[wifi] Optimize WiFi network storage with FixedVector (#11458)

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

esphome/components/wifi/__init__.py

@@ -378,14 +378,19 @@ async def to_code(config):
     # Track if any network uses Enterprise authentication
     has_eap = False
 
-    def add_sta(ap, network):
-        ip_config = network.get(CONF_MANUAL_IP, config.get(CONF_MANUAL_IP))
-        cg.add(var.add_sta(wifi_network(network, ap, ip_config)))
+    # Initialize FixedVector with the count of networks
+    networks = config.get(CONF_NETWORKS, [])
+    if networks:
+        cg.add(var.init_sta(len(networks)))
 
-    for network in config.get(CONF_NETWORKS, []):
-        if CONF_EAP in network:
-            has_eap = True
-        cg.with_local_variable(network[CONF_ID], WiFiAP(), add_sta, network)
+        def add_sta(ap: cg.MockObj, network: dict) -> None:
+            ip_config = network.get(CONF_MANUAL_IP, config.get(CONF_MANUAL_IP))
+            cg.add(var.add_sta(wifi_network(network, ap, ip_config)))
+
+        for network in networks:
+            if CONF_EAP in network:
+                has_eap = True
+            cg.with_local_variable(network[CONF_ID], WiFiAP(), add_sta, network)
 
     if CONF_AP in config:
         conf = config[CONF_AP]

esphome/components/wifi/wifi_component.cpp

@@ -330,9 +330,11 @@ float WiFiComponent::get_loop_priority() const {
   return 10.0f;  // before other loop components
 }
 
+void WiFiComponent::init_sta(size_t count) { this->sta_.init(count); }
 void WiFiComponent::add_sta(const WiFiAP &ap) { this->sta_.push_back(ap); }
 void WiFiComponent::set_sta(const WiFiAP &ap) {
   this->clear_sta();
+  this->init_sta(1);
   this->add_sta(ap);
 }
 void WiFiComponent::clear_sta() { this->sta_.clear(); }

esphome/components/wifi/wifi_component.h

@@ -219,6 +219,7 @@ class WiFiComponent : public Component {
 
   void set_sta(const WiFiAP &ap);
   WiFiAP get_sta() { return this->selected_ap_; }
+  void init_sta(size_t count);
   void add_sta(const WiFiAP &ap);
   void clear_sta();
 
@@ -393,7 +394,7 @@ class WiFiComponent : public Component {
 #endif
 
   std::string use_address_;
-  std::vector<WiFiAP> sta_;
+  FixedVector<WiFiAP> sta_;
   std::vector<WiFiSTAPriority> sta_priorities_;
   wifi_scan_vector_t<WiFiScanResult> scan_result_;
   WiFiAP selected_ap_;

esphome/core/helpers.h

@@ -194,12 +194,8 @@ template<typename T> class FixedVector {
     size_ = 0;
   }
 
- public:
-  FixedVector() = default;
-
-  /// Constructor from initializer list - allocates exact size needed
-  /// This enables brace initialization: FixedVector<int> v = {1, 2, 3};
-  FixedVector(std::initializer_list<T> init_list) {
+  // Helper to assign from initializer list (shared by constructor and assignment operator)
+  void assign_from_initializer_list_(std::initializer_list<T> init_list) {
     init(init_list.size());
     size_t idx = 0;
     for (const auto &item : init_list) {
@@ -209,6 +205,13 @@ template<typename T> class FixedVector {
     size_ = init_list.size();
   }
 
+ public:
+  FixedVector() = default;
+
+  /// Constructor from initializer list - allocates exact size needed
+  /// This enables brace initialization: FixedVector<int> v = {1, 2, 3};
+  FixedVector(std::initializer_list<T> init_list) { assign_from_initializer_list_(init_list); }
+
   ~FixedVector() { cleanup_(); }
 
   // Disable copy operations (avoid accidental expensive copies)
@@ -234,6 +237,15 @@ template<typename T> class FixedVector {
     return *this;
   }
 
+  /// Assignment from initializer list - avoids temporary and move overhead
+  /// This enables: FixedVector<int> v; v = {1, 2, 3};
+  FixedVector &operator=(std::initializer_list<T> init_list) {
+    cleanup_();
+    reset_();
+    assign_from_initializer_list_(init_list);
+    return *this;
+  }
+
   // Allocate capacity - can be called multiple times to reinit
   void init(size_t n) {
     cleanup_();