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    <p>It sounds like you're looking for a <a href="https://en.wikipedia.org/wiki/Finite-state_machine" rel="nofollow">state machine</a>. There's a lot of reading you can do on state machines, but basically they represent a thing that can be in one of any number of <em>states</em>; the state of the machine can be changed by triggering appropriate <em>events</em> on the machine.</p> <p>In your case, <code>calling</code> and <code>onUserCalling</code> are the <em>states</em>, and <code>call</code>, <code>answer</code>, <code>reject</code>, etc. are the <em>events</em>. By representing this as a state machine, you can keep track of which state a user is in and what states they can transition to based on the events.</p> <p>As an example, <a href="http://plunker.co/edit/sUJNGo32XhOzrhPXqNDp?p=preview" rel="nofollow">here is some code on Plunker that demonstrates such a state machine</a>. (Ignore the fact that the user interface interaction was built with AngularJS.) The state machine is defined via a library called <a href="https://github.com/jakesgordon/javascript-state-machine" rel="nofollow">JavaScript State Machine</a> and is defined as such:</p> <pre class="lang-javascript prettyprint-override"><code>StateMachine.create({ initial: 'ready', events: [ { name: 'placeCall', from: 'ready', to: 'calling' }, { name: 'receiveCall', from: 'ready', to: 'callWaiting' }, { name: 'callAnswered', from: 'calling', to: 'inCall' }, { name: 'callRejected', from: 'calling', to: 'ready' }, { name: 'answerCall', from: 'callWaiting', to: 'inCall' }, { name: 'rejectCall', from: 'callWaiting', to: 'ready' }, { name: 'hangUp', from: ['calling', 'inCall'], to: 'ready' } ] }); </code></pre> <p>This says that when the machine is in the <code>ready</code> state, we can trigger the <code>placeCall</code> event to change the state <code>calling</code>, or we can trigger the <code>receiveCall</code> event to change the state to <code>callWaiting</code>. Once the state has been changed to <code>calling</code>, we can move to either the <code>inCall</code> state or back to <code>ready</code> by triggering the <code>callAnswered</code> or <code>callRejected</code> events, respectively.</p> <p>The state machine library can look into the events definition and tell you which events are allowed to be fired based on the current state of the machine. In the example, I've used this to only enable the buttons that will fire allowed events.</p> <p>Most state machine libraries, including the JavaScript one I've used here, allow you to supply callbacks when you enter or leave certain events; this particular library also allows asynchronous event transitions, which may help when using the library with async Node.js code. You can find more information <a href="https://github.com/jakesgordon/javascript-state-machine#callbacks" rel="nofollow">in the readme</a>.</p>
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    1. USMichelle Tilley
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    1. COThanks for your reply. It really helps! I will try to use your lib to implement what I want.
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    2. COI'm glad you found it useful. It's worth mentioning that there are many state machine implementations out there; a Google search will find you several I'm sure. The one I mentioned isn't mine, but I use it for my own JavaScript stuff.
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    3. COUnderstood, thanks:)
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