Rename Screens to Activities and restructure files (#44)

## Summary

* This PR drastically reshapes the structure of the codebase, moving
from the concept of "Screens" to "Activities", restructing the files and
setting up the concept of subactivities.
* This should help with keep the main file clean and containing all
functional logic in the relevant activity.
* CrossPointState is now also a global singleton which should help with
accessing it from within activities.

## Additional Context

* This is probably going to be a bit disruptive for people with open
PRs, sorry 😞

src/activities/reader/EpubReaderActivity.cpp

@@ -0,0 +1,395 @@
+#include "EpubReaderActivity.h"
+
+#include <Epub/Page.h>
+#include <GfxRenderer.h>
+#include <SD.h>
+
+#include "Battery.h"
+#include "CrossPointSettings.h"
+#include "EpubReaderChapterSelectionActivity.h"
+#include "config.h"
+
+namespace {
+constexpr int pagesPerRefresh = 15;
+constexpr unsigned long skipChapterMs = 700;
+constexpr float lineCompression = 0.95f;
+constexpr int marginTop = 8;
+constexpr int marginRight = 10;
+constexpr int marginBottom = 22;
+constexpr int marginLeft = 10;
+}  // namespace
+
+void EpubReaderActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<EpubReaderActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void EpubReaderActivity::onEnter() {
+  if (!epub) {
+    return;
+  }
+
+  renderingMutex = xSemaphoreCreateMutex();
+
+  epub->setupCacheDir();
+
+  if (SD.exists((epub->getCachePath() + "/progress.bin").c_str())) {
+    File f = SD.open((epub->getCachePath() + "/progress.bin").c_str());
+    uint8_t data[4];
+    f.read(data, 4);
+    currentSpineIndex = data[0] + (data[1] << 8);
+    nextPageNumber = data[2] + (data[3] << 8);
+    Serial.printf("[%lu] [ERS] Loaded cache: %d, %d\n", millis(), currentSpineIndex, nextPageNumber);
+    f.close();
+  }
+
+  // Trigger first update
+  updateRequired = true;
+
+  xTaskCreate(&EpubReaderActivity::taskTrampoline, "EpubReaderActivityTask",
+              8192,               // Stack size
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+}
+
+void EpubReaderActivity::onExit() {
+  // Wait until not rendering to delete task to avoid killing mid-instruction to EPD
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+  section.reset();
+  epub.reset();
+}
+
+void EpubReaderActivity::loop() {
+  // Pass input responsibility to sub activity if exists
+  if (subAcitivity) {
+    subAcitivity->loop();
+    return;
+  }
+
+  // Enter chapter selection activity
+  if (inputManager.wasPressed(InputManager::BTN_CONFIRM)) {
+    // Don't start activity transition while rendering
+    xSemaphoreTake(renderingMutex, portMAX_DELAY);
+    subAcitivity.reset(new EpubReaderChapterSelectionActivity(
+        this->renderer, this->inputManager, epub, currentSpineIndex,
+        [this] {
+          subAcitivity->onExit();
+          subAcitivity.reset();
+          updateRequired = true;
+        },
+        [this](const int newSpineIndex) {
+          if (currentSpineIndex != newSpineIndex) {
+            currentSpineIndex = newSpineIndex;
+            nextPageNumber = 0;
+            section.reset();
+          }
+          subAcitivity->onExit();
+          subAcitivity.reset();
+          updateRequired = true;
+        }));
+    subAcitivity->onEnter();
+    xSemaphoreGive(renderingMutex);
+  }
+
+  if (inputManager.wasPressed(InputManager::BTN_BACK)) {
+    onGoBack();
+    return;
+  }
+
+  const bool prevReleased =
