2020-07-30 17:24:38 +02:00
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# Local echo (developer docs)
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The React SDK provides some local echo functionality to allow for components to do something
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quickly and fall back when it fails. This is all available in the `local-echo` directory within
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`stores`.
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Echo is handled in EchoChambers, with `GenericEchoChamber` being the base implementation for all
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chambers. The `EchoChamber` class is provided as semantic access to a `GenericEchoChamber`
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implementation, such as the `RoomEchoChamber` (which handles echoable details of a room).
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Anything that can be locally echoed will be provided by the `GenericEchoChamber` implementation.
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The echo chamber will also need to deal with external changes, and has full control over whether
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2022-12-12 12:24:14 +01:00
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or not something has successfully been echoed.
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2020-07-30 17:24:38 +02:00
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An `EchoContext` is provided to echo chambers (usually with a matching type: `RoomEchoContext`
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gets provided to a `RoomEchoChamber` for example) with details about their intended area of
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effect, as well as manage `EchoTransaction`s. An `EchoTransaction` is simply a unit of work that
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needs to be locally echoed.
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The `EchoStore` manages echo chamber instances, builds contexts, and is generally less semantically
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accessible than the `EchoChamber` class. For separation of concerns, and to try and keep things
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tidy, this is an intentional design decision.
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2022-12-12 12:24:14 +01:00
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**Note**: The local echo stack uses a "whenable" pattern, which is similar to thenables and
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2020-07-30 17:24:38 +02:00
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`EventEmitter`. Whenables are ways of actioning a changing condition without having to deal
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with listeners being torn down. Once the reference count of the Whenable causes garbage collection,
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the Whenable's listeners will also be torn down. This is accelerated by the `IDestroyable` interface
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usage.
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## Audit functionality
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The UI supports a "Server isn't responding" dialog which includes a partial audit log-like
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structure to it. This is partially the reason for added complexity of `EchoTransaction`s
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and `EchoContext`s - this information feeds the UI states which then provide direct retry
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mechanisms.
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The `EchoStore` is responsible for ensuring that the appropriate non-urgent toast (lower left)
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is set up, where the dialog then drives through the contexts and transactions.
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