LibWeb: Start building a new Resource class to share more resources

A Resource represents a resource that we're loading, have loaded or
will soon load. Basically, it's a downloadable resource that can be
shared by multiple clients.

A typical usecase is multiple <img> elements with the same src.
In a future patch, we will try to make sure that those <img> elements
get the same Resource if possible. This will reduce network usage,
memory usage, and CPU usage. :^)

For now, this first patch simply introduces the mechanism.

You get a Resource by calling ResourceLoader::load_resource().
To get notified about changes to a Resource's load status, you inherit
from ResourceClient and implement the callbacks you're interested in.

This patch turns HTMLImageElement into a ResourceClient.

Libraries/LibWeb/Loader/Resource.h

@@ -0,0 +1,105 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/ByteBuffer.h>
+#include <AK/HashMap.h>
+#include <AK/HashTable.h>
+#include <AK/Noncopyable.h>
+#include <AK/RefCounted.h>
+#include <AK/URL.h>
+#include <LibWeb/Forward.h>
+
+namespace Web {
+
+class ResourceClient;
+
+class Resource : public RefCounted<Resource> {
+    AK_MAKE_NONCOPYABLE(Resource);
+    AK_MAKE_NONMOVABLE(Resource);
+
+public:
+    static NonnullRefPtr<Resource> create(Badge<ResourceLoader>, const URL&);
+    ~Resource();
+
+    bool is_loaded() const { return m_loaded; }
+
+    bool is_failed() const { return m_failed; }
+    const String& error() const { return m_error; }
+
+    bool has_encoded_data() const { return !m_encoded_data.is_null(); }
+
+    const URL& url() const { return m_url; }
+    const ByteBuffer& encoded_data() const { return m_encoded_data; }
+
+    void register_client(Badge<ResourceClient>, ResourceClient&);
+    void unregister_client(Badge<ResourceClient>, ResourceClient&);
+
+    template<typename Callback>
+    void for_each_client(Callback callback)
+    {
+        // FIXME: This should use some kind of smart pointer to ResourceClient!
+        Vector<ResourceClient*, 16> clients_copy;
+        clients_copy.ensure_capacity(m_clients.size());
+        for (auto* client : m_clients)
+            clients_copy.append(client);
+        for (auto* client : clients_copy)
+            callback(*client);
+    }
+
+    void did_load(Badge<ResourceLoader>, const ByteBuffer& data, const HashMap<String, String, CaseInsensitiveStringTraits>& headers);
+    void did_fail(Badge<ResourceLoader>, const String& error);
+
+private:
+    explicit Resource(const URL&);
+
+    URL m_url;
+    ByteBuffer m_encoded_data;
+    bool m_loaded { false };
+    bool m_failed { false };
+    String m_error;
+    HashMap<String, String, CaseInsensitiveStringTraits> m_response_headers;
+    HashTable<ResourceClient*> m_clients;
+};
+
+class ResourceClient {
+public:
+    virtual ~ResourceClient();
+
+    virtual void resource_did_load() { }
+    virtual void resource_did_fail() { }
+
+protected:
+    Resource* resource() { return m_resource; }
+    const Resource* resource() const { return m_resource; }
+    void set_resource(Resource*);
+
+private:
+    RefPtr<Resource> m_resource;
+};
+
+}