+      inputManager.wasReleased(InputManager::BTN_UP) || inputManager.wasReleased(InputManager::BTN_LEFT);
+  const bool nextReleased =
+      inputManager.wasReleased(InputManager::BTN_DOWN) || inputManager.wasReleased(InputManager::BTN_RIGHT);
+
+  if (!prevReleased && !nextReleased) {
+    return;
+  }
+
+  // any botton press when at end of the book goes back to the last page
+  if (currentSpineIndex > 0 && currentSpineIndex >= epub->getSpineItemsCount()) {
+    currentSpineIndex = epub->getSpineItemsCount() - 1;
+    nextPageNumber = UINT16_MAX;
+    updateRequired = true;
+    return;
+  }
+
+  const bool skipChapter = inputManager.getHeldTime() > skipChapterMs;
+
+  if (skipChapter) {
+    // We don't want to delete the section mid-render, so grab the semaphore
+    xSemaphoreTake(renderingMutex, portMAX_DELAY);
+    nextPageNumber = 0;
+    currentSpineIndex = nextReleased ? currentSpineIndex + 1 : currentSpineIndex - 1;
+    section.reset();
+    xSemaphoreGive(renderingMutex);
+    updateRequired = true;
+    return;
+  }
+
+  // No current section, attempt to rerender the book
+  if (!section) {
+    updateRequired = true;
+    return;
+  }
+
+  if (prevReleased) {
+    if (section->currentPage > 0) {
+      section->currentPage--;
+    } else {
+      // We don't want to delete the section mid-render, so grab the semaphore
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      nextPageNumber = UINT16_MAX;
+      currentSpineIndex--;
+      section.reset();
+      xSemaphoreGive(renderingMutex);
+    }
+    updateRequired = true;
+  } else {
+    if (section->currentPage < section->pageCount - 1) {
+      section->currentPage++;
+    } else {
+      // We don't want to delete the section mid-render, so grab the semaphore
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      nextPageNumber = 0;
+      currentSpineIndex++;
+      section.reset();
+      xSemaphoreGive(renderingMutex);
+    }
+    updateRequired = true;
+  }
+}
+
+void EpubReaderActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      renderScreen();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+// TODO: Failure handling
+void EpubReaderActivity::renderScreen() {
+  if (!epub) {
+    return;
+  }
+
+  // edge case handling for sub-zero spine index
+  if (currentSpineIndex < 0) {
+    currentSpineIndex = 0;
+  }
+  // based bounds of book, show end of book screen
+  if (currentSpineIndex > epub->getSpineItemsCount()) {
+    currentSpineIndex = epub->getSpineItemsCount();
+  }
+
+  // Show end of book screen
+  if (currentSpineIndex == epub->getSpineItemsCount()) {
+    renderer.clearScreen();
+    renderer.drawCenteredText(READER_FONT_ID, 300, "End of book", true, BOLD);
+    renderer.displayBuffer();
+    return;
+  }
+
+  if (!section) {
+    const auto filepath = epub->getSpineItem(currentSpineIndex);
+    Serial.printf("[%lu] [ERS] Loading file: %s, index: %d\n", millis(), filepath.c_str(), currentSpineIndex);
+    section = std::unique_ptr<Section>(new Section(epub, currentSpineIndex, renderer));
+    if (!section->loadCacheMetadata(READER_FONT_ID, lineCompression, marginTop, marginRight, marginBottom, marginLeft,
+                                    SETTINGS.extraParagraphSpacing)) {
+      Serial.printf("[%lu] [ERS] Cache not found, building...\n", millis());
+
+      {
+        renderer.grayscaleRevert();
+
+        const int textWidth = renderer.getTextWidth(READER_FONT_ID, "Indexing...");
+        constexpr int margin = 20;
+        // Round all coordinates to 8 pixel boundaries
