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clojurescriptom

om get-props vs get-state

I am trying to grasp the purpose of the two OM functions get-state and get-props. Have a look at the following example: 

(defn example [app owner]
  (reify
    om/IInitState
    (render-state [this state]
      (println "app-state: " app )
      (println "state: " state )
      (println "get-props: " (om/get-props owner) )
      (println "get-state: " (om/get-state owner) )
      (dom/div nil "hello"))))

You will notice that app and state contain exactly what get-props and get-state return, which seems at the first glance pretty redundant.

Now, not all the lifecycle functions (e.g. IWillMount ) pass the state argument, so when you need it in such circumstances, it's obvious that you need to invoke om/get-state to get access to it.

However, for the app-state it looks different to me. You always have the app-state cursor available in all functions since it's a top-level argument of the function, even if you need it in callbacks you can just pass it around. Most examples/tutorials make use of get-state but I can't find an example of get-props. Is get-props redundant? Where would I use it?

And one more thing related to this construct. In React we have props and state, but in OM we have app-state and state (internal state) which confused me when learning OM. Props are passed from parent to child in React, likewise in OM we pass app-state (cursors) to children. Are the following observations valid?

  • app-state is the OM equivalent of React' props
  • props is React is just data, while app-state in OM is data wrapped in cursors
  • this means that OM doesn't have props, only app-state cursors, hence the function get-props really means get-app-state
Solution

  • According to the docs, get-props is mostly (or exclusively) needed in the IWillReceiveProps phase. will-receive-props gets a next-props argument, which contains the future app-state/props. get-props gives you the current app-state/props so that you can compare the two.

    From the Om docs for IWillReceiveProps:

    In your implementation if you wish to detect prop transitions you must use om.core/get-props to get the previous props. This is because your component constructor function is called with the updated props.

    So the rest of the time, get-props isn't necessary because, as mentioned in the question, you have access to the cursor.