core/java/android/webkit/WebMessagePort.java

/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * 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 android.webkit;

import android.annotation.SystemApi;
import android.os.Handler;

/**
 * The Java representation of the
 * <a href="https://html.spec.whatwg.org/multipage/comms.html#messageport">
 * HTML5 message ports.</a>
 *
 * A Message port represents one endpoint of a Message Channel. In Android
 * webview, there is no separate Message Channel object. When a message channel
 * is created, both ports are tangled to each other and started, and then
 * returned in a MessagePort array, see {@link WebView#createWebMessageChannel}
 * for creating a message channel.
 *
 * When a message port is first created or received via transfer, it does not
 * have a WebMessageCallback to receive web messages. The messages are queued until
 * a WebMessageCallback is set.
 *
 * A message port should be closed when it is not used by the embedder application
 * anymore. A closed port cannot be transferred or cannot be reopened to send
 * messages. Close can be called multiple times.
 *
 * When a port is transferred to JS, it cannot be used to send or receive messages
 * at the Java side anymore. Different from HTML5 Spec, a port cannot be transferred
 * if one of these has ever happened: i. a message callback was set, ii. a message was
 * posted on it. A transferred port cannot be closed by the application, since
 * the ownership is also transferred.
 *
 * It is possible to transfer both ports of a channel to JS, for example for
 * communication between subframes.
 */
public abstract class WebMessagePort {

    /**
     * The listener for handling MessagePort events. The message callback
     * methods are called on the main thread. If the embedder application
     * wants to receive the messages on a different thread, it can do this
     * by passing a Handler in
     *  {@link WebMessagePort#setWebMessageCallback(WebMessageCallback, Handler)}.
     * In the latter case, the application should be extra careful for thread safety
     * since WebMessagePort methods should be called on main thread.
     */
    public static abstract class WebMessageCallback {
        /**
         * Message callback for receiving onMessage events.
         *
         * @param port  the WebMessagePort that the message is destined for
         * @param message  the message from the entangled port.
         */
        public void onMessage(WebMessagePort port, WebMessage message) { }
    }

    /**
     * Constructor.
     * @hide
     */
    @SystemApi
    public WebMessagePort() { }

    /**
     * Post a WebMessage to the entangled port.
     *
     * @param message  the message from Java to JS.
     *
     * @throws IllegalStateException If message port is already transferred or closed.
     */
    public abstract void postMessage(WebMessage message);

    /**
     * Close the message port and free any resources associated with it.
     */
    public abstract void close();

    /**
     * Sets a callback to receive message events on the main thread.
     *
     * @param callback  the message callback.
     */
    public abstract void setWebMessageCallback(WebMessageCallback callback);

    /**
     * Sets a callback to receive message events on the handler that is provided
     * by the application.
     *
     * @param callback  the message callback.
     * @param handler   the handler to receive the message messages.
     */
    public abstract void setWebMessageCallback(WebMessageCallback callback, Handler handler);
}