+        const int x = ((GfxRenderer::getScreenWidth() - textWidth - margin * 2) / 2 + 7) / 8 * 8;
+        constexpr int y = 56;
+        const int w = (textWidth + margin * 2 + 7) / 8 * 8;
+        const int h = (renderer.getLineHeight(READER_FONT_ID) + margin * 2 + 7) / 8 * 8;
+        renderer.fillRect(x, y, w, h, false);
+        renderer.drawText(READER_FONT_ID, x + margin, y + margin, "Indexing...");
+        renderer.drawRect(x + 5, y + 5, w - 10, h - 10);
+        // EXPERIMENTAL: Still suffers from ghosting
+        renderer.displayWindow(x, y, w, h);
+        pagesUntilFullRefresh = 0;
+      }
+
+      section->setupCacheDir();
+      if (!section->persistPageDataToSD(READER_FONT_ID, lineCompression, marginTop, marginRight, marginBottom,
+                                        marginLeft, SETTINGS.extraParagraphSpacing)) {
+        Serial.printf("[%lu] [ERS] Failed to persist page data to SD\n", millis());
+        section.reset();
+        return;
+      }
+    } else {
+      Serial.printf("[%lu] [ERS] Cache found, skipping build...\n", millis());
+    }
+
+    if (nextPageNumber == UINT16_MAX) {
+      section->currentPage = section->pageCount - 1;
+    } else {
+      section->currentPage = nextPageNumber;
+    }
+  }
+
+  renderer.clearScreen();
+
+  if (section->pageCount == 0) {
+    Serial.printf("[%lu] [ERS] No pages to render\n", millis());
+    renderer.drawCenteredText(READER_FONT_ID, 300, "Empty chapter", true, BOLD);
+    renderStatusBar();
+    renderer.displayBuffer();
+    return;
+  }
+
+  if (section->currentPage < 0 || section->currentPage >= section->pageCount) {
+    Serial.printf("[%lu] [ERS] Page out of bounds: %d (max %d)\n", millis(), section->currentPage, section->pageCount);
+    renderer.drawCenteredText(READER_FONT_ID, 300, "Out of bounds", true, BOLD);
+    renderStatusBar();
+    renderer.displayBuffer();
+    return;
+  }
+
+  {
+    auto p = section->loadPageFromSD();
+    if (!p) {
+      Serial.printf("[%lu] [ERS] Failed to load page from SD - clearing section cache\n", millis());
+      section->clearCache();
+      section.reset();
+      return renderScreen();
+    }
+    const auto start = millis();
+    renderContents(std::move(p));
+    Serial.printf("[%lu] [ERS] Rendered page in %dms\n", millis(), millis() - start);
+  }
+
+  File f = SD.open((epub->getCachePath() + "/progress.bin").c_str(), FILE_WRITE);
+  uint8_t data[4];
+  data[0] = currentSpineIndex & 0xFF;
+  data[1] = (currentSpineIndex >> 8) & 0xFF;
+  data[2] = section->currentPage & 0xFF;
+  data[3] = (section->currentPage >> 8) & 0xFF;
+  f.write(data, 4);
+  f.close();
+}
+
+void EpubReaderActivity::renderContents(std::unique_ptr<Page> page) {
+  page->render(renderer, READER_FONT_ID);
+  renderStatusBar();
+  if (pagesUntilFullRefresh <= 1) {
+    renderer.displayBuffer(EInkDisplay::HALF_REFRESH);
+    pagesUntilFullRefresh = pagesPerRefresh;
+  } else {
+    renderer.displayBuffer();
+    pagesUntilFullRefresh--;
+  }
+
+  // Save bw buffer to reset buffer state after grayscale data sync
+  renderer.storeBwBuffer();
+
+  // grayscale rendering
+  // TODO: Only do this if font supports it
+  {
+    renderer.clearScreen(0x00);
+    renderer.setRenderMode(GfxRenderer::GRAYSCALE_LSB);
+    page->render(renderer, READER_FONT_ID);
+    renderer.copyGrayscaleLsbBuffers();
+
+    // Render and copy to MSB buffer
+    renderer.clearScreen(0x00);
+    renderer.setRenderMode(GfxRenderer::GRAYSCALE_MSB);
+    page->render(renderer, READER_FONT_ID);
+    renderer.copyGrayscaleMsbBuffers();
+
