Package org.eclipse.core.databinding.observable.value

Class ComputedValue<T>

  • Type Parameters:
    T - the type of value being observed
    All Implemented Interfaces:
    IObservable, IObservableValue<T>
    Direct Known Subclasses:
    AggregateValidationStatus
    public abstract class ComputedValue<T>
extends AbstractObservableValue<T>
    A Lazily calculated value that automatically computes and registers listeners on its dependencies as long as all of its dependencies are IObservable objects. Any change to one of the observable dependencies causes the value to be recomputed.

    This class is thread safe. All state accessing methods must be invoked from the current realm. Methods for adding and removing listeners may be invoked from any thread.

    Example: compute the sum of all elements in an IObservableList < Integer >. 

     final IObservableList addends = WritableValue.withValueType(Integer.TYPE);
 addends.add(Integer.valueOf(0));
 addends.add(Integer.valueOf(1));
 addends.add(Integer.valueOf(2));

 IObservableValue sum = new ComputedValue() {
        protected Object calculate() {
                int sum = 0;
                for (Iterator it = addends.iterator(); it.hasNext();) {
                        Integer addend = (Integer) it.next();
                        sum += addend.intValue();
                }
                return sum;
        }
 };

 System.out.println(sum.getValue()); // => 3

 addends.add(Integer.valueOf(10));
 System.out.println(sum.getValue()); // => 13
    Since:
    1.0

    • Constructor Detail

      • ComputedValue

        public ComputedValue()

      • ComputedValue

        public ComputedValue​(Object valueType)
        Parameters:
        valueType - can be null

      • ComputedValue

        public ComputedValue​(Realm realm)
        Parameters:
        realm - the realm to use; not null

      • ComputedValue

        public ComputedValue​(Realm realm,
                     Object valueType)
        Parameters:
        realm - the realm to use; not null
        valueType - can be null

    • Method Detail

      • calculate

        protected abstract T calculate()
        Subclasses must override this method to provide the object's value. Any dependencies used to calculate the value must be IObservable, and implementers must use one of the interface methods tagged TrackedGetter for ComputedValue to recognize it as a dependency.
        Returns:
        the object's value

      • makeDirty

        protected final void makeDirty()

      • isStale

        public boolean isStale()
        Description copied from interface: IObservable
        Returns whether the state of this observable is stale and is expected to change soon. A non-stale observable that becomes stale will notify its stale listeners. A stale object that becomes non-stale does so by changing its state and notifying its change listeners, it does not notify its stale listeners about becoming non-stale. Clients that do not expect asynchronous changes may ignore staleness of observable objects.
        Specified by:
        isStale in interface IObservable
        Overrides:
        isStale in class AbstractObservableValue<T>
        Returns:
        true if this observable's state is stale and will change soon.

      • getValueType

        public Object getValueType()
        Description copied from interface: IObservableValue
        The value type of this observable value, or null if this observable value is untyped.
        Returns:
        the value type, or null

      • hasListeners

        protected boolean hasListeners()
        Since:
        1.1

      • addChangeListener

        public void addChangeListener​(IChangeListener listener)
        Description copied from interface: IObservable
        Adds the given change listener to the list of change listeners. Change listeners are notified about changes of the state of this observable in a generic way, without specifying the change that happened. To get the changed state, a change listener needs to query for the current state of this observable.
        Specified by:
        addChangeListener in interface IObservable
        Overrides:
        addChangeListener in class AbstractObservable
        Parameters:
        listener - the listener to add; not null

      • dispose

        public void dispose()
        Description copied from interface: IObservable
        Disposes of this observable object, removing all listeners registered with this object, and all listeners this object might have registered on other objects.
        Specified by:
        dispose in interface IObservable
        Overrides:
        dispose in class AbstractObservable

      • addListener

        protected void addListener​(Object listenerType,
                           IObservablesListener listener)
        Parameters:
        listenerType - arbitrary object to identify a type of the listener
        listener - the listener to add; not null

      • removeListener

        protected void removeListener​(Object listenerType,
                              IObservablesListener listener)
        Parameters:
        listenerType - arbitrary object to identify a type of the listener
        listener - the listener to remove; not null

      • firstListenerAdded

        protected void firstListenerAdded()

      • lastListenerRemoved

        protected void lastListenerRemoved()

      • getRealm

        public Realm getRealm()
        Returns:
        Returns the realm.