First working version.

launcher/src/main/java/nl/andrewl/aos2_launcher/MainViewController.java

@@ -118,12 +118,10 @@ public class MainViewController {
 					throwable.printStackTrace();
 					return new ArrayList<>();
 				})
-				.thenAccept(newServers -> {
+				.thenAccept(newServers -> Platform.runLater(() -> {
-					Platform.runLater(() -> {
 					this.servers.clear();
 					this.servers.addAll(newServers);
-					});
+				}));
-				});
 	}
 
 	@FXML
@@ -150,7 +148,20 @@ public class MainViewController {
 
 	@FXML
 	public void play() {
-
+		Profile profile = this.selectedProfile.get();
+		Server server = this.selectedServer.get();
+		VersionFetcher.INSTANCE.ensureVersionIsDownloaded(profile.getClientVersion())
+				.thenAccept(path -> {
+					try {
+						Process p = new ProcessBuilder()
+								.command("java", "-jar", path.toAbsolutePath().toString())
+								.directory(Launcher.BASE_DIR.toFile())
+								.inheritIO()
+								.start();
+					} catch (IOException e) {
+						e.printStackTrace();
+					}
+				});
 	}
 
 	private void selectProfile(ProfileView view) {

launcher/src/main/java/nl/andrewl/aos2_launcher/SystemVersionValidator.java

@@ -0,0 +1,35 @@
+package nl.andrewl.aos2_launcher;
+
+public class SystemVersionValidator {
+	private static final String os = System.getProperty("os.name").trim().toLowerCase();
+	private static final String arch = System.getProperty("os.arch").trim().toLowerCase();
+
+	private static final boolean OS_WINDOWS = os.contains("win");
+	private static final boolean OS_MAC = os.contains("mac");
+	private static final boolean OS_LINUX = os.contains("nix") || os.contains("nux") || os.contains("aix");
+
+	private static final boolean ARCH_X86 = arch.equals("x86");
+	private static final boolean ARCH_X86_64 = arch.equals("x86_64");
+	private static final boolean ARCH_AMD64 = arch.equals("amd64");
+	private static final boolean ARCH_AARCH64 = arch.equals("aarch64");
+	private static final boolean ARCH_ARM = arch.equals("arm");
+	private static final boolean ARCH_ARM32 = arch.equals("arm32");
+
+	public static String getPreferredVersionSuffix() {
+		if (OS_LINUX) {
+			if (ARCH_AARCH64) return "linux-aarch64";
+			if (ARCH_AMD64) return "linux-amd64";
+			if (ARCH_ARM) return "linux-arm";
+			if (ARCH_ARM32) return "linux-arm32";
+		} else if (OS_MAC) {
+			if (ARCH_AARCH64) return "macos-aarch64";
+			if (ARCH_X86_64) return "macos-x86_64";
+		} else if (OS_WINDOWS) {
+			if (ARCH_AARCH64) return "windows-aarch64";
+			if (ARCH_AMD64) return "windows-amd64";
+			if (ARCH_X86) return "windows-x86";
+		}
+		System.err.println("Couldn't determine the preferred OS/ARCH version. Defaulting to windows-amd64.");
+		return "windows-amd64";
+	}
+}

