rewrote discovery and registration code to be composable and higher level

src/main/java/me/clip/placeholderapi/util/Futures.java

@@ -0,0 +1,63 @@
+package me.clip.placeholderapi.util;
+
+import org.bukkit.Bukkit;
+import org.bukkit.plugin.Plugin;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Collection;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.function.BiConsumer;
+import java.util.stream.Collector;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+public final class Futures
+{
+
+	private Futures()
+	{}
+
+
+	public static <T> void onMainThread(@NotNull final Plugin plugin, @NotNull final CompletableFuture<T> future, @NotNull final BiConsumer<T, Throwable> consumer)
+	{
+		future.whenComplete((value, exception) -> {
+			if (Bukkit.isPrimaryThread())
+			{
+				consumer.accept(value, exception);
+			}
+			else
+			{
+				Bukkit.getScheduler().runTask(plugin, () -> consumer.accept(value, exception));
+			}
+		});
+	}
+
+
+	@NotNull
+	public static <T> Collector<CompletableFuture<T>, ?, CompletableFuture<List<T>>> collector()
+	{
+		return Collectors.collectingAndThen(Collectors.toList(), Futures::of);
+	}
+
+
+	@NotNull
+	public static <T> CompletableFuture<List<T>> of(@NotNull final Stream<CompletableFuture<T>> futures)
+	{
+		return of(futures.collect(Collectors.toList()));
+	}
+
+	@NotNull
+	public static <T> CompletableFuture<List<T>> of(@NotNull final Collection<CompletableFuture<T>> futures)
+	{
+		return CompletableFuture.allOf(futures.toArray(new CompletableFuture[0]))
+								.thenApplyAsync($ -> awaitCompletion(futures));
+	}
+
+	@NotNull
+	private static <T> List<T> awaitCompletion(@NotNull final Collection<CompletableFuture<T>> futures)
+	{
+		return futures.stream().map(CompletableFuture::join).collect(Collectors.toList());
+	}
+
+}