ListenerKeeper

ListenerKeeper.java

package com.preplay.android.util;

import android.content.Context;
import android.support.v4.app.Fragment;
import android.view.View;

import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

public class ListenerKeeper<T> {

    private final Set<WeakReference<T>> mListWeakReference = new HashSet<WeakReference<T>>();

    private final Set<T> mListHardReference = new HashSet<T>();

    public void add(T listener) {
        if (checkNullListener(listener)) {
            return;
        }
        if (shouldBeWeakReference(listener)) {
            addWeakReference(listener);
        } else {
            addHardReference(listener);
        }
    }

    public void addWeakReference(T listener) {
        if (checkNullListener(listener)) {
            return;
        }
        synchronized (mListWeakReference) {
            Iterator<WeakReference<T>> iterator = mListWeakReference.iterator();
            while (iterator.hasNext()) {
                WeakReference<T> weakReference = iterator.next();
                T reference = weakReference.get();
                if (listener.equals(reference)) {
                    // already in our list, don't add it again
                    return;
                }
            }
            mListWeakReference.add(new WeakReference<T>(listener));
        }
    }

    public void remove(T listener) {
        if (checkNullListener(listener)) {
            return;
        }
        removeWeakListener(listener);
        removeHardListener(listener);
    }

    public void removeAll() {
        removeAllWeakListeners();
        removeAllHardListeners();
    }

    public List<T> generateListListener() {
        List<T> list = new ArrayList<T>();
        addHardReferenceTo(list);
        addWeakReferencesTo(list);
        return list;
    }

    public boolean hasListeners() {
        synchronized (mListHardReference) {
            if (!mListHardReference.isEmpty()) {
                return true;
            }
        }
        synchronized (mListWeakReference) {
            if (!mListWeakReference.isEmpty()) {
                return true;
            }
        }
        return false;
    }

    private void removeAllHardListeners() {
        synchronized (mListHardReference) {
            mListHardReference.clear();
        }
    }

    private void removeAllWeakListeners() {
        synchronized (mListWeakReference) {
            mListWeakReference.clear();
        }
    }

    private void removeHardListener(T listener) {
        synchronized (mListHardReference) {
            mListHardReference.remove(listener);
        }
    }

    private void removeWeakListener(T listener) {
        synchronized (mListWeakReference) {
            Iterator<WeakReference<T>> iterator = mListWeakReference.iterator();
            while (iterator.hasNext()) {
                WeakReference<T> weakReference = iterator.next();
                T reference = weakReference.get();
                if (listener.equals(reference)) {
                    iterator.remove();
                }
            }
        }
    }

    private void addHardReferenceTo(List<T> list) {
        synchronized (mListHardReference) {
            list.addAll(mListHardReference);
        }
    }

    private void addWeakReferencesTo(List<T> list) {
        synchronized (mListWeakReference) {
            Iterator<WeakReference<T>> iterator = mListWeakReference.iterator();
            while (iterator.hasNext()) {
                WeakReference<T> weakReference = iterator.next();
                T reference = weakReference.get();
                if (reference == null) {
                    iterator.remove();
                } else {
                    list.add(reference);
                }
            }
        }
    }

    private void addHardReference(T listener) {
        synchronized (mListHardReference) {
            mListHardReference.add(listener);
        }
    }

    private boolean shouldBeWeakReference(T listener) {
        boolean shouldBeWeek = listener instanceof Context;
        if (AndroidUtil.isAndroidUpTo(11)) {
            shouldBeWeek |= (listener instanceof android.app.Fragment);
        }
        shouldBeWeek |= (listener instanceof Fragment);
        shouldBeWeek |= (listener instanceof View);
        return shouldBeWeek;
    }

    private boolean checkNullListener(T listener) {
        if (listener == null) {
            PPLog.e("You tried to add a null listener");
            return true;
        }
        return false;
    }
}

Call "generateListListener()" to have a list of listener when necessary.