launcher/src/main/java/nl/andrewl/aos2_launcher/VersionFetcher.java

@@ -0,0 +1,168 @@
+package nl.andrewl.aos2_launcher;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonObject;
+import nl.andrewl.aos2_launcher.model.ClientVersionRelease;
+
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.time.Duration;
+import java.time.LocalDateTime;
+import java.time.OffsetDateTime;
+import java.time.ZoneId;
+import java.time.format.DateTimeFormatter;
+import java.util.*;
+import java.util.concurrent.CompletableFuture;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+public class VersionFetcher {
+	private static final String BASE_GITHUB_URL = "https://api.github.com/repos/andrewlalis/ace-of-shades-2";
+
+	public static final VersionFetcher INSTANCE = new VersionFetcher();
+
+	private final List<ClientVersionRelease> availableReleases;
+
+	private final HttpClient httpClient = HttpClient.newBuilder().followRedirects(HttpClient.Redirect.NORMAL).build();
+	private boolean loaded = false;
+	private CompletableFuture<List<ClientVersionRelease>> activeReleaseFetchFuture;
+
+	public VersionFetcher() {
+		this.availableReleases = new ArrayList<>();
+		System.out.println(System.getProperty("os.name"));
+		System.out.println(System.getProperty("os.arch"));
+	}
+
+	public CompletableFuture<ClientVersionRelease> getRelease(String versionTag) {
+		return getAvailableReleases().thenApply(releases -> releases.stream()
+				.filter(r -> r.tag().equals(versionTag))
+				.findFirst().orElse(null));
+	}
+
+	public CompletableFuture<List<ClientVersionRelease>> getAvailableReleases() {
+		if (loaded) {
+			return CompletableFuture.completedFuture(Collections.unmodifiableList(availableReleases));
+		}
+
+		System.out.println("Fetching the list of available releases...");
+		return fetchReleasesFromGitHub();
+	}
+
+	private CompletableFuture<List<ClientVersionRelease>> fetchReleasesFromGitHub() {
+		if (activeReleaseFetchFuture != null) return activeReleaseFetchFuture;
+		HttpRequest req = HttpRequest.newBuilder(URI.create(BASE_GITHUB_URL + "/releases"))
+				.timeout(Duration.ofSeconds(3))
+				.GET()
+				.build();
+		activeReleaseFetchFuture = httpClient.sendAsync(req, HttpResponse.BodyHandlers.ofInputStream())
+				.thenApplyAsync(resp -> {
+					if (resp.statusCode() == 200) {
+						JsonArray releasesArray = new Gson().fromJson(new InputStreamReader(resp.body()), JsonArray.class);
+						availableReleases.clear();
+						for (var element : releasesArray) {
+							if (element.isJsonObject()) {
+								JsonObject obj = element.getAsJsonObject();
+								String tag = obj.get("tag_name").getAsString();
+								String apiUrl = obj.get("url").getAsString();
+								String assetsUrl = obj.get("assets_url").getAsString();
+								OffsetDateTime publishedAt = OffsetDateTime.parse(obj.get("published_at").getAsString(), DateTimeFormatter.ISO_OFFSET_DATE_TIME);
+								LocalDateTime localPublishedAt = publishedAt.atZoneSameInstant(ZoneId.systemDefault()).toLocalDateTime();
+								availableReleases.add(new ClientVersionRelease(tag, apiUrl, assetsUrl, localPublishedAt));
+							}
+						}
+						availableReleases.sort(Comparator.comparing(ClientVersionRelease::publishedAt).reversed());
+						loaded = true;
+						return availableReleases;
+					} else {
+						throw new RuntimeException("Error while requesting releases.");
+					}
+				});
+		return activeReleaseFetchFuture;
+	}
+
+	public List<String> getDownloadedVersions() {
+		try (var s = Files.list(Launcher.VERSIONS_DIR)) {
+			return s.filter(this::isVersionFile)
+					.map(this::extractVersion)
+					.toList();
+		} catch (IOException e) {
+			e.printStackTrace();
+			return Collections.emptyList();
+		}
+	}
+
+	public CompletableFuture<Path> ensureVersionIsDownloaded(String versionTag) {
+		try (var s = Files.list(Launcher.VERSIONS_DIR)) {
+			Optional<Path> optionalFile = s.filter(f -> isVersionFile(f) && versionTag.equals(extractVersion(f)))
+					.findFirst();
+			if (optionalFile.isPresent()) return CompletableFuture.completedFuture(optionalFile.get());
+		} catch (IOException e) {
+			return CompletableFuture.failedFuture(e);
+		}
+		return getRelease(versionTag)
+				.thenComposeAsync(this::downloadVersion);
+	}
+
+	private CompletableFuture<Path> downloadVersion(ClientVersionRelease release) {
+		System.out.println("Downloading version " + release.tag());
+		HttpRequest req = HttpRequest.newBuilder(URI.create(release.assetsUrl()))
+				.GET().timeout(Duration.ofSeconds(3)).build();
+		CompletableFuture<JsonObject> downloadUrlFuture = httpClient.sendAsync(req, HttpResponse.BodyHandlers.ofInputStream())
+				.thenApplyAsync(resp -> {
+					if (resp.statusCode() == 200) {
+						JsonArray assetsArray = new Gson().fromJson(new InputStreamReader(resp.body()), JsonArray.class);
+						String preferredVersionSuffix = SystemVersionValidator.getPreferredVersionSuffix();
+						String regex = "aos2-client-\\d+\\.\\d+\\.\\d+-" + preferredVersionSuffix + "\\.jar";
+						for (var asset : assetsArray) {
+							JsonObject assetObj = asset.getAsJsonObject();
+							String name = assetObj.get("name").getAsString();
+							if (name.matches(regex)) {
+								System.out.println("Found matching asset: " + name);
+								return assetObj;
+							}
+						}
+						throw new RuntimeException("Couldn't find a matching release asset.");
+					} else {
+						throw new RuntimeException("Error while requesting release assets.");
+					}
+				});
+		return downloadUrlFuture.thenComposeAsync(asset -> {
+			String url = asset.get("browser_download_url").getAsString();
+			String fileName = asset.get("name").getAsString();
+			HttpRequest downloadRequest = HttpRequest.newBuilder(URI.create(url))
+					.GET().timeout(Duration.ofMinutes(5)).build();
+			Path file = Launcher.VERSIONS_DIR.resolve(fileName);
+			System.out.printf("Downloading %s to %s.%n", fileName, file.toAbsolutePath());
+			return httpClient.sendAsync(downloadRequest, HttpResponse.BodyHandlers.ofFile(file))
+					.thenApplyAsync(resp -> {
+						System.out.println(resp);
+						return resp.body();
+					});
+		});
+	}
+
+	private boolean isVersionDownloaded(String versionTag) {
+		return getDownloadedVersions().contains(versionTag);
+	}
+
+	private boolean isVersionFile(Path p) {
+		return Files.isRegularFile(p) && p.getFileName().toString()
+				.matches("aos2-client-\\d+\\.\\d+\\.\\d+-.+\\.jar");
+	}
+
+	private String extractVersion(Path file) {
+		Pattern pattern = Pattern.compile("\\d+\\.\\d+\\.\\d+");
+		Matcher matcher = pattern.matcher(file.getFileName().toString());
+		if (matcher.find()) {
+			return "v" + matcher.group();
+		}
+		throw new IllegalArgumentException("File doesn't contain a valid version pattern.");
+	}
+}

