LibWeb: Add an initial implementation for Web::FileAPI::FileReader

Some steps are still to be implemented, namely:
 * Properly aborting the read algorithm
 * Handling BinaryString type properly
 * Setting error on any error

But as it stands, this is enough functionality for the basic case of
reading the contents of a blob using the FileReader API.

Userland/Libraries/LibWeb/FileAPI/FileReader.h

@@ -0,0 +1,116 @@
+/*
+ * Copyright (c) 2023, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <LibWeb/Bindings/PlatformObject.h>
+#include <LibWeb/DOM/EventTarget.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/WebIDL/ExceptionOr.h>
+
+namespace Web::FileAPI {
+
+// https://w3c.github.io/FileAPI/#dfn-filereader
+class FileReader : public DOM::EventTarget {
+    WEB_PLATFORM_OBJECT(FileReader, EventTarget);
+
+public:
+    using Result = Variant<Empty, String, JS::Handle<JS::ArrayBuffer>>;
+
+    virtual ~FileReader() override;
+
+    [[nodiscard]] static JS::NonnullGCPtr<FileReader> create(JS::Realm&);
+    static JS::NonnullGCPtr<FileReader> construct_impl(JS::Realm&);
+
+    // async read methods
+    WebIDL::ExceptionOr<void> read_as_array_buffer(Blob&);
+    WebIDL::ExceptionOr<void> read_as_binary_string(Blob&);
+    WebIDL::ExceptionOr<void> read_as_text(Blob&, Optional<String> const& encoding = {});
+    WebIDL::ExceptionOr<void> read_as_data_url(Blob&);
+
+    void abort();
+
+    // states
+    enum class State : u16 {
+        // The FileReader object has been constructed, and there are no pending reads. None of the read methods have been called.
+        // This is the default state of a newly minted FileReader object, until one of the read methods have been called on it.
+        Empty = 0,
+
+        // A File or Blob is being read. One of the read methods is being processed, and no error has occurred during the read.
+        Loading = 1,
+
+        // The entire File or Blob has been read into memory, OR a file read error occurred, OR the read was aborted using abort().
+        // The FileReader is no longer reading a File or Blob.
+        // If readyState is set to DONE it means at least one of the read methods have been called on this FileReader.
+        Done = 2,
+    };
+
+    // https://w3c.github.io/FileAPI/#dom-filereader-readystate
+    State ready_state() const { return m_state; }
+
+    // File or Blob data
+
+    // https://w3c.github.io/FileAPI/#dom-filereader-result
+    Result result() const { return m_result; }
+
+    // https://w3c.github.io/FileAPI/#dom-filereader-error
+    JS::GCPtr<WebIDL::DOMException> error() const { return m_error; }
+
+    // event handler attributes
+    void set_onloadstart(WebIDL::CallbackType*);
+    WebIDL::CallbackType* onloadstart();
+
+    void set_onprogress(WebIDL::CallbackType*);
+    WebIDL::CallbackType* onprogress();
+
+    void set_onload(WebIDL::CallbackType*);
+    WebIDL::CallbackType* onload();
+
+    void set_onabort(WebIDL::CallbackType*);
+    WebIDL::CallbackType* onabort();
+
+    void set_onerror(WebIDL::CallbackType*);
+    WebIDL::CallbackType* onerror();
+
+    void set_onloadend(WebIDL::CallbackType*);
+    WebIDL::CallbackType* onloadend();
+
+protected:
+    FileReader(JS::Realm&, ByteBuffer);
+
+    virtual void initialize(JS::Realm&) override;
+
+    virtual void visit_edges(JS::Cell::Visitor&) override;
+
+private:
+    explicit FileReader(JS::Realm&);
+
+    enum class Type {
+        ArrayBuffer,
+        BinaryString,
+        Text,
+        DataURL,
+    };
+
+    WebIDL::ExceptionOr<void> read_operation(Blob&, Type, Optional<String> const& encoding_name = {});
+
+    static WebIDL::ExceptionOr<Result> blob_package_data(JS::Realm& realm, ByteBuffer, FileReader::Type type, Optional<String> const&, Optional<String> const& encoding_name);
+
+    // A FileReader has an associated state, that is "empty", "loading", or "done". It is initially "empty".
+    // https://w3c.github.io/FileAPI/#filereader-state
+    State m_state { State::Empty };
+
+    // A FileReader has an associated result (null, a DOMString or an ArrayBuffer). It is initially null.
+    // https://w3c.github.io/FileAPI/#filereader-result
+    Result m_result;
+
+    // A FileReader has an associated error (null or a DOMException). It is initially null.
+    // https://w3c.github.io/FileAPI/#filereader-error
+    JS::GCPtr<WebIDL::DOMException> m_error;
+};
+
+}