android/hardware/camera2/dispatch/HandlerDispatcher.java

/*
 * Copyright (C) 2014 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.hardware.camera2.dispatch;

import android.hardware.camera2.utils.UncheckedThrow;
import android.os.Handler;
import android.util.Log;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

import static com.android.internal.util.Preconditions.*;

/**
 * Forward all interface calls into a handler by posting it as a {@code Runnable}.
 *
 * <p>All calls will return immediately; functions with return values will return a default
 * value of {@code null}, {@code 0}, or {@code false} where that value is legal.</p>
 *
 * <p>Any exceptions thrown on the handler while trying to invoke a method
 * will be re-thrown. Throwing checked exceptions on a handler which doesn't expect any
 * checked exceptions to be thrown will result in "undefined" behavior
 * (although in practice it is usually thrown as normal).</p>
 */
public class HandlerDispatcher<T> implements Dispatchable<T> {

    private static final String TAG = "HandlerDispatcher";

    private final Dispatchable<T> mDispatchTarget;
    private final Handler mHandler;

    /**
     * Create a dispatcher that forwards it's dispatch calls by posting
     * them onto the {@code handler} as a {@code Runnable}.
     *
     * @param dispatchTarget the destination whose method calls will be redirected into the handler
     * @param handler all calls into {@code dispatchTarget} will be posted onto this handler
     * @param <T> the type of the element you want to wrap.
     * @return a dispatcher that will forward it's dispatch calls to a handler
     */
    public HandlerDispatcher(Dispatchable<T> dispatchTarget, Handler handler) {
        mDispatchTarget = checkNotNull(dispatchTarget, "dispatchTarget must not be null");
        mHandler = checkNotNull(handler, "handler must not be null");
    }

    @Override
    public Object dispatch(final Method method, final Object[] args) throws Throwable {
        mHandler.post(new Runnable() {
            @Override
            public void run() {
                try {
                    mDispatchTarget.dispatch(method, args);
                } catch (InvocationTargetException e) {
                    Throwable t = e.getTargetException();
                    // Potential UB. Hopefully 't' is a runtime exception.
                    UncheckedThrow.throwAnyException(t);
                } catch (IllegalAccessException e) {
                    // Impossible
                    Log.wtf(TAG, "IllegalAccessException while invoking " + method, e);
                } catch (IllegalArgumentException e) {
                    // Impossible
                    Log.wtf(TAG, "IllegalArgumentException while invoking " + method, e);
                } catch (Throwable e) {
                    UncheckedThrow.throwAnyException(e);
                }
            }
        });

        // TODO handle primitive return values that would avoid NPE if unboxed
        return null;
    }
}