Transformations.switchMap()

先看源码：

/*
 * Copyright (C) 2017 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 androidx.lifecycle;

import androidx.annotation.MainThread;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.arch.core.util.Function;

/**
 * Transformation methods for {@link LiveData}.
 * <p>
 * These methods permit functional composition and delegation of {@link LiveData} instances. The
 * transformations are calculated lazily, and will run only when the returned {@link LiveData} is
 * observed. Lifecycle behavior is propagated from the input {@code source} {@link LiveData} to the
 * returned one.
 */
@SuppressWarnings("WeakerAccess")
public class Transformations {

    private Transformations() {
    }

    /**
     * Returns a {@code LiveData} mapped from the input {@code source} {@code LiveData} by applying
     * {@code mapFunction} to each value set on {@code source}.
     * <p>
     * This method is analogous to {@link io.reactivex.Observable#map}.
     * <p>
     * {@code transform} will be executed on the main thread.
     * <p>
     * Here is an example mapping a simple {@code User} struct in a {@code LiveData} to a
     * {@code LiveData} containing their full name as a {@code String}.
     *
     * <pre>
     * LiveData<User> userLiveData = ...;
     * LiveData<String> userFullNameLiveData =
     *     Transformations.map(
     *         userLiveData,
     *         user -> user.firstName + user.lastName);
     * });
     * </pre>
     *
     * @param source      the {@code LiveData} to map from
     * @param mapFunction a function to apply to each value set on {@code source} in order to set
     *