docs: Add info about custom Dispatchers

docs/docs/worklets.md

@@ -6,10 +6,10 @@ import TabItem from '@theme/TabItem';
 
 # Worklets/Threading
 
-Nitro itself is fully runtime-agnostic, which means every [Hybrid Object](hybrid-object) can be used from any JS Runtime or Worklet Context.
+Nitro itself is fully runtime-agnostic, which means every [Hybrid Object](hybrid-objects) can be used from any JS Runtime or Worklet Context.
 
 This allows the caller to call into native Nitro Modules from libraries like [react-native-worklets-core](https://github.com/margelo/react-native-worklets-core), or [react-native-reanimated](https://github.com/software-mansion/react-native-reanimated).
-You can use a Nitro [Hybrid Object](hybrid-object) on the default React JS context, on the UI context, or on any other background worklet context.
+You can use a Nitro [Hybrid Object](hybrid-objects) on the default React JS context, on the UI context, or on any other background worklet context.
 
 <Tabs groupId="worklet-library">
   <TabItem value="rnwc" label="Worklets Core" default>
@@ -51,3 +51,33 @@ A _boxed_ Hybrid Object is a native `jsi::HostObject`, which is supported by wor
 4. The result of `.unbox()` is the original `HybridObject` - you can now call any methods on it as usual.
 
 In future versions of [react-native-worklets-core](https://github.com/margelo/react-native-worklets-core) or [react-native-reanimated](https://github.com/software-mansion/react-native-reanimated) we expect fullly automatic `jsi::NativeState` support, which will make boxing obsolete.
+
+## Dispatcher
+
+All synchronous APIs of Nitro work ✨ automagically ✨ on any runtime, but asynchronous APIs (Promises and callbacks) require a `Dispatcher`.
+
+If you call an asynchronous API on a runtime that Nitro doesn't know, it likely doesn't have a `Dispatcher`, so it doesn't know how to call back to the JS Thread after the asynchronous operation has finished (Promise resolve or callback call).
+
+If you created that `jsi::Runtime`, you need to create a `Dispatcher` for it and implement `runSync` and `runAsync`:
+
+```cpp
+#include <NitroModules/Dispatcher.hpp>
+using namespace margelo::nitro;
+
+class MyRuntimeDispatcher: public Dispatcher {
+public:
+  void runSync(std::function<void()>&& function) override;
+  void runAsync(std::function<void()>&& function) override;
+};
+```
+
+Then, simply install this Dispatcher into your runtime:
+
+```cpp
+auto myDispatcher = std::make_shared<MyRuntimeDispatcher>();
+Dispatcher::installRuntimeGlobalDispatcher(myRuntime, myDispatcher);
+```
+
+This needs to be done once, ideally immediately as soon as possible after creating the `jsi::Runtime`.
+
+Your `runSync` and `runAsync` implementations must run the given `function` on the same Thread that the `jsi::Runtime` was created on - see [`CallInvokerDispatcher.hpp`](https://github.com/mrousavy/nitro/blob/main/packages/react-native-nitro-modules/cpp/threading/CallInvokerDispatcher.hpp) for an example.