[blog] Add breaking change blog post for Fan entity preset mode optimizations (#69)

Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

Author: bdraco

docs/blog/posts/2025-11-07-fan-entity-preset-modes.md

@@ -0,0 +1,304 @@
+---
+date: 2025-11-07
+authors:
+  - bdraco
+comments: true
+---
+
+# Fan Entity Class: Preset Mode Flash Storage and Order Preservation
+
+ESPHome 2025.11.0 introduces a memory optimization to the `Fan` entity class that also changes how preset modes are ordered. This affects external components implementing custom fan devices and may change the display order of preset modes in Home Assistant.
+
+<!-- more -->
+
+## Background
+
+**[PR #11483](https://github.com/esphome/esphome/pull/11483): Store Preset Modes in Flash**
+Changes preset mode storage from `std::set<std::string>` (heap, alphabetically sorted) to `std::vector<const char*>` (flash, preserves YAML order). Saves ~80 bytes for the `std::set` structure plus at least 24 bytes overhead per preset (more for longer strings). Strings move from heap to flash memory. This is particularly important for ESP8266 devices with limited heap.
+
+## What's Changing
+
+### For ESPHome 2025.11.0 and Later
+
+**Storage Changes (Breaking - [PR #11483](https://github.com/esphome/esphome/pull/11483)):**
+```cpp
+// OLD - std::set in heap, alphabetically sorted
+std::set<std::string> preset_modes_;
+traits.set_supported_preset_modes(modes);  // std::set parameter
+
+// NEW - std::vector of const char* in flash, preserves order
+std::vector<const char *> preset_modes_;
+traits.set_supported_preset_modes({"Low", "Medium", "High"});  // initializer_list
+```
+
+**User-Facing Change:**
+Preset modes now appear in Home Assistant in the **order you define them in YAML**, not alphabetically. This makes Fan consistent with all other components (select options, climate presets, etc.).
+
+## Who This Affects
+
+**External components** that:
+- Explicitly create `std::set<std::string>` and pass it to `set_supported_preset_modes()` in C++
+- Store or manipulate fan preset mode lists in member variables
+
+**Note:** Most components already use the correct syntax since Python code generation produces initializer lists like `traits.set_supported_preset_modes({"Low", "Medium", "High"})`. This only affects external components that manually create sets in C++ code.
+
+**YAML users** may notice:
+- Preset mode order in Home Assistant changes to match YAML order instead of alphabetical
+- This is a **behavioral change** - you now control the display order
+
+**Standard YAML configurations** work without code changes, but the display order may change.
+
+## User-Facing Behavior Change
+
+### Preset Mode Display Order
+
+Previously, fan preset modes were **always sorted alphabetically** regardless of YAML order (only fan presets had this limitation). Now they preserve YAML order like all other components.
+
+**Example YAML:**
+```yaml
+fan:
+  - platform: template
+    name: "Bedroom Fan"
+    preset_modes:
+      - "Turbo"
+      - "Normal"
+      - "Sleep"
+```
+
+**Before (alphabetical sort):** Normal → Sleep → Turbo
+**After (YAML order):** Turbo → Normal → Sleep
+
+**Action:** If you want a specific order in Home Assistant, arrange preset modes in your YAML in that order.
+
+## Migration Guide for External Components
+
+### 1. Update Container Type (Required Now)
+
+```cpp
+// OLD
+#include <set>
+std::set<std::string> preset_modes_;
+
+// NEW
+#include <vector>
+std::vector<const char *> preset_modes_;
+```
+
+### 2. Update Setter Signatures (Required Now)
+
+```cpp
+// OLD
+void set_preset_modes(const std::set<std::string> &presets) {
+  this->preset_modes_ = presets;
+}
+
+// NEW - use initializer list for string literals
+void set_preset_modes(std::initializer_list<const char *> presets) {
+  this->preset_modes_ = presets;
+}
+```
+
+### 3. Update Trait Calls (If You Explicitly Created Sets)
+
+**Note:** Most components already pass initializer lists directly and don't need changes. This only affects code that explicitly creates `std::set` variables.
+
+```cpp
+// OLD - explicitly creating std::set (uncommon)
+std::set<std::string> modes = {"Low", "Medium", "High"};
+traits.set_supported_preset_modes(modes);
+
+// NEW - initializer list with string literals (most components already did this)
+traits.set_supported_preset_modes({"Low", "Medium", "High"});
+```
+
+### 4. Update Lookups (Required Now)
+
+```cpp
+// OLD - std::set::find
+if (this->preset_modes_.find(mode) != this->preset_modes_.end()) {
+  // mode is supported
+}
+
+// NEW - linear search with strcmp
+bool found = false;
+for (const char *m : this->preset_modes_) {
+  if (strcmp(m, mode.c_str()) == 0) {
+    found = true;
+    break;
+  }
+}
+if (found) {
+  // mode is supported
+}
+
+// Or use std::find_if (cleaner but adds STL overhead)
+auto it = std::find_if(this->preset_modes_.begin(), this->preset_modes_.end(),
+                       [&mode](const char *m) { return strcmp(m, mode.c_str()) == 0; });
+if (it != this->preset_modes_.end()) {
