FileReaderSync

备注： 此特性仅在 Web Worker（不包括 Service Worker）中可用。

FileReaderSync 接口允许同步读取 File 或 Blob 对象。此接口仅在 worker 中可用，因为它支持可能导致潜在的阻塞的同步 I/O。

构造函数

FileReaderSync()

返回一个新的 FileReaderSync 对象。

实例属性

该接口没有任何属性。

实例方法

FileReaderSync.readAsArrayBuffer()

此方法将指定的 Blob 或 File 转换为将输入数据表示为二进制字符串的 ArrayBuffer。

FileReaderSync.readAsBinaryString() 已弃用

此方法将指定的 Blob 或 File 转换为表示输入数据的二进制字符串。此方法已弃用，请考虑使用 readAsArrayBuffer() 代替。

FileReaderSync.readAsText()

此方法将指定的 Blob 或 File 转换为将输入数据表示为文本字符串的字符串。可选的 encoding 参数指示要使用的编码（例如，iso-8859-1 或 UTF-8）。如果不存在，该方法将对其应用检测算法以确定其编码。

FileReaderSync.readAsDataURL()

此方法将指定的 Blob 或 File 转换为将输入数据表示为 data URL 的字符串。

FileReaderSync在c++代码中实现：

1、FileReaderSync接口定义 third_party\blink\renderer\core\fileapi\file_reader_sync.idl 

     

/*
 * 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.
 */

// https://w3c.github.io/FileAPI/#FileReaderSync

[
    Exposed=(DedicatedWorker,SharedWorker)
] interface FileReaderSync {
    [CallWith=ExecutionContext, Measure] constructor();
    [RaisesException] ArrayBuffer readAsArrayBuffer(Blob blob);
    [RaisesException] DOMString readAsBinaryString(Blob blob);
    [RaisesException] DOMString readAsText(Blob blob, optional DOMString label);
    [RaisesException] DOMString readAsDataURL(Blob blob);
};

2、FileReaderSync接口接口实现

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

third_party\blink\renderer\core\fileapi\file_reader_sync.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_SYNC_H_
#define THIRD_PARTY_BLINK_RENDERER_CORE_FILEAPI_FILE_READER_SYNC_H_

#include "third_party/blink/renderer/core/execution_context/execution_context_lifecycle_observer.h"
#include "third_party/blink/renderer/core/fileapi/file_reader_client.h"
#include "third_party/blink/renderer/core/fileapi/file_reader_data.h"
#include "third_party/blink/renderer/core/typed_arrays/dom_array_buffer.h"
#include "third_party/blink/renderer/platform/bindings/script_wrappable.h"
#include "third_party/blink/renderer/platform/heap/garbage_collected.h"
#include "third_party/blink/renderer/platform/wtf/text/wtf_string.h"

namespace base {
class SingleThreadTaskRunner;
}

namespace blink {

class Blob;
class DOMArrayBuffer;
class ExceptionState;
class ExecutionContext;

class FileReaderSync final : public ScriptWrappable {
  DEFINE_WRAPPERTYPEINFO();

 public:
  static FileReaderSync* Create(ExecutionContext* context) {
    return MakeGarbageCollected<FileReaderSync>(context);
  }

  explicit FileReaderSync(ExecutionContext*);

  DOMArrayBuffer* readAsArrayBuffer(Blob*, ExceptionState&);
  String readAsBinaryString(Blob*, ExceptionState&);
  String readAsText(Blob* blob, ExceptionState& ec) {
    return readAsText(blob, "", ec);
  }
  String readAsText(Blob*, const String& encoding, ExceptionState&);
  String readAsDataURL(Blob*, ExceptionState&);

  void Trace(Visitor* visitor) const override {
    ScriptWrappable::Trace(visitor);
  }

 private:
  absl::optional<FileReaderData> Load(const Blob&, ExceptionState&);

  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
};

}  // namespace blink

#endif  // THIRD_PARTY_BLINK_RENDERER_CORE_FILEAPI_FILE_READER_SYNC_H_