FileReader

备注： 此特性在 Web Worker 中可用。

FileReader 接口允许 Web 应用程序异步读取存储在用户计算机上的文件（或原始数据缓冲区）的内容，使用 File 或 Blob 对象指定要读取的文件或数据。

文件对象可以从用户使用 <input> 元素选择文件而返回的 FileList 对象中获取，或者从拖放操作的 DataTransfer 对象中获取。

FileReader 只能访问用户明确选择的文件内容，无论是使用 HTML <input type="file"> 元素还是通过拖放。它不能用于从用户的文件系统中按路径名读取文件。要按路径名读取客户端文件系统上的文件，请使用文件系统访问 API。要读取服务器端文件，请使用 fetch()，如果跨源读取，则需要 CORS 权限。

EventTargetFileReader

构造函数

FileReader()

返回一个新的 FileReader 对象。

有关详细信息和示例，请参阅在 Web 应用程序中使用文件。

实例属性

FileReader.error 只读

一个表示在读取文件时发生的错误的 DOMException。

FileReader.readyState 只读

表示FileReader状态的数字。取值如下：

常量名	值	描述
EMPTY	0	还没有加载任何数据。
LOADING	1	数据正在被加载。
DONE	2	已完成全部的读取请求。

FileReader.result 只读

文件的内容。该属性仅在读取操作完成后才有效，数据的格式取决于使用哪个方法来启动读取操作。

实例方法

FileReader.abort()

中止读取操作。在返回时，readyState 属性为 DONE。

FileReader.readAsArrayBuffer()

开始读取指定的 Blob 中的内容，一旦完成，result 属性中将包含一个表示文件数据的 ArrayBuffer 对象。

FileReader.readAsBinaryString() 已弃用

开始读取指定的 Blob 中的内容。一旦完成，result 属性中将包含一个表示文件中的原始二进制数据的字符串。

FileReader.readAsDataURL()

开始读取指定的 Blob 中的内容。一旦完成，result 属性中将包含一个表示文件数据的 data: URL。

FileReader.readAsText()

开始读取指定的 Blob 中的内容。一旦完成，result 属性中将包含一个表示所读取的文件内容的字符串。可以指定可选的编码名称。

事件

使用 addEventListener() 方法或通过将事件侦听器分配给此接口的 oneventname 属性来侦听这些事件。一旦不再使用 FileReader，请使用 removeEventListener() 删除事件侦听器，以避免内存泄漏。

abort

当读取被中止时触发，例如因为程序调用了 FileReader.abort() 方法。

error

当读取由于错误而失败时触发。

load

读取成功完成时触发。

loadend

读取完成时触发，无论成功与否。

loadstart

读取开始时触发。

progress

读取数据时定期触发。

前端中这些接口在c++中代码实现如下：

1、FileReader接口定义 third_party\blink\renderer\core\fileapi\file_reader.idl

/** Copyright (C) 2010 Google Inc.  All rights reserved.* Copyright (C) 2011 Apple Inc. All rights reserved.** Redistribution and use in source and binary forms, with or without* modification, are permitted provided that the following conditions are* met:**     * Redistributions of source code must retain the above copyright* notice, this list of conditions and the following disclaimer.*     * 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.*     * Neither the name of Google Inc. nor the names of its* contributors may be used to endorse or promote products derived from* this software without specific prior written permission.** 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* OWNER 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.*/// https://w3c.github.io/FileAPI/#APIASynch[ActiveScriptWrappable,Exposed=(Window,Worker)
] interface FileReader : EventTarget {[CallWith=ExecutionContext] constructor();// async read methods[RaisesException] void readAsArrayBuffer(Blob blob);[RaisesException] void readAsBinaryString(Blob blob);[RaisesException] void readAsText(Blob blob, optional DOMString label);[RaisesException] void readAsDataURL(Blob blob);void abort();// statesconst unsigned short EMPTY = 0;const unsigned short LOADING = 1;const unsigned short DONE = 2;[ImplementedAs=getReadyState] readonly attribute unsigned short readyState;// File or Blob datareadonly attribute (DOMString or ArrayBuffer)? result;readonly attribute DOMException? error;// event handler attributesattribute EventHandler onloadstart;attribute EventHandler onprogress;attribute EventHandler onload;attribute EventHandler onabort;attribute EventHandler onerror;attribute EventHandler onloadend;
};