+    // display grayscale part
+    renderer.displayGrayBuffer();
+    renderer.setRenderMode(GfxRenderer::BW);
+  }
+
+  // restore the bw data
+  renderer.restoreBwBuffer();
+}
+
+void EpubReaderActivity::renderStatusBar() const {
+  constexpr auto textY = 776;
+
+  // Calculate progress in book
+  float sectionChapterProg = static_cast<float>(section->currentPage) / section->pageCount;
+  uint8_t bookProgress = epub->calculateProgress(currentSpineIndex, sectionChapterProg);
+
+  // Right aligned text for progress counter
+  const std::string progress = std::to_string(section->currentPage + 1) + "/" + std::to_string(section->pageCount) +
+                               "  " + std::to_string(bookProgress) + "%";
+  const auto progressTextWidth = renderer.getTextWidth(SMALL_FONT_ID, progress.c_str());
+  renderer.drawText(SMALL_FONT_ID, GfxRenderer::getScreenWidth() - marginRight - progressTextWidth, textY,
+                    progress.c_str());
+
+  // Left aligned battery icon and percentage
+  const uint16_t percentage = battery.readPercentage();
+  const auto percentageText = std::to_string(percentage) + "%";
+  const auto percentageTextWidth = renderer.getTextWidth(SMALL_FONT_ID, percentageText.c_str());
+  renderer.drawText(SMALL_FONT_ID, 20 + marginLeft, textY, percentageText.c_str());
+
+  // 1 column on left, 2 columns on right, 5 columns of battery body
+  constexpr int batteryWidth = 15;
+  constexpr int batteryHeight = 10;
+  constexpr int x = marginLeft;
+  constexpr int y = 783;
+
+  // Top line
+  renderer.drawLine(x, y, x + batteryWidth - 4, y);
+  // Bottom line
+  renderer.drawLine(x, y + batteryHeight - 1, x + batteryWidth - 4, y + batteryHeight - 1);
+  // Left line
+  renderer.drawLine(x, y, x, y + batteryHeight - 1);
+  // Battery end
+  renderer.drawLine(x + batteryWidth - 4, y, x + batteryWidth - 4, y + batteryHeight - 1);
+  renderer.drawLine(x + batteryWidth - 3, y + 2, x + batteryWidth - 1, y + 2);
+  renderer.drawLine(x + batteryWidth - 3, y + batteryHeight - 3, x + batteryWidth - 1, y + batteryHeight - 3);
+  renderer.drawLine(x + batteryWidth - 1, y + 2, x + batteryWidth - 1, y + batteryHeight - 3);
+
+  // The +1 is to round up, so that we always fill at least one pixel
+  int filledWidth = percentage * (batteryWidth - 5) / 100 + 1;
+  if (filledWidth > batteryWidth - 5) {
+    filledWidth = batteryWidth - 5;  // Ensure we don't overflow
+  }
+  renderer.fillRect(x + 1, y + 1, filledWidth, batteryHeight - 2);
+
+  // Centered chatper title text
+  // Page width minus existing content with 30px padding on each side
+  const int titleMarginLeft = 20 + percentageTextWidth + 30 + marginLeft;
+  const int titleMarginRight = progressTextWidth + 30 + marginRight;
+  const int availableTextWidth = GfxRenderer::getScreenWidth() - titleMarginLeft - titleMarginRight;
+  const int tocIndex = epub->getTocIndexForSpineIndex(currentSpineIndex);
+
+  std::string title;
+  int titleWidth;
+  if (tocIndex == -1) {
+    title = "Unnamed";
+    titleWidth = renderer.getTextWidth(SMALL_FONT_ID, "Unnamed");
+  } else {
+    const auto tocItem = epub->getTocItem(tocIndex);
+    title = tocItem.title;
+    titleWidth = renderer.getTextWidth(SMALL_FONT_ID, title.c_str());
+    while (titleWidth > availableTextWidth) {
+      title = title.substr(0, title.length() - 8) + "...";
+      titleWidth = renderer.getTextWidth(SMALL_FONT_ID, title.c_str());
+    }
+  }
+
+  renderer.drawText(SMALL_FONT_ID, titleMarginLeft + (availableTextWidth - titleWidth) / 2, textY, title.c_str());
+}

