Improved Scheduler API (#696)

* Improved Scheduler API

- Added `Scheduler#builder(plugin)`
This method allows a more simplified builder while maintaining the main requirement of the executor plugin
- Added `Scheduler#taskByPlugin(plugin)`
Allows to obtain the tasks that a plugin has sent to execute and that are currently active
- Added `TaskBuilder#task(Consumer<SchuledTask>)`
Allows to specify a task with access to the task itself with the ability to cancel itself

* Applied requested changes

- Removed tasks builder method
- Added `Scheduler#buildTask(plugin, Consumer<ScheduledTask>)`

* Removed some unused imports

* Applied suggested change

* Fix possible test bug

* Applied more suggested changes

* Fixed tests inside tasks

api/src/main/java/com/velocitypowered/api/command/Command.java

@@ -7,11 +7,7 @@
 
 package com.velocitypowered.api.command;
 
-import com.google.common.collect.ImmutableList;
 import com.velocitypowered.api.proxy.Player;
-import java.util.List;
-import java.util.concurrent.CompletableFuture;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
 /**
  * Represents a command that can be executed by a {@link CommandSource}

api/src/main/java/com/velocitypowered/api/permission/PermissionSubject.java

@@ -8,7 +8,6 @@
 package com.velocitypowered.api.permission;
 
 import net.kyori.adventure.permission.PermissionChecker;
-import net.kyori.adventure.util.TriState;
 
 /**
  * Represents a object that has a set of queryable permissions.

api/src/main/java/com/velocitypowered/api/proxy/ProxyServer.java

@@ -23,7 +23,6 @@ import java.util.Collection;
 import java.util.Optional;
 import java.util.UUID;
 import net.kyori.adventure.audience.Audience;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
 /**
  * Provides an interface to a Minecraft server proxy.

api/src/main/java/com/velocitypowered/api/scheduler/ScheduledTask.java

@@ -7,6 +7,8 @@
 
 package com.velocitypowered.api.scheduler;
 
+import org.jetbrains.annotations.NotNull;
+
 /**
  * Represents a task that is scheduled to run on the proxy.
  */
@@ -17,7 +19,7 @@ public interface ScheduledTask {
    *
    * @return the plugin that scheduled this task
    */
-  Object plugin();
+  @NotNull Object plugin();
 
   /**
    * Returns the current status of this task.

api/src/main/java/com/velocitypowered/api/scheduler/Scheduler.java

@@ -8,8 +8,12 @@
 package com.velocitypowered.api.scheduler;
 
 import java.time.Duration;
+import java.util.Collection;
 import java.util.concurrent.TimeUnit;
+import java.util.function.Consumer;
+
 import org.checkerframework.common.value.qual.IntRange;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents a scheduler to execute tasks on the proxy.
@@ -23,7 +27,24 @@ public interface Scheduler {
    * @param runnable the task to run when scheduled
    * @return the task builder
    */
-  TaskBuilder buildTask(Object plugin, Runnable runnable);
+  TaskBuilder buildTask(@NotNull Object plugin, @NotNull Runnable runnable);
+
+  /**
+   * Initializes a new {@link TaskBuilder} for creating a task on the proxy.
+   *
+   * @param plugin the plugin to request the task for
+   * @param consumer the task to be run when scheduled with the capacity to cancel itself
+   * @return the task builder
+   */
+  TaskBuilder buildTask(@NotNull Object plugin, @NotNull Consumer<ScheduledTask> consumer);
+
+  /**
+   * Get the {@link ScheduledTask} for a specific plugin.
+   *
+   * @param plugin the plugin object
+   * @return the list of {@link ScheduledTask} corresponding to a specific plugin
+   */
+  @NotNull Collection<ScheduledTask> tasksByPlugin(@NotNull Object plugin);
 
   /**
    * Represents a fluent interface to schedule tasks on the proxy.
@@ -37,7 +58,7 @@ public interface Scheduler {
      * @param unit the unit of time for {@code time}
      * @return this builder, for chaining
      */
-    TaskBuilder delay(@IntRange(from = 0) long time, TimeUnit unit);
+    TaskBuilder delay(@IntRange(from = 0) long time, @NotNull TimeUnit unit);
 
     /**
      * Specifies that the task should delay its execution by the specified amount of time.
@@ -45,7 +66,7 @@ public interface Scheduler {
      * @param duration the duration of the delay
      * @return this builder, for chaining
      */
-    default TaskBuilder delay(Duration duration) {
+    default TaskBuilder delay(@NotNull Duration duration) {
       return delay(duration.toMillis(), TimeUnit.MILLISECONDS);
     }
 
@@ -57,7 +78,7 @@ public interface Scheduler {
      * @param unit the unit of time for {@code time}
      * @return this builder, for chaining
      */
-    TaskBuilder repeat(@IntRange(from = 0) long time, TimeUnit unit);
+    TaskBuilder repeat(@IntRange(from = 0) long time, @NotNull TimeUnit unit);
 
     /**
      * Specifies that the task should continue running after waiting for the specified amount, until
@@ -66,7 +87,7 @@ public interface Scheduler {
      * @param duration the duration of the delay
      * @return this builder, for chaining
      */
-    default TaskBuilder repeat(Duration duration) {
+    default TaskBuilder repeat(@NotNull Duration duration) {
       return repeat(duration.toMillis(), TimeUnit.MILLISECONDS);
     }