ESPHome 2026.4.0: Callback registration methods should use templates

[bdraco]

Summary

ESPHome 2026.4.0 replaces std::function with a lightweight Callback struct in CallbackManager (PR #14853). Existing code continues to compile, but callback registration methods that take std::function now use a less efficient heap-allocation path.

Recommended migration

Convert callback registration methods from std::function to templates:

// Before (still works, but now less efficient)
void add_on_state_callback(std::function<void(float)> &&callback) {
  this->state_callback_.add(std::move(callback));
}

// After (optimal — zero heap allocation for [this] lambdas)
template<typename F> void add_on_state_callback(F &&callback) {
  this->state_callback_.add(std::forward<F>(callback));
}

If you had a separate .cpp definition, move the body inline into the header (templates must be defined in headers).

The template approach is backward compatible with older ESPHome versions — no version guard needed.

References

• ESPHome PR: [core] Replace std::function with lightweight Callback in CallbackManager esphome/esphome#14853
• Developer blog post: Add blog post: CallbackManager lightweight Callback (#14853) esphome/developers.esphome.io#118