Trigger Trampolines Eliminated with build_callback_automation

Apr 9, 2026

Common entity trigger classes have been replaced with lightweight forwarder structs that fit inline in the callback system. The new build_callback_automation() API eliminates per-trigger object allocations. Several entity callback signatures have also changed to pass state as an argument.

This is a breaking change for external components in ESPHome 2026.4.0 and later.

Background

PR #15174: Eliminate trigger trampolines with deduplicated forwarder structs
PR #15198: alarm_control_panel — Migrate triggers to callback automation
PR #15199: lock — Migrate LockStateTrigger to callback automation
PR #15200: media_player — Migrate triggers to callback automation

Previously, each automation trigger created a separate C++ object that existed solely to forward a callback to an Automation. For example:

button press → callback → ButtonPressTrigger::trigger() → Automation::trigger()

Now a lightweight forwarder struct collapses this into the callback itself:

button press → callback → TriggerForwarder::operator()() → Automation::trigger()

The forwarder fits in the Callback::ctx_ field — no additional storage needed.

Memory savings

Config	Platform	RAM Saved	Flash Saved
ratgdo (garage door)	ESP8266	88 bytes	224 bytes
multi-sensor device	ESP32-IDF	208 bytes	280 bytes

What’s Changing

1. Callback signatures changed

Several entity callback signatures changed to pass state as an argument, enabling single-pointer forwarders:

// alarm_control_panel — Before
void add_on_state_callback(std::function<void()> &&callback);
// After
template<typename F> void add_on_state_callback(F &&callback);
// Callback signature: void(AlarmControlPanelState)

// lock — Before
void add_on_state_callback(std::function<void()> &&callback);
// After
template<typename F> void add_on_state_callback(F &&callback);
// Callback signature: void(LockState)

// media_player — Before
void add_on_state_callback(std::function<void()> &&callback);
// After
template<typename F> void add_on_state_callback(F &&callback);
// Callback signature: void(MediaPlayerState)

2. Automation::trigger_ field removed

The trigger_ protected field on Automation (set in constructor, never read) has been removed.

Who This Affects

External components registering callbacks on alarm_control_panel, lock, or media_player — must update callback signature to accept the state parameter
External components accessing Automation::trigger_ — this field no longer exists

Migration Guide

Callback signature changes

// alarm_control_panel — Before
this->parent_->add_on_state_callback([this]() {
  auto state = this->parent_->get_state();
  // ...
});

// After
this->parent_->add_on_state_callback([this](AlarmControlPanelState state) {
  // state is passed directly, no need to call get_state()
});

// lock — Before
this->parent_->add_on_state_callback([this]() {
  auto state = this->parent_->state;
  // ...
});

// After
this->parent_->add_on_state_callback([this](LockState state) {
  // state is passed directly
});

// media_player — Before
this->parent_->add_on_state_callback([this]() {
  auto state = this->parent_->state;
  // ...
});

// After
this->parent_->add_on_state_callback([this](MediaPlayerState state) {
  // state is passed directly
});

Python codegen migration

If your external component uses build_automation() with trigger classes, migrate to build_callback_automations() (PR #15246):

# Before
from esphome import automation

MyStateTrigger = my_ns.class_("MyStateTrigger", automation.Trigger.template(cg.bool_))
MyPressTrigger = my_ns.class_("MyPressTrigger", automation.Trigger.template())

CONFIG_SCHEMA = cv.Schema({
    cv.Optional(CONF_ON_STATE): automation.validate_automation(
        {cv.GenerateID(CONF_TRIGGER_ID): cv.declare_id(MyStateTrigger)}
    ),
    cv.Optional(CONF_ON_PRESS): automation.validate_automation(
        {cv.GenerateID(CONF_TRIGGER_ID): cv.declare_id(MyPressTrigger)}
    ),
})

async def to_code(config):
    for conf in config.get(CONF_ON_STATE, []):
        trigger = cg.new_Pvariable(conf[CONF_TRIGGER_ID], var)
        await automation.build_automation(trigger, [(bool, "x")], conf)
    for conf in config.get(CONF_ON_PRESS, []):
        trigger = cg.new_Pvariable(conf[CONF_TRIGGER_ID], var)
        await automation.build_automation(trigger, [], conf)

# After — no trigger classes needed, no CONF_TRIGGER_ID in schema
from esphome import automation

_CALLBACK_AUTOMATIONS = (
    automation.CallbackAutomation(CONF_ON_STATE, "add_on_state_callback", [(bool, "x")]),
    automation.CallbackAutomation(CONF_ON_PRESS, "add_on_press_callback"),
)

CONFIG_SCHEMA = cv.Schema({
    cv.Optional(CONF_ON_STATE): automation.validate_automation({}),
    cv.Optional(CONF_ON_PRESS): automation.validate_automation({}),
})

async def to_code(config):
    await automation.build_callback_automations(var, config, _CALLBACK_AUTOMATIONS)

build_automation() and all Trigger subclasses remain available for triggers that need mutable state beyond a single Automation* pointer.

Timeline

ESPHome 2026.4.0 (April 2026): Trigger classes removed, callback signatures changed
No deprecation period — these are signature changes and class removals

Finding Code That Needs Updates

# Find alarm_control_panel/lock/media_player callback registrations
grep -rn 'add_on_state_callback' your_component/

Questions?

If you have questions about migrating your external component, please ask in:

ESPHome Discord - #devs channel
ESPHome GitHub Discussions

Comments

Feel free to leave a comment here to discuss this post with others. You can ask questions, share your experience, or suggest improvements. If you have a question about a specific feature or issue, please consider using the ESPHome Discord. Stick to English and follow ESPHome's code of conduct. These comments exist on a discussion on GitHub, so you can also comment there directly if you prefer.