Feature Request: Standalone `rxMutation` Function

[rainerhahnekamp]

Create a standalone version of rxMutation that can be used independently of SignalStore and contains the full mutation feature with RxJS-support.

Background

Currently, rxMutation is designed to work within the SignalStore context as part of the withMutations feature. While this works well for SignalStore-based applications, there's a need for a standalone version that can be used in various contexts without requiring the full SignalStore setup.

API Design Considerations

Based on internal discussions, several API design questions need to be addressed:

1. Implementation Details

Question: Why expose callCount in Mutation? This seems like an implementation detail.

Proposal: Make callCount internal or provide it only when explicitly requested.

2. Helper Functions

Proposal: Add convenient helper functions:

export interface RxMutation<Input, Value> {
  (input: Input): Promise<MutationResult<Value>>;
  value: Signal<Value | undefined>;
  error: Signal<unknown>;
  status: Signal<MutationStatus>;
  
  // Proposed helpers
  isPending(): boolean;
  isFulfilled(): boolean;
  hasValue(): boolean;
}

3. Value Access

Question: How to access current value from outside?

Solution: Ensure the value signal is available.

4. Simplified Overload

Proposal: Add overload that only requires the operation:

// Instead of
const mutation = rxMutation({
  executor: (id: string) => httpClient.delete(`/user/${id}`)
});

// Allow
const mutation = rxMutation((id: string) => httpClient.delete(`/user/${id}`));

This feature was discussed in #203

"TODO: Generally speaking, once we make rxMutation standalone, most tests of rxMutation will cover already what we have here."

[manfredsteyer]

Like it!