launcher/src/main/java/nl/andrewl/aos2_launcher/model/ClientVersionRelease.java

@@ -0,0 +1,11 @@
+package nl.andrewl.aos2_launcher.model;
+
+import java.time.LocalDateTime;
+import java.time.ZonedDateTime;
+
+public record ClientVersionRelease (
+		String tag,
+		String apiUrl,
+		String assetsUrl,
+		LocalDateTime publishedAt
+) {}

launcher/src/main/java/nl/andrewl/aos2_launcher/view/EditProfileDialog.java

@@ -11,6 +11,8 @@ import javafx.scene.Parent;
 import javafx.scene.control.*;
 import javafx.stage.Modality;
 import javafx.stage.Window;
+import nl.andrewl.aos2_launcher.VersionFetcher;
+import nl.andrewl.aos2_launcher.model.ClientVersionRelease;
 import nl.andrewl.aos2_launcher.model.Profile;
 
 import java.io.IOException;
@@ -41,8 +43,15 @@ public class EditProfileDialog extends Dialog<Profile> {
 					.or(clientVersionChoiceBox.valueProperty().isNull());
 			nameField.setText(profile.getName());
 			descriptionTextArea.setText(profile.getDescription());
-			clientVersionChoiceBox.setItems(FXCollections.observableArrayList("v1.2.0", "v1.3.0", "v1.4.0"));
+			VersionFetcher.INSTANCE.getAvailableReleases().thenAccept(releases -> {
-			clientVersionChoiceBox.setValue(profile.getClientVersion());
+				Platform.runLater(() -> {
+					clientVersionChoiceBox.setItems(FXCollections.observableArrayList(releases.stream().map(ClientVersionRelease::tag).toList()));
+					String lastRelease = releases.size() == 0 ? null : releases.get(0).tag();
+					if (lastRelease != null) {
+						clientVersionChoiceBox.setValue(lastRelease);
+					}
+				});
+			});
 
 			DialogPane pane = new DialogPane();
 			pane.setContent(parent);