+  // mode is supported
+}
+```
+
+**Note:** `std::find_if` is cleaner but adds STL template overhead. For ESP8266 devices with tight flash constraints, prefer the manual loop approach. For typical fan preset counts (3-6 items), linear search performance is negligible and the simpler approach is fine.
+
+### 5. Remove Unnecessary Includes
+
+```cpp
+// Remove:
+#include <set>
+```
+
+## Complete Migration Example
+
+**Before:**
+```cpp
+#include <set>
+
+class MyFan : public fan::Fan {
+ public:
+  void set_preset_modes(const std::set<std::string> &modes) {
+    this->preset_modes_ = modes;
+  }
+
+  fan::FanTraits get_traits() override {
+    auto traits = fan::FanTraits();
+    traits.set_supported_preset_modes(this->preset_modes_);
+    return traits;
+  }
+
+  void control(const fan::FanCall &call) override {
+    if (!call.get_preset_mode().empty()) {
+      std::string mode = call.get_preset_mode();
+      if (this->preset_modes_.find(mode) != this->preset_modes_.end()) {
+        // Set mode
+      }
+    }
+  }
+
+ protected:
+  std::set<std::string> preset_modes_;
+};
+```
+
+**After:**
+```cpp
+#include <vector>
+
+class MyFan : public fan::Fan {
+ public:
+  void set_preset_modes(std::initializer_list<const char *> modes) {
+    this->preset_modes_ = modes;
+  }
+
+  fan::FanTraits get_traits() override {
+    auto traits = fan::FanTraits();
+    traits.set_supported_preset_modes(this->preset_modes_);
+    return traits;
+  }
+
+  void control(const fan::FanCall &call) override {
+    if (!call.get_preset_mode().empty()) {
+      const std::string &mode = call.get_preset_mode();
+      auto it = std::find_if(this->preset_modes_.begin(), this->preset_modes_.end(),
+                             [&mode](const char *m) { return strcmp(m, mode.c_str()) == 0; });
+      if (it != this->preset_modes_.end()) {
+        // Set mode
+      }
+    }
+  }
+
+ protected:
+  std::vector<const char *> preset_modes_;
+};
+```
+
+## Lifetime Safety for Preset Modes
+
+All `const char*` pointers must point to memory that lives for the component's lifetime:
+
+**Safe patterns:**
+```cpp
+// 1. String literals (preferred) - stored in flash
+traits.set_supported_preset_modes({"Low", "Medium", "High"});
+
+// 2. Static constants
+static const char *const PRESET_LOW = "Low";
+traits.set_supported_preset_modes({PRESET_LOW});
+
+// 3. C arrays
+static constexpr const char *const PRESETS[] = {"Low", "Medium", "High"};
+traits.set_supported_preset_modes({PRESETS[0], PRESETS[1], PRESETS[2]});
+```
+
+**Unsafe patterns (DO NOT USE):**
+```cpp
+// WRONG - temporary string
+std::string temp = "Low";
+traits.set_supported_preset_modes({temp.c_str()});  // Dangling pointer!
+
+// WRONG - local array
+const char *modes[] = {"Low", "High"};
+traits.set_supported_preset_modes({modes[0], modes[1]});  // Array destroyed!
+```
+
+**For dynamic modes (rare):**
+```cpp
+#include "esphome/core/helpers.h"
+
+class MyFan : public fan::Fan {
+ protected:
+  // Storage for strings (must persist)
+  FixedVector<std::string> preset_strings_;
+  // Pointers into preset_strings_
+  std::vector<const char *> preset_modes_;
+
+  void setup() override {
+    // Read dynamic presets
+    this->preset_strings_.init(mode_count);
+    for (size_t i = 0; i < mode_count; i++) {
+      this->preset_strings_.push_back(this->read_mode_from_device(i));
+    }
+
+    // Build pointer array
+    this->preset_modes_.clear();
+    for (const auto &s : this->preset_strings_) {
+      this->preset_modes_.push_back(s.c_str());
+    }
+
+    // Set traits
+    this->traits_.set_supported_preset_modes(this->preset_modes_);
+  }
+};
+```
+
+**Important:** The `preset_strings_` member must outlive `preset_modes_` and never be resized or modified after the pointers are assigned, as this would invalidate the pointers in `preset_modes_`. `FixedVector` guarantees this by allocating all storage upfront with `init()` and never reallocating.
+
+## Timeline
+
+- **ESPHome 2025.11.0 (November 2025):**
+  - Storage change is active (breaking change for external components)
+  - Preset mode order changes to YAML order (user-facing behavior change)
+
+## Finding Code That Needs Updates
+
+Search your external component code for these patterns:
+
+```bash
+# Find std::set usage for fan preset modes
+grep -r 'std::set<.*string>.*preset' --include='*.cpp' --include='*.h'
+
+# Find set_supported_preset_modes calls
+grep -r 'set_supported_preset_modes' --include='*.cpp' --include='*.h'
+
+# Find preset_modes_ member variables
+grep -r 'preset_modes_' --include='*.cpp' --include='*.h'
+```
+
+## Questions?
+
+If you have questions about these changes or need help migrating your external component, please ask in the [ESPHome Discord](https://discord.gg/KhAMKrd) or open a [discussion on GitHub](https://github.com/esphome/esphome/discussions).
+
+## Related Documentation
+
+- [Fan Component Documentation](https://esphome.io/components/fan/index.html)
+- [PR #11483: Store Preset Modes in Flash](https://github.com/esphome/esphome/pull/11483)