feat: add shared payment runtime

.changelog/shared-payment-runtime.md

@@ -0,0 +1,5 @@
+---
+pympp: minor
+---
+
+Added shared client-side primitives for payment-method matching, events, and credential creation.

README.md

@@ -50,6 +50,18 @@ async with Client(methods=[tempo(account=account, intents={"charge": ChargeInten
     response = await client.get("https://mpp.dev/api/ping/paid")
 ```
 
+Custom integrations can share method matching, events, and credential creation:
+
+```python
+from mpp.runtime import PaymentRuntime
+
+async with PaymentRuntime([method]) as runtime:
+    challenge, method = runtime.match_challenge(challenges)
+    credential = await runtime.create_credential(challenge, method)
+```
+
+The runtime borrows its methods and runs them on the caller's event loop.
+
 ## Examples
 
 | Example | Description |

src/mpp/client/transport.py

@@ -11,7 +11,7 @@
 
 import logging
 from datetime import UTC, datetime
-from typing import TYPE_CHECKING, Any, Protocol, runtime_checkable
+from typing import TYPE_CHECKING, Any
 
 import httpx
 
@@ -28,24 +28,14 @@
     EventHandler,
     Unsubscribe,
 )
+from mpp.runtime import Method
 
 logger = logging.getLogger(__name__)
 
 if TYPE_CHECKING:
     from collections.abc import Sequence
 
 
-@runtime_checkable
-class Method(Protocol):
-    """Payment method interface for client-side credential creation."""
-
-    name: str
-
-    async def create_credential(self, challenge: Challenge) -> Credential:
-        """Create a credential to satisfy the given challenge."""
-        ...
-
-
 def _client_payment_failed_payload(
     *,
     challenge: Challenge | None,

src/mpp/runtime.py

@@ -0,0 +1,152 @@
+"""Shared client-side payment primitives."""
+
+from __future__ import annotations
+
+from collections.abc import Mapping, Sequence
+from typing import Any, Protocol, Self, runtime_checkable
+
+from mpp import Challenge, Credential
+from mpp.events import (
+    CHALLENGE_RECEIVED,
+    CREDENTIAL_CREATED,
+    EventDispatcher,
+    EventPayload,
+)
+
+
+@runtime_checkable
+class Method(Protocol):
+    """Client-side payment method."""
+
+    name: str
+
+    async def create_credential(self, challenge: Challenge) -> Credential:
+        """Create a credential for a challenge."""
+        ...
+
+
+class PaymentRuntime:
+    """Match methods and create credentials on the caller's event loop.
+
+    Methods are borrowed: the runtime neither enters nor closes them.
+    """
+
+    def __init__(
+        self,
+        methods: Sequence[Method] = (),
+        *,
+        events: EventDispatcher | None = None,
+    ) -> None:
+        self.methods = tuple(methods)
+        for method in self.methods:
+            if not _is_method(method):
+                raise TypeError("methods must contain payment Methods")
+            _method_intents(method)
+        self.events = events or EventDispatcher()
+        self._closed = False
+
+    def start(self) -> Self:
+        """Open the runtime, or return it if already open."""
+        if self._closed:
+            raise RuntimeError("PaymentRuntime is closed")
+        return self
+
+    def __enter__(self) -> Self:
+        return self.start()
+
+    def __exit__(self, *_args: Any) -> None:
+        self.close()
+
+    async def __aenter__(self) -> Self:
+        return self.start()
+
+    async def __aexit__(self, *_args: Any) -> None:
+        self.close()
+
+    def match_challenge(
+        self,
+        challenges: Sequence[Challenge],
+        *,
+        prefer_method_order: bool = True,
+        allow_name_only: bool = False,
+    ) -> tuple[Challenge, Method]:
+        """Return the first compatible challenge and method."""
+        pairs = (
+            ((challenge, method) for method in self.methods for challenge in challenges)
+            if prefer_method_order
+            else ((challenge, method) for challenge in challenges for method in self.methods)
+        )
+        for challenge, method in pairs:
+            if challenge.method == method.name and (
+                allow_name_only or challenge.intent in _method_intents(method)
+            ):
+                return challenge, method
+
+        offered = [challenge.method for challenge in challenges]
+        installed = [method.name for method in self.methods]
+        raise ValueError(
+            f"No compatible payment method. Server offered: {offered}, client has: {installed}"
+        )
+
+    async def create_credential(
+        self,
+        challenge: Challenge,
+        method: Method,
+        *,
+        event_payload: dict[str, Any] | None = None,
+    ) -> Credential:
+        """Create a credential and emit its lifecycle events."""
+        self.start()
+        if not any(candidate is method for candidate in self.methods):
+            raise ValueError("Method is not installed in this PaymentRuntime")
+        if challenge.method != method.name or challenge.intent not in _method_intents(method):
+            raise ValueError(
+                f"Method {method.name!r} does not support {challenge.method!r}/{challenge.intent!r}"
+            )
+        payload = {
+            **(event_payload or {}),
+            "challenge": challenge,
+            "method": method,
+        }
+        payload.setdefault("challenges", [challenge])
+        supplied = await self.events.emit(
+            CHALLENGE_RECEIVED,
+            payload,
+            first_result=True,
+        )
+        credential = (
+            supplied
+            if isinstance(supplied, Credential)
+            else await method.create_credential(challenge)
+        )
+        await self.events.emit(
+            CREDENTIAL_CREATED,
+            {**payload, "credential": credential},
+        )
+        return credential
+
+    async def emit_event(self, name: str, payload: EventPayload) -> Any:
+        """Emit an event on the caller's event loop."""
+        self.start()
+        return await self.events.emit(name, payload)
+
+    def close(self) -> None:
+        """Prevent new runtime operations."""
+        self._closed = True
+
+
+def _is_method(value: Any) -> bool:
+    return isinstance(getattr(value, "name", None), str) and callable(
+        getattr(value, "create_credential", None)
+    )
+
+
+def _method_intents(method: Method) -> Mapping[str, Any]:
+    intents = getattr(method, "intents", None)
+    if intents is None:
+        intents = getattr(method, "_intents", None)
+    if intents is None:
+        return {"charge": None}
+    if not isinstance(intents, Mapping):
+        raise TypeError("Method intents must be a Mapping")
+    return intents