTick(function () {
                newPromise.promiseDispatch.apply(newPromise, message); // 执行 resolve
            });
        }, void 0);

        messages = void 0;
        progressListeners = void 0;
    }

    deferred.promise = promise;
    deferred.resolve = function (value) {
        if (resolvedPromise) {
            return;
        }

        become(Q(value));
    };

    deferred.fulfill = function (value) {
        if (resolvedPromise) {
            return;
        }

        become(fulfill(value));
    };
    deferred.reject = function (reason) {
        if (resolvedPromise) {
            return;
        }

        become(reject(reason));
    };
    deferred.notify = function (progress) {
        if (resolvedPromise) {
            return;
        }

        array_reduce(progressListeners, function (undefined, progressListener) {
            Q.nextTick(function () {
                progressListener(progress);
            });
        }, void 0);
    };

    return deferred;
}


Q.makePromise = Promise;
function Promise(descriptor, fallback, inspect) {
    if (fallback === void 0) {
        fallback = function (op) {
            return reject(new Error(
                "Promise does not support operation: " + op
            ));
        };
    }
    if (inspect === void 0) {
        inspect = function () {
            return {state: "unknown"};
        };
    }

    var promise = object_create(Promise.prototype);

    promise.promiseDispatch = function (resolve, op, args) {
        var result;
        try {
            if (descriptor[op]) {
                result = descriptor[op].apply(promise, args);
            } else {
                result = fallback.call(promise, op, args);
            }
        } catch (exception) {
            result = reject(exception);
        }
        if (resolve) {
            resolve(result);
        }
    };

    promise.inspect = inspect;

    // XXX deprecated `valueOf` and `exception` support
    if (inspect) {
        var inspected = inspect();
        if (inspected.state === "rejected") {
            promise.exception = inspected.reason;
        }

        promise.valueOf = function () {
            var inspected = inspect();
            if (inspected.state === "pending" ||
                inspected.state === "rejected") {
                return promise;
            }
            return inspected.value;
        };
    }

    return promise;
}

// then 方法
Promise.prototype.then = function (fulfilled, rejected, progressed) {
    var self = this;
    var deferred = defer();
    var done = false;   // ensure the untrusted promise makes at most a
                        // single call to one of the callbacks

    function _fulfilled(value) {
        try {
            return typeof fulfilled === "function" ? fulfilled(value) : value;
        } catch (exception) {
            return reject(exception);
        }
    }

    function _rejected(exception) {
        if (typeof rejected === "function") {
            makeStackTraceLong(exception, self);
            try {
                return rejected(exception);
            } catch (newException) {
                return reject(newException);
            }
        }
        return reject(exception);
    }

    function _progressed(value) {
        return typeof progressed === "function" ? progressed(value) : value;
    }

    // 在下一次 执行
    Q.nextTick(function () {
        self.promiseDispatch(function (value) {
            if (done) {
                return;
            }
            done = true;

            deferred.resolve(_fulfilled(value));
        }, "when", [function (exception) {
            if (done) {
                return;
            }
            done = true;

            deferred.resolve(_rejected(exception));
        }]);
    });

    // Progress propagator need to be attached in the current tick.
    self.promiseDispatch(void 0, "when", [void 0, function (value) {
        var newValue;
        var threw = false;
        try {
            newValue = _progressed(value);
        } catch (e) {
            threw = true;
            if (Q.onerror) {
                Q.onerror(e);
            } else {
                throw e;
            }
        }

        if (!threw) {
            deferred.notify(newValue);
        }
    }]);

    return deferred.promise;
};


//catch 方法
Promise.prototype["catch"] = function (rejected) {
    return this.then(void 0, rejected);
};