239 lines
8.0 KiB
JavaScript
239 lines
8.0 KiB
JavaScript
flectra.define('web.concurrency', function (require) {
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"use strict";
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/**
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* Concurrency Utils
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*
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* This file contains a short collection of useful helpers designed to help with
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* everything concurrency related in Flectra.
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*
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* The basic concurrency primitives in Flectra JS are the callback, and the
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* promises. Promises (deferred) are more composable, so we usually use them
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* whenever possible. We use the jQuery implementation.
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*
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* Those functions are really nothing special, but are simply the result of how
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* we solved some concurrency issues, when we noticed that a pattern emerged.
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*/
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var Class = require('web.Class');
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return {
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/**
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* The jquery implementation for $.when has a (most of the time) useful
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* property: it is synchronous, if the deferred is resolved immediately.
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*
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* This means that when we execute $.when(def), then all registered
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* callbacks will be executed before the next line is executed. This is
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* useful quite often, but in some rare cases, we might want to force an
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* async behavior. This is the purpose of this function, which simply adds a
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* setTimeout before resolving the deferred.
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*
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* @returns {Deferred}
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*/
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asyncWhen: function () {
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var async = false;
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var def = $.Deferred();
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$.when.apply($, arguments).done(function() {
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var args = arguments;
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var action = function() {
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def.resolve.apply(def, args);
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};
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if (async) {
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action();
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} else {
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setTimeout(action, 0);
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}
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}).fail(function() {
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var args = arguments;
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var action = function() {
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def.reject.apply(def, args);
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};
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if (async) {
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action();
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} else {
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setTimeout(action, 0);
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}
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});
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async = true;
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return def;
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},
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/**
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* Returns a deferred resolved after 'wait' milliseconds
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*
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* @param {int} [wait=0] the delay in ms
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* @return {Deferred}
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*/
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delay: function (wait) {
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var def = $.Deferred();
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setTimeout(def.resolve, wait);
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return def;
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},
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/**
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* The DropMisordered abstraction is useful for situations where you have
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* a sequence of operations that you want to do, but if one of them
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* completes after a subsequent operation, then its result is obsolete and
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* should be ignored.
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*
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* Note that is is kind of similar to the DropPrevious abstraction, but
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* subtly different. The DropMisordered operations will all resolves if
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* they complete in the correct order.
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*/
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DropMisordered: Class.extend({
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/**
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* @constructor
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*
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* @param {boolean} [failMisordered=false] whether mis-ordered responses
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* should be failed or just ignored
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*/
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init: function (failMisordered) {
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// local sequence number, for requests sent
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this.lsn = 0;
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// remote sequence number, seqnum of last received request
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this.rsn = -1;
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this.failMisordered = failMisordered || false;
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},
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/**
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* Adds a deferred (usually an async request) to the sequencer
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*
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* @param {Deferred} deferred to ensure add
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* @returns {Deferred}
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*/
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add: function (deferred) {
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var res = $.Deferred();
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var self = this, seq = this.lsn++;
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deferred.done(function () {
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if (seq > self.rsn) {
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self.rsn = seq;
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res.resolve.apply(res, arguments);
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} else if (self.failMisordered) {
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res.reject();
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}
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}).fail(function () {
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res.reject.apply(res, arguments);
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});
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return res.promise();
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},
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}),
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/**
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* The DropPrevious abstraction is useful when you have a sequence of
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* operations that you want to execute, but you only care of the result of
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* the last operation.
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*
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* For example, let us say that we have a _fetch method on a widget which
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* fetches data. We want to rerender the widget after. We could do this::
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*
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* this._fetch().then(function (result) {
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* self.state = result;
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* self.render();
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* });
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*
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* Now, we have at least two problems:
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*
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* - if this code is called twice and the second _fetch completes before the
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* first, the end state will be the result of the first _fetch, which is
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* not what we expect
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* - in any cases, the user interface will rerender twice, which is bad.
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*
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* Now, if we have a DropPrevious::
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*
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* this.dropPrevious = new DropPrevious();
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*
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* Then we can wrap the _fetch in a DropPrevious and have the expected
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* result::
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*
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* this.dropPrevious
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* .add(this._fetch())
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* .then(function (result) {
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* self.state = result;
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* self.render();
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* });
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*/
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DropPrevious: Class.extend({
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/**
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* Registers a new deferred and rejects the previous one
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*
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* @param {Deferred} deferred the new deferred
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* @returns {Promise}
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*/
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add: function (deferred) {
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if (this.current_def) { this.current_def.reject(); }
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var res = $.Deferred();
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deferred.then(res.resolve, res.reject);
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this.current_def = res;
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return res.promise();
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}
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}),
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/**
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* A (Flectra) mutex is a primitive for serializing computations. This is
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* useful to avoid a situation where two computations modify some shared
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* state and cause some corrupted state.
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*
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* Imagine that we have a function to fetch some data _load(), which returns
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* a deferred which resolves to something useful. Now, we have some code
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* looking like this::
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*
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* return this._load().then(function (result) {
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* this.state = result;
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* });
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*
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* If this code is run twice, but the second execution ends before the
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* first, then the final state will be the result of the first call to
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* _load. However, if we have a mutex::
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*
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* this.mutex = new Mutex();
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*
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* and if we wrap the calls to _load in a mutex::
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*
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* return this.mutex.exec(function() {
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* return this._load().then(function (result) {
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* this.state = result;
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* });
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* });
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*
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* Then, it is guaranteed that the final state will be the result of the
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* second execution.
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*
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* A Mutex has to be a class, and not a function, because we have to keep
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* track of some internal state.
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*/
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Mutex: Class.extend({
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init: function () {
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this.def = $.Deferred().resolve();
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},
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/**
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* Add a computation to the queue, it will be executed as soon as the
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* previous computations are completed.
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*
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* @param {function} action a function which may return a deferred
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* @returns {Deferred}
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*/
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exec: function (action) {
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var current = this.def;
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var next = this.def = $.Deferred();
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return current.then(function() {
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return $.when(action()).always(function() {
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next.resolve();
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});
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});
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}
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}),
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/**
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* Rejects a deferred as soon as a reference deferred is either resolved or
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* rejected
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*
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* @param {Deferred} [target_def] the deferred to potentially reject
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* @param {Deferred} [reference_def] the reference target
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* @returns {Deferred}
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*/
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rejectAfter: function (target_def, reference_def) {
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var res = $.Deferred();
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target_def.then(res.resolve, res.reject);
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reference_def.always(res.reject);
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return res.promise();
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}
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};
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});
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