Add cache generalization & Snapshots for synchronous support #37

z4kn4fein · 2023-08-22T18:51:42Z

Describe the purpose of your pull request

This PR contains the following:

Enables the generalized caching feature.
Removes deprecated functions:
- getVariationId() and getAllVariationIds(), alternative: getValueDetails() / getAllValueDetails().
- refresh(), alternative: forceRefresh().
- Init function of PollingModes.autoPoll() that had an onConfigChanged callback parameter. Alternative for subscribing to config changes: Hooks.
- Each ***Sync() method that used DispatchSemaphore for synchronizing mainly asynchronous operations. For an alternative, see the next bullet point.
Introduces the concept of Snapshots. They are for capturing the actual state of the feature flag data used by the SDK. A snapshot allows synchronous operations on the captured context.
```
let snapshot = configCatClient.snapshot()
let isMyFeatureEnabled = snapshot.getValue(for: "isMyFeatureEnabled", defaultValue: false)
```
Snapshots are created from the actual state of the SDK; therefore it's crucial to know whether the SDK has valid feature flag data to work on. To determine whether it's safe to create snapshots, the SDK already provides an onClientReady hook. It's being changed to accept a state parameter to give details about the SDK's initialization state.
```
client.hooks.addOnReady { state in
  // the state parameter indicates what is the SDK's initialization state
}
```
These are the possible state values:
- noFlagData: This means the SDK has no feature flag data to work on (it didn't get anything from the cache or from the network)
- hasLocalOverrideFlagDataOnly: The SDK was initialized with localOnly flag overrides.
- hasCachedFlagDataOnly: The SDK has feature flag data from only the cache. It can happen when the SDK is configured with PollingModes.manualPoll() and there wasn't a client.forceRefresh() call yet. Another example could be an SDK configured with PollingModes.autoPoll(), but it can't reach the ConfigCat CDN so it falls back to the cache.
- hasUpToDateFlagData: The SDK is initialized with up-to-date feature flag data.
This functionality was extended with a new awaitable method waitForReady() that asynchronously waits for the onClientReady hook to fire and also returns with the SDK's initialization state.
```
let state = await client.waitForReady()
```