Add the on_page_change display trigger (#1687)

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

esphome/components/display/display_buffer.h

@@ -81,6 +81,7 @@ class Font;
 class Image;
 class DisplayBuffer;
 class DisplayPage;
+class DisplayOnPageChangeTrigger;
 
 using display_writer_t = std::function<void(DisplayBuffer &)>;
 
@@ -298,6 +299,8 @@ class DisplayBuffer {
 
   const DisplayPage *get_active_page() const { return this->page_; }
 
+  void add_on_page_change_trigger(DisplayOnPageChangeTrigger *t) { this->on_page_change_triggers_.push_back(t); }
+
   /// Internal method to set the display rotation with.
   void set_rotation(DisplayRotation rotation);
 
@@ -318,6 +321,8 @@ class DisplayBuffer {
   DisplayRotation rotation_{DISPLAY_ROTATION_0_DEGREES};
   optional<display_writer_t> writer_{};
   DisplayPage *page_{nullptr};
+  DisplayPage *previous_page_{nullptr};
+  std::vector<DisplayOnPageChangeTrigger *> on_page_change_triggers_;
 };
 
 class DisplayPage {
@@ -465,5 +470,17 @@ template<typename... Ts> class DisplayIsDisplayingPageCondition : public Conditi
   DisplayPage *page_;
 };
 
+class DisplayOnPageChangeTrigger : public Trigger<DisplayPage *, DisplayPage *> {
+ public:
+  explicit DisplayOnPageChangeTrigger(DisplayBuffer *parent) { parent->add_on_page_change_trigger(this); }
+  void process(DisplayPage *from, DisplayPage *to);
+  void set_from(DisplayPage *p) { this->from_ = p; }
+  void set_to(DisplayPage *p) { this->to_ = p; }
+
+ protected:
+  DisplayPage *from_{nullptr};
+  DisplayPage *to_{nullptr};
+};
+
 }  // namespace display
 }  // namespace esphome