feat: dynamic initialState (#435)

* feat: `initialState` to be derived from `Context`

Useful with path parameters.

* chore: changesets

* nit: support async too

.changeset/twelve-rocks-push.md

@@ -0,0 +1,8 @@
+---
+"frog": patch
+---
+
+Implemented a feature where `initialState` can be a callback receiving Hono's `Context`.
+
+This is particularly useful when dealing with path parameters to dynamically initiate state.
+This state will also be accessible in `c.previousState` in the Image Handler.

src/frog-base.tsx

@@ -146,7 +146,10 @@ export type FrogConstructorParameters<
      * }
      * ```
      */
-    initialState?: _state | undefined
+    initialState?:
+      | ((c: Context<env>) => _state | Promise<_state>)
+      | _state
+      | undefined
     /**
      * Origin URL of the server instance.
      *
@@ -270,7 +273,10 @@ export class FrogBase<
 > {
   // Note: not using native `private` fields to avoid tslib being injected
   // into bundled code.
-  _initialState: env['State'] = undefined as env['State']
+  _initialState:
+    | ((c: Context<env>) => _state | Promise<_state>)
+    | _state
+    | undefined = undefined
   /** Path for assets. */
   assetsPath: string
   /** Base path of the server instance. */
@@ -596,7 +602,10 @@ export class FrogBase<
           secret: this.secret,
           verify,
         }),
-        initialState: this._initialState,
+        initialState:
+          typeof this._initialState === 'function'
+            ? await (this._initialState as any)(c)
+            : this._initialState,
         origin,
       })
 
@@ -1000,7 +1009,10 @@ export class FrogBase<
         context: await requestBodyToImageContext(c, {
           secret: this.secret,
         }),
-        initialState: this._initialState,
+        initialState:
+          typeof this._initialState === 'function'
+            ? await (this._initialState as any)(c)
+            : this._initialState,
       })
 
       const response = await handler(context)

src/frog.tsx

@@ -40,7 +40,7 @@ export class Frog<
   //
   _state = env['State'],
 > extends FrogBase<env, schema, basePath, _state> {
-  constructor(params: FrogConstructorParameters) {
+  constructor(params: FrogConstructorParameters<env, basePath, _state>) {
     super(params as any)
 
     const frame = this.frame