src/activities/reader/EpubReaderActivity.h

@@ -0,0 +1,35 @@
+#pragma once
+#include <Epub.h>
+#include <Epub/Section.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include "../Activity.h"
+
+class EpubReaderActivity final : public Activity {
+  std::shared_ptr<Epub> epub;
+  std::unique_ptr<Section> section = nullptr;
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  std::unique_ptr<Activity> subAcitivity = nullptr;
+  int currentSpineIndex = 0;
+  int nextPageNumber = 0;
+  int pagesUntilFullRefresh = 0;
+  bool updateRequired = false;
+  const std::function<void()> onGoBack;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void renderScreen();
+  void renderContents(std::unique_ptr<Page> p);
+  void renderStatusBar() const;
+
+ public:
+  explicit EpubReaderActivity(GfxRenderer& renderer, InputManager& inputManager, std::unique_ptr<Epub> epub,
+                              const std::function<void()>& onGoBack)
+      : Activity(renderer, inputManager), epub(std::move(epub)), onGoBack(onGoBack) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+};

src/activities/reader/EpubReaderChapterSelectionActivity.cpp

@@ -0,0 +1,107 @@
+#include "EpubReaderChapterSelectionActivity.h"
+
+#include <GfxRenderer.h>
+#include <SD.h>
+
+#include "config.h"
+
+constexpr int PAGE_ITEMS = 24;
+constexpr int SKIP_PAGE_MS = 700;
+
+void EpubReaderChapterSelectionActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<EpubReaderChapterSelectionActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void EpubReaderChapterSelectionActivity::onEnter() {
+  if (!epub) {
+    return;
+  }
+
+  renderingMutex = xSemaphoreCreateMutex();
+  selectorIndex = currentSpineIndex;
+
+  // Trigger first update
+  updateRequired = true;
+  xTaskCreate(&EpubReaderChapterSelectionActivity::taskTrampoline, "EpubReaderChapterSelectionActivityTask",
+              2048,               // Stack size
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+}
+
+void EpubReaderChapterSelectionActivity::onExit() {
+  // Wait until not rendering to delete task to avoid killing mid-instruction to EPD
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+}
+
+void EpubReaderChapterSelectionActivity::loop() {
+  const bool prevReleased =
+      inputManager.wasReleased(InputManager::BTN_UP) || inputManager.wasReleased(InputManager::BTN_LEFT);
+  const bool nextReleased =
+      inputManager.wasReleased(InputManager::BTN_DOWN) || inputManager.wasReleased(InputManager::BTN_RIGHT);
+
+  const bool skipPage = inputManager.getHeldTime() > SKIP_PAGE_MS;
+
+  if (inputManager.wasPressed(InputManager::BTN_CONFIRM)) {
+    onSelectSpineIndex(selectorIndex);
+  } else if (inputManager.wasPressed(InputManager::BTN_BACK)) {
+    onGoBack();
+  } else if (prevReleased) {
+    if (skipPage) {
+      selectorIndex =
+          ((selectorIndex / PAGE_ITEMS - 1) * PAGE_ITEMS + epub->getSpineItemsCount()) % epub->getSpineItemsCount();
+    } else {
+      selectorIndex = (selectorIndex + epub->getSpineItemsCount() - 1) % epub->getSpineItemsCount();
+    }
+    updateRequired = true;
+  } else if (nextReleased) {
+    if (skipPage) {
+      selectorIndex = ((selectorIndex / PAGE_ITEMS + 1) * PAGE_ITEMS) % epub->getSpineItemsCount();
+    } else {
+      selectorIndex = (selectorIndex + 1) % epub->getSpineItemsCount();
+    }
+    updateRequired = true;
+  }
+}
+
+void EpubReaderChapterSelectionActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      renderScreen();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void EpubReaderChapterSelectionActivity::renderScreen() {
+  renderer.clearScreen();
+
+  const auto pageWidth = renderer.getScreenWidth();
+  renderer.drawCenteredText(READER_FONT_ID, 10, "Select Chapter", true, BOLD);
+
+  const auto pageStartIndex = selectorIndex / PAGE_ITEMS * PAGE_ITEMS;
+  renderer.fillRect(0, 60 + (selectorIndex % PAGE_ITEMS) * 30 + 2, pageWidth - 1, 30);
+  for (int i = pageStartIndex; i < epub->getSpineItemsCount() && i < pageStartIndex + PAGE_ITEMS; i++) {
+    const int tocIndex = epub->getTocIndexForSpineIndex(i);
+    if (tocIndex == -1) {
+      renderer.drawText(UI_FONT_ID, 20, 60 + (i % PAGE_ITEMS) * 30, "Unnamed", i != selectorIndex);
+    } else {
+      auto item = epub->getTocItem(tocIndex);
+      renderer.drawText(UI_FONT_ID, 20 + (item.level - 1) * 15, 60 + (i % PAGE_ITEMS) * 30, item.title.c_str(),
+                        i != selectorIndex);
+    }
+  }
+
+  renderer.displayBuffer();
+}