2、FileReader接口实现

third_party\blink\renderer\core\fileapi\file_reader.h

third_party\blink\renderer\core\fileapi\file_reader.cc

/** Copyright (C) 2010 Google Inc.  All rights reserved.** Redistribution and use in source and binary forms, with or without* modification, are permitted provided that the following conditions are* met:**     * Redistributions of source code must retain the above copyright* notice, this list of conditions and the following disclaimer.*     * 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.*     * Neither the name of Google Inc. nor the names of its* contributors may be used to endorse or promote products derived from* this software without specific prior written permission.** 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* OWNER 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.*/#ifndef THIRD_PARTY_BLINK_RENDERER_CORE_FILEAPI_FILE_READER_H_
#define THIRD_PARTY_BLINK_RENDERER_CORE_FILEAPI_FILE_READER_H_#include "base/timer/elapsed_timer.h"
#include "third_party/blink/renderer/bindings/core/v8/active_script_wrappable.h"
#include "third_party/blink/renderer/core/core_export.h"
#include "third_party/blink/renderer/core/dom/events/event_target.h"
#include "third_party/blink/renderer/core/execution_context/execution_context_lifecycle_observer.h"
#include "third_party/blink/renderer/core/fileapi/file_error.h"
#include "third_party/blink/renderer/core/fileapi/file_read_type.h"
#include "third_party/blink/renderer/core/fileapi/file_reader_client.h"
#include "third_party/blink/renderer/core/fileapi/file_reader_loader.h"
#include "third_party/blink/renderer/core/probe/async_task_context.h"
#include "third_party/blink/renderer/platform/heap/garbage_collected.h"
#include "third_party/blink/renderer/platform/wtf/forward.h"namespace blink {class Blob;
class ExceptionState;
class ExecutionContext;
class V8UnionArrayBufferOrString;
enum class FileErrorCode;class CORE_EXPORT FileReader final : public EventTarget,public ActiveScriptWrappable<FileReader>,public ExecutionContextLifecycleObserver,public FileReaderAccumulator {DEFINE_WRAPPERTYPEINFO();public:static FileReader* Create(ExecutionContext*);explicit FileReader(ExecutionContext*);~FileReader() override;enum ReadyState { kEmpty = 0, kLoading = 1, kDone = 2 };void readAsArrayBuffer(Blob*, ExceptionState&);void readAsBinaryString(Blob*, ExceptionState&);void readAsText(Blob*, const String& encoding, ExceptionState&);void readAsText(Blob*, ExceptionState&);void readAsDataURL(Blob*, ExceptionState&);void abort();ReadyState getReadyState() const { return state_; }DOMException* error() { return error_.Get(); }V8UnionArrayBufferOrString* result() const;probe::AsyncTaskContext* async_task_context() { return &async_task_context_; }// ExecutionContextLifecycleObservervoid ContextDestroyed() override;// ScriptWrappablebool HasPendingActivity() const final;// EventTargetconst AtomicString& InterfaceName() const override;ExecutionContext* GetExecutionContext() const override {return ExecutionContextLifecycleObserver::GetExecutionContext();}// FileReaderClientFileErrorCode DidStartLoading() override;FileErrorCode DidReceiveData() override;void DidFinishLoading(FileReaderData contents) override;void DidFail(FileErrorCode) override;DEFINE_ATTRIBUTE_EVENT_LISTENER(loadstart, kLoadstart)DEFINE_ATTRIBUTE_EVENT_LISTENER(progress, kProgress)DEFINE_ATTRIBUTE_EVENT_LISTENER(load, kLoad)DEFINE_ATTRIBUTE_EVENT_LISTENER(abort, kAbort)DEFINE_ATTRIBUTE_EVENT_LISTENER(error, kError)DEFINE_ATTRIBUTE_EVENT_LISTENER(loadend, kLoadend)void Trace(Visitor*) const override;private:class ThrottlingController;void Terminate();void ReadInternal(Blob*, FileReadType, ExceptionState&);void FireEvent(const AtomicString& type);void ExecutePendingRead();ReadyState state_;// Internal loading state, which could differ from ReadyState as it's// for script-visible state while this one's for internal state.enum LoadingState {kLoadingStateNone,kLoadingStatePending,kLoadingStateLoading,kLoadingStateAborted};LoadingState loading_state_;bool still_firing_events_;String blob_type_;scoped_refptr<BlobDataHandle> blob_data_handle_;FileReadType read_type_;String encoding_;probe::AsyncTaskContext async_task_context_;Member<FileReaderLoader> loader_;Member<V8UnionArrayBufferOrString> result_ = nullptr;Member<DOMException> error_;absl::optional<base::ElapsedTimer> last_progress_notification_time_;
};}  // namespace blink#endif  // THIRD_PARTY_BLINK_RENDERER_CORE_FILEAPI_FILE_READER_H_