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Store Ordered Values in Topics

The Topic API provides the CollaborationList data structure to store ordered values and subscribe to their changes.

A CollaborationList can be obtained by its name in the Topic connection activation callback.

// NOTE: In a real application, use the user id of the logged in user
// instead
String userId = System.identityHashCode(UI.getCurrent()) + "";
UserInfo localUser = new UserInfo(userId, "User " + userId);
CollaborationEngine.getInstance().openTopicConnection(this, "my-topic",
        localUser, connection -> {
            CollaborationList list = connection.getNamedList("my-list");
            return null;
        });

When you have obtained the list instance, you can insert values at the end of the list with the insertLast() method.

ListOperationResult<Void> result = list.insertLast("foo");

Since the insertion is asynchronous, the method returns a result object from which you can get a CompletableFuture that you can use to determine when the insertion has been done.

ListOperationResult<Void> result = list.insertLast("foo");
result.getCompletableFuture().thenAccept(v -> {
    // do something when the insertion completes
});

The result object also provides the ListKey of the inserted item, which you can use to replace the item itself, get its current value or remove it from the list.

ListOperationResult<Void> result = list.insertLast("foo");
ListKey key = result.getKey();

// Replace an item using its key
list.set(key, "bar");

// Get an item value using its key
String bar = list.getItem(key, String.class);


// Remove an item using its key
list.remove(key);

Advanced List Operations

The ListOperation class allows you to prepare a list operation that can then be applied to the list using the CollaborationList::apply method.

The reason you may want to use this class is to define certain conditions that must be met when the operation is attempted. If a condition is not met, the operation will not be completed. This is useful to protect against duplicate operations.

Static methods of ListOperation

  • insertFirst() Value will be inserted at the beginning of the list.

  • insertLast() Value will be inserted at the end of the list.

  • insertBefore() Value will be inserted immediately before a specified key.

  • insertAfter() Value will be inserted immediately after a specified key.

  • insertBetween() Shorthand for insertAfter(value, keyBefore).ifNext(keyBefore, keyAfter).

Conditional methods of ListOperation

  • ifFirst(key) The specified key must be the first in the list.

  • ifLast(key) The specified key must be the last in the list.

  • ifPrev(key, keyPrev) The specified keyPrev must be immediately before key.

  • ifNext(key, keyNext) The specified keyNext must be immediately after key.

  • ifEmpty() The list must have no entries.

  • ifNotEmpty() The list must have at least one entry.

// Insert an item at the end of the list
// Only succeeds if key is currently last
ListOperation operation1 = ListOperation
        .insertLast("baz")
        .ifLast(key);
ListOperationResult<Boolean> result1 =
        list.apply(operation1);
ListKey key1 = result1.getKey();

// Insert an item before key1
// Only succeeds if key1 is currently last
ListOperation operation2 = ListOperation
        .insertBefore(key1, "qux")
        .ifLast(key1);
ListOperationResult<Boolean> result2 =
        list.apply(operation2);
ListKey key2 = result2.getKey();

// Insert an item between two keys
// Only succeeds if the keys are consecutive
ListOperation operation3 = ListOperation
        .insertBetween(key2, key1, "xyz");
ListOperationResult<Boolean> result3 =
        list.apply(operation3);

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