[mipi_spi] Honour show_test_card when pages or a lambda are set (#20807)

Co-authored-by: clydebarrow <2366188+clydebarrow@users.noreply.github.com>

esphome/components/display/display.cpp

@@ -843,7 +843,6 @@ constexpr uint8_t TESTCARD_FONT[3][8] PROGMEM = {{0x41, 0x7F, 0x7F, 0x09, 0x19,
 void Display::test_card() {
   int w = get_width(), h = get_height(), image_w, image_h;
   this->clear();
-  this->show_test_card_ = false;
   image_w = std::min(w - 20, 310);
   image_h = std::min(h - 20, 255);
   int shift_x = (w - image_w) / 2;

esphome/components/mipi_spi/mipi_spi.h

@@ -570,12 +570,12 @@ class MipiSpiBuffer
       if (this->auto_clear_enabled_) {
         this->clear();
       }
-      if (this->page_ != nullptr) {
+      if (this->show_test_card_) {
+        this->test_card();
+      } else if (this->page_ != nullptr) {
         this->page_->get_writer()(*this);
       } else if (this->writer_.has_value()) {
         (*this->writer_)(*this);
-      } else {
-        this->test_card();
       }
 #if ESPHOME_LOG_LEVEL == ESPHOME_LOG_LEVEL_VERBOSE
       esph_log_v(TAG, "Drawing from line %d took %dms", this->start_line_, millis() - lap);

tests/components/display/test_test_card.cpp

@@ -0,0 +1,80 @@
+#include <gtest/gtest.h>
+
+#include <algorithm>
+#include <cstdint>
+#include <new>
+#include <vector>
+
+#include "esphome/components/display/display.h"
+#include "esphome/core/application.h"
+
+namespace esphome::display::testing {
+
+// Records the colour of every pixel written, so a drawn test card shows up as non-black pixels.
+class TestCardDisplay : public Display {
+ public:
+  void draw_pixel_at(int x, int y, Color color) override { this->colors_.push_back(color.raw_32); }
+  void update() override { this->do_update_(); }
+  DisplayType get_display_type() override { return DisplayType::DISPLAY_TYPE_COLOR; }
+
+  bool drew_test_card() const {
+    return std::any_of(this->colors_.begin(), this->colors_.end(), [](uint32_t c) { return c != 0; });
+  }
+  void reset() { this->colors_.clear(); }
+
+ protected:
+  int get_width_internal() override { return 64; }
+  int get_height_internal() override { return 64; }
+  std::vector<uint32_t> colors_;
+};
+
+namespace {
+class TestCard : public ::testing::Test {
+ protected:
+  void SetUp() override {
+    // test_card() stops the poller, which needs the scheduler; the test main does not construct App.
+    static const bool APP_CONSTRUCTED = (new (&App) Application(), true);
+    (void) APP_CONSTRUCTED;
+  }
+  int writer_calls_{0};
+  display_writer_t count_writer_() {
+    return [this](Display &) { this->writer_calls_++; };
+  }
+};
+}  // namespace
+
+TEST_F(TestCard, DrawnOnEveryUpdateInsteadOfLambda) {
+  TestCardDisplay display;
+  display.set_writer(this->count_writer_());
+  display.show_test_card();
+  for (int i = 0; i != 3; i++) {
+    display.reset();
+    display.update();
+    EXPECT_TRUE(display.drew_test_card()) << "update " << i;
+  }
+  EXPECT_EQ(this->writer_calls_, 0);
+}
+
+TEST_F(TestCard, DrawnOnEveryUpdateInsteadOfPages) {
+  TestCardDisplay display;
+  DisplayPage page(this->count_writer_());
+  display.set_pages({&page});
+  display.show_test_card();
+  for (int i = 0; i != 3; i++) {
+    display.reset();
+    display.update();
+    EXPECT_TRUE(display.drew_test_card()) << "update " << i;
+  }
+  EXPECT_EQ(this->writer_calls_, 0);
+}
+
+TEST_F(TestCard, LambdaRunsWithoutTestCard) {
+  TestCardDisplay display;
+  display.set_writer(this->count_writer_());
+  display.update();
+  display.update();
+  EXPECT_FALSE(display.drew_test_card());
+  EXPECT_EQ(this->writer_calls_, 2);
+}
+
+}  // namespace esphome::display::testing