src/activities/reader/EpubReaderChapterSelectionActivity.h

@@ -0,0 +1,38 @@
+#pragma once
+#include <Epub.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <memory>
+
+#include "../Activity.h"
+
+class EpubReaderChapterSelectionActivity final : public Activity {
+  std::shared_ptr<Epub> epub;
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  int currentSpineIndex = 0;
+  int selectorIndex = 0;
+  bool updateRequired = false;
+  const std::function<void()> onGoBack;
+  const std::function<void(int newSpineIndex)> onSelectSpineIndex;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void renderScreen();
+
+ public:
+  explicit EpubReaderChapterSelectionActivity(GfxRenderer& renderer, InputManager& inputManager,
+                                              const std::shared_ptr<Epub>& epub, const int currentSpineIndex,
+                                              const std::function<void()>& onGoBack,
+                                              const std::function<void(int newSpineIndex)>& onSelectSpineIndex)
+      : Activity(renderer, inputManager),
+        epub(epub),
+        currentSpineIndex(currentSpineIndex),
+        onGoBack(onGoBack),
+        onSelectSpineIndex(onSelectSpineIndex) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+};

src/activities/reader/FileSelectionActivity.cpp

@@ -0,0 +1,147 @@
+#include "FileSelectionActivity.h"
+
+#include <GfxRenderer.h>
+#include <SD.h>
+
+#include "config.h"
+
+void sortFileList(std::vector<std::string>& strs) {
+  std::sort(begin(strs), end(strs), [](const std::string& str1, const std::string& str2) {
+    if (str1.back() == '/' && str2.back() != '/') return true;
+    if (str1.back() != '/' && str2.back() == '/') return false;
+    return lexicographical_compare(
+        begin(str1), end(str1), begin(str2), end(str2),
+        [](const char& char1, const char& char2) { return tolower(char1) < tolower(char2); });
+  });
+}
+
+void FileSelectionActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<FileSelectionActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void FileSelectionActivity::loadFiles() {
+  files.clear();
+  selectorIndex = 0;
+  auto root = SD.open(basepath.c_str());
+  for (File file = root.openNextFile(); file; file = root.openNextFile()) {
+    auto filename = std::string(file.name());
+    if (filename[0] == '.') {
+      file.close();
+      continue;
+    }
+
+    if (file.isDirectory()) {
+      files.emplace_back(filename + "/");
+    } else if (filename.substr(filename.length() - 5) == ".epub") {
+      files.emplace_back(filename);
+    }
+    file.close();
+  }
+  root.close();
+  sortFileList(files);
+}
+
+void FileSelectionActivity::onEnter() {
+  renderingMutex = xSemaphoreCreateMutex();
