/**
 * Use the {@link Looper} for the current thread with the specified callback interface
 * and set whether the handler should be asynchronous.
 *
 * Handlers are synchronous by default unless this constructor is used to make
 * one that is strictly asynchronous.
 *
 * Asynchronous messages represent interrupts or events that do not require global ordering
 * with respect to synchronous messages.  Asynchronous messages are not subject to
 * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
 *
 * @param callback The callback interface in which to handle messages, or null.
 * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
 * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
 *
 * @hide
 */
public Handler(@Nullable Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
        final Class<? extends Handler> klass = getClass();
        if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                (klass.getModifiers() & Modifier.STATIC) == 0) {
            Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                klass.getCanonicalName());
        }
    }
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

/**
 * Use the provided {@link Looper} instead of the default one and take a callback
 * interface in which to handle messages.  Also set whether the handler
 * should be asynchronous.
 *
 * Handlers are synchronous by default unless this constructor is used to make
 * one that is strictly asynchronous.
 *
 * Asynchronous messages represent interrupts or events that do not require global ordering
 * with respect to synchronous messages.  Asynchronous messages are not subject to
 * the synchronization barriers introduced by conditions such as display vsync.
 *
 * @param looper The looper, must not be null.
 * @param callback The callback interface in which to handle messages, or null.
 * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
 * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
 *
 * @hide
 */
@UnsupportedAppUsage
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}