common/src/com/android/tv/common/WeakHandler.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 com.android.tv.common;

import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.support.annotation.NonNull;
import java.lang.ref.WeakReference;

/**
 * A Handler that keeps a {@link WeakReference} to an object.
 *
 * <p>Use this to prevent leaking an Activity or other Context while messages are still pending.
 * When you extend this class you <strong>MUST NOT</strong> use a non static inner class, or the
 * containing object will still be leaked.
 *
 * <p>See <a href="http://android-developers.blogspot.com/2009/01/avoiding-memory-leaks.html">
 * Avoiding memory leaks</a>.
 */
public abstract class WeakHandler<T> extends Handler {
    private final WeakReference<T> mRef;

    /**
     * Constructs a new handler with a weak reference to the given referent using the provided
     * Looper instead of the default one.
     *
     * @param looper The looper, must not be null.
     * @param ref the referent to track
     */
    public WeakHandler(@NonNull Looper looper, T ref) {
        super(looper);
        mRef = new WeakReference<>(ref);
    }

    /**
     * Constructs a new handler with a weak reference to the given referent.
     *
     * @param ref the referent to track
     */
    public WeakHandler(T ref) {
        mRef = new WeakReference<>(ref);
    }

    /** Calls {@link #handleMessage(Message, Object)} if the WeakReference is not cleared. */
    @Override
    public final void handleMessage(Message msg) {
        T referent = mRef.get();
        if (referent == null) {
            return;
        }
        handleMessage(msg, referent);
    }

    /**
     * Subclasses must implement this to receive messages.
     *
     * <p>If the WeakReference is cleared this method will no longer be called.
     *
     * @param msg the message to handle
     * @param referent the referent. Guaranteed to be non null.
     */
    protected abstract void handleMessage(Message msg, @NonNull T referent);
}