+
+  basepath = "/";
+  loadFiles();
+  selectorIndex = 0;
+
+  // Trigger first update
+  updateRequired = true;
+
+  xTaskCreate(&FileSelectionActivity::taskTrampoline, "FileSelectionActivityTask",
+              2048,               // Stack size
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+}
+
+void FileSelectionActivity::onExit() {
+  // Wait until not rendering to delete task to avoid killing mid-instruction to EPD
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+  files.clear();
+}
+
+void FileSelectionActivity::loop() {
+  const bool prevPressed =
+      inputManager.wasPressed(InputManager::BTN_UP) || inputManager.wasPressed(InputManager::BTN_LEFT);
+  const bool nextPressed =
+      inputManager.wasPressed(InputManager::BTN_DOWN) || inputManager.wasPressed(InputManager::BTN_RIGHT);
+
+  if (inputManager.wasPressed(InputManager::BTN_CONFIRM)) {
+    if (files.empty()) {
+      return;
+    }
+
+    if (basepath.back() != '/') basepath += "/";
+    if (files[selectorIndex].back() == '/') {
+      basepath += files[selectorIndex].substr(0, files[selectorIndex].length() - 1);
+      loadFiles();
+      updateRequired = true;
+    } else {
+      onSelect(basepath + files[selectorIndex]);
+    }
+  } else if (inputManager.wasPressed(InputManager::BTN_BACK)) {
+    if (basepath != "/") {
+      basepath = basepath.substr(0, basepath.rfind('/'));
+      if (basepath.empty()) basepath = "/";
+      loadFiles();
+      updateRequired = true;
+    } else {
+      // At root level, go back home
+      onGoHome();
+    }
+  } else if (prevPressed) {
+    selectorIndex = (selectorIndex + files.size() - 1) % files.size();
+    updateRequired = true;
+  } else if (nextPressed) {
+    selectorIndex = (selectorIndex + 1) % files.size();
+    updateRequired = true;
+  }
+}
+
+void FileSelectionActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      render();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void FileSelectionActivity::render() const {
+  renderer.clearScreen();
+
+  const auto pageWidth = GfxRenderer::getScreenWidth();
+  renderer.drawCenteredText(READER_FONT_ID, 10, "CrossPoint Reader", true, BOLD);
+
+  // Help text
+  renderer.drawText(SMALL_FONT_ID, 20, GfxRenderer::getScreenHeight() - 30, "Press BACK for Home");
+
+  if (files.empty()) {
+    renderer.drawText(UI_FONT_ID, 20, 60, "No EPUBs found");
+  } else {
+    // Draw selection
+    renderer.fillRect(0, 60 + selectorIndex * 30 + 2, pageWidth - 1, 30);
+
+    for (size_t i = 0; i < files.size(); i++) {
+      const auto file = files[i];
+      renderer.drawText(UI_FONT_ID, 20, 60 + i * 30, file.c_str(), i != selectorIndex);
+    }
+  }
+
+  renderer.displayBuffer();
+}

