Cursors

Cursors are a concept that appears in many database systems and are used to iterate over and access data in a controlled manner. A cursor in SignalDB is a pointer to a specific set of rows. It provides an interface to interact with items while offering capabilities like reactivity, transformation, observation of changes, and more.

You don't have to create a cursor by yourself. SignalDB is handling that for you and returns the cursor from a .find() call.

The following methods are available in the cursor class:

forEach(callback: (item: U) => void) (reactive)

Iterates over each item in the cursor, applying the given callback function.

• Parameters:
  • callback: A function that gets executed for each item.

map<V>(callback: (item: U) => V) (reactive)

Maps each item in the cursor to a new array using the provided callback function.

• Parameters:
  • callback: A function that transforms each item.
• Returns
  • An array of transformed items

fetch() (reactive)

Fetches all the items in the cursor and returns them.

• Returns
  • An array of items

count() (reactive)

Counts the number of items in the cursor.

• Returns
  • The count of items

observeChanges(callbacks: ObserveCallbacks<U>, skipInitial = false)

This method allows observation of changes in the cursor items. It uses callbacks to notify of different events like addition, removal, changes, etc.

• Parameters
  • callbacks: An object of Callback functions for different observation events.
    • added(item: T)gets called when a new item was added to the cursor
    • addedBefore(item: T, before: T)gets called when a new item was added to the cursor and also indicates the position of the new item
    • changed(item: T)gets called when an item in the cursor was changed
    • movedBefore(item: T, before: T)gets called when an item moved its position in the cursor
    • removed(item: T)gets called when an item was removed from the cursor
  • skipInitial: A boolean to decide whether to skip the initial observation event.
• Returns
  • A function that, when called, stops observing the changes.

requery()

Re-queries the cursor to fetch items and check observers for any changes.

cleanup()

The cleanup method is used to invoke all the cleanup callbacks. This helps in managing resources and ensuring efficient garbage collection. You have to call this method, if you're using a reactivity adapter, that doesn't support automatic cleanup.