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According to the docs
CustomProperties.saveAsync saves item-specific custom properties to the server. Does it also internally perform the same logic as MessageCompose.saveAsync?
We have business logic that requires the draft to be saved and a custom property to be set with a single button action. I am trying to understand if both calls are necessary or if the order matters.
MessageCompose.saveAsync() then set the custom property and save with CustomProperties.saveAsync() CustomProperties.saveAsync(), and then call MessageCompose.saveAsync() CustomProperties.saveAsync(), (dont call MessageCompose.saveAsync()` since the draft automatically is saved and this is redundant). 
From playing around with it, it seems to me the following is true:
CustomProperties.saveAsync() will save the item as a draft when it has not yet been saved CustomProperties.saveAsync() will not save the item again if it has already been saved This is just an observation, but i am hoping someone can tell me that i am wrong (or right).
For now I am assuming the correct order is:
Option 1: Call MessageCompose.saveAsync() then set the custom property and save with CustomProperties.saveAsync()