src/activities/reader/FileSelectionActivity.h

@@ -0,0 +1,35 @@
+#pragma once
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "../Activity.h"
+
+class FileSelectionActivity final : public Activity {
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  std::string basepath = "/";
+  std::vector<std::string> files;
+  int selectorIndex = 0;
+  bool updateRequired = false;
+  const std::function<void(const std::string&)> onSelect;
+  const std::function<void()> onGoHome;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void render() const;
+  void loadFiles();
+
+ public:
+  explicit FileSelectionActivity(GfxRenderer& renderer, InputManager& inputManager,
+                                 const std::function<void(const std::string&)>& onSelect,
+                                 const std::function<void()>& onGoHome)
+      : Activity(renderer, inputManager), onSelect(onSelect), onGoHome(onGoHome) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+};

src/activities/reader/ReaderActivity.cpp

@@ -0,0 +1,68 @@
+#include "ReaderActivity.h"
+
+#include <SD.h>
+
+#include "CrossPointState.h"
+#include "Epub.h"
+#include "EpubReaderActivity.h"
+#include "FileSelectionActivity.h"
+#include "activities/util/FullScreenMessageActivity.h"
+
+std::unique_ptr<Epub> ReaderActivity::loadEpub(const std::string& path) {
+  if (!SD.exists(path.c_str())) {
+    Serial.printf("[%lu] [   ] File does not exist: %s\n", millis(), path.c_str());
+    return nullptr;
+  }
+
+  auto epub = std::unique_ptr<Epub>(new Epub(path, "/.crosspoint"));
+  if (epub->load()) {
+    return epub;
+  }
+
+  Serial.printf("[%lu] [   ] Failed to load epub\n", millis());
+  return nullptr;
+}
+
+void ReaderActivity::onSelectEpubFile(const std::string& path) {
+  exitActivity();
+  enterNewActivity(new FullScreenMessageActivity(renderer, inputManager, "Loading..."));
+
+  auto epub = loadEpub(path);
+  if (epub) {
+    APP_STATE.openEpubPath = path;
+    APP_STATE.saveToFile();
+    onGoToEpubReader(std::move(epub));
+  } else {
+    exitActivity();
+    enterNewActivity(new FullScreenMessageActivity(renderer, inputManager, "Failed to load epub", REGULAR,
+                                                   EInkDisplay::HALF_REFRESH));
+    delay(2000);
+    onGoToFileSelection();
+  }
+}
+
+void ReaderActivity::onGoToFileSelection() {
+  exitActivity();
+  enterNewActivity(new FileSelectionActivity(
+      renderer, inputManager, [this](const std::string& path) { onSelectEpubFile(path); }, onGoBack));
+}
+
+void ReaderActivity::onGoToEpubReader(std::unique_ptr<Epub> epub) {
+  exitActivity();
+  enterNewActivity(new EpubReaderActivity(renderer, inputManager, std::move(epub), [this] { onGoToFileSelection(); }));
+}
+
+void ReaderActivity::onEnter() {
+  if (initialEpubPath.empty()) {
+    onGoToFileSelection();
+    return;
+  }
+
+  auto epub = loadEpub(initialEpubPath);
+  if (!epub) {
+    onGoBack();
+    return;
+  }
+
+  onGoToEpubReader(std::move(epub));
+}

src/activities/reader/ReaderActivity.h

@@ -0,0 +1,24 @@
+#pragma once
+#include <memory>
+
+#include "../ActivityWithSubactivity.h"
+
+class Epub;
+
+class ReaderActivity final : public ActivityWithSubactivity {
+  std::string initialEpubPath;
+  const std::function<void()> onGoBack;
+  static std::unique_ptr<Epub> loadEpub(const std::string& path);
+
+  void onSelectEpubFile(const std::string& path);
+  void onGoToFileSelection();
+  void onGoToEpubReader(std::unique_ptr<Epub> epub);
+
+ public:
+  explicit ReaderActivity(GfxRenderer& renderer, InputManager& inputManager, std::string initialEpubPath,
+                          const std::function<void()>& onGoBack)
+      : ActivityWithSubactivity(renderer, inputManager),
+        initialEpubPath(std::move(initialEpubPath)),
+        onGoBack(onGoBack) {}
+  void onEnter() override;
+};