base/android/java/src/org/chromium/base/supplier/LazyOneshotSupplier.java

// Copyright 2023 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

package org.chromium.base.supplier;

import androidx.annotation.Nullable;

import org.chromium.base.Callback;

/**
 * Wraps a lazy-loaded nullable object, notifying observers a single time when the dependency
 * becomes available. This intentionally doesn't extend {@link OneshotSupplier} to support the
 * supplied value being null.
 *
 * @param <T> The type of the wrapped object.
 */
public interface LazyOneshotSupplier<T> {
    /**
     * Add a callback that's called when the object owned by this supplier is available. If the
     * object is already available, the callback will be called at the end of the current message
     * loop.
     *
     * @param callback The callback to be called.
     */
    void onAvailable(Callback<T> callback);

    /**
     * Returns the value currently held or <code>null</code> when none is held. Use {@link
     * #hasValue} to tell if the value is intentionally null.
     */
    @Nullable
    T get();

    /** Returns whether the supplier holds a value currently. */
    boolean hasValue();

    /**
     * Creates a supplier using a lambda closure to hold onto the given value. Should only be used
     * when the value already exists or in tests, as otherwise it defeats the purpose of the lazy
     * part of this supplier.
     */
    static <T> LazyOneshotSupplier<T> fromValue(T value) {
        return new LazyOneshotSupplierImpl<>() {
            @Override
            public void doSet() {
                set(value);
            }
        };
    }

    /**
     * Allows callers to inline a lambda to satisfy the implementation of this object. The supplier
     * must be able to run and complete synchronously at any point.
     */
    static <T> LazyOneshotSupplier<T> fromSupplier(Supplier<T> supplier) {
        return new LazyOneshotSupplierImpl<>() {
            @Override
            public void doSet() {
                set(supplier.get());
            }
        };
    }
}