com.google.gwt.event.logical.shared.ValueChangeEvent.java Source code

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Here is the source code for com.google.gwt.event.logical.shared.ValueChangeEvent.java

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/*
 * Copyright 2008 Google Inc.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */
package com.google.gwt.event.logical.shared;

import com.google.gwt.event.shared.GwtEvent;

/**
 * Represents a value change event.
 * 
 * @param <T> the value about to be changed
 */
public class ValueChangeEvent<T> extends GwtEvent<ValueChangeHandler<T>> {

    /**
     * Handler type.
     */
    private static Type<ValueChangeHandler<?>> TYPE;

    /**
     * Fires a value change event on all registered handlers in the handler
     * manager. If no such handlers exist, this method will do nothing.
     * 
     * @param <T> the old value type
     * @param source the source of the handlers
     * @param value the value
     */
    public static <T> void fire(HasValueChangeHandlers<T> source, T value) {
        if (TYPE != null) {
            ValueChangeEvent<T> event = new ValueChangeEvent<T>(value);
            source.fireEvent(event);
        }
    }

    /**
     * Fires value change event if the old value is not equal to the new value.
     * Use this call rather than making the decision to short circuit yourself for
     * safe handling of null.
     * 
     * @param <T> the old value type
     * @param source the source of the handlers
     * @param oldValue the oldValue, may be null
     * @param newValue the newValue, may be null
     */
    public static <T> void fireIfNotEqual(HasValueChangeHandlers<T> source, T oldValue, T newValue) {
        if (shouldFire(source, oldValue, newValue)) {
            ValueChangeEvent<T> event = new ValueChangeEvent<T>(newValue);
            source.fireEvent(event);
        }
    }

    /**
     * Gets the type associated with this event.
     * 
     * @return returns the handler type
     */
    public static Type<ValueChangeHandler<?>> getType() {
        if (TYPE == null) {
            TYPE = new Type<ValueChangeHandler<?>>();
        }
        return TYPE;
    }

    /**
     * Convenience method to allow subtypes to know when they should fire a value
     * change event in a null-safe manner.
     * 
     * @param <T> value type
     * @param source the source
     * @param oldValue the old value
     * @param newValue the new value
     * @return whether the event should be fired
     */
    protected static <T> boolean shouldFire(HasValueChangeHandlers<T> source, T oldValue, T newValue) {
        return TYPE != null && oldValue != newValue && (oldValue == null || !oldValue.equals(newValue));
    }

    private final T value;

    /**
     * Creates a value change event.
     * 
     * @param value the value
     */
    protected ValueChangeEvent(T value) {
        this.value = value;
    }

    // The instance knows its BeforeSelectionHandler is of type I, but the TYPE
    // field itself does not, so we have to do an unsafe cast here.
    @SuppressWarnings({ "unchecked", "rawtypes" })
    @Override
    public final Type<ValueChangeHandler<T>> getAssociatedType() {
        return (Type) TYPE;
    }

    /**
     * Gets the value.
     * 
     * @return the value
     */
    public T getValue() {
        return value;
    }

    @Override
    public String toDebugString() {
        return super.toDebugString() + getValue();
    }

    @Override
    protected void dispatch(ValueChangeHandler<T> handler) {
        handler.onValueChange(this);
    }
}