Demonstrate the functionality of the File System Provider API for archives.

chrome/common/extensions/docs/examples/api/fileSystemProvider/archive/background.js

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+'use strict';
+
+// For file examples with metadata inside, see example1.fake and example2.fake.

[mtomasz, 2014/06/30 08:30:30]

... -> Metadata is stored in files as serialized to JSON maps. See contents of
example1.fake and example2.fake.

[cmihail, 2014/07/01 00:07:05]

Done.

+
+// Multiple volumes can be opened at the same time. The key is the
+// fileSystemId, which is the same as the file entry.name. The value

[mtomasz, 2014/06/30 08:30:31]

file -> file's

[cmihail, 2014/07/01 00:07:05]

Done.

+// is a Volume object.
+var volumes = {};
+
+// Defines a volume object that contains information about mounted devices.
+// openedFiles parameter is optional and can be undefined.
+function Volume(entry, mount, metadata, openedFiles) {

[mtomasz, 2014/06/30 08:30:31]

openedFiles -> opt_openedFiles

[cmihail, 2014/07/01 00:07:05]

Done.

+  // Used for restoring file entry.

[mtomasz, 2014/06/30 08:30:30]

... -> Used for restoring the opened file entry after resuming the event page.

[cmihail, 2014/07/01 00:07:05]

Done.

+  this.entry_ = entry;
+
+  // The volume metadata.

[mtomasz, 2014/06/30 08:30:31]

Let's add a comment:
Date object is serialized in JSON as string.

[cmihail, 2014/07/01 00:07:05]

Done.

+  for (var item in metadata) {

[mtomasz, 2014/06/30 08:30:31]

I think it is better not to modify input arguments, especially if they are a
reference. How about:

this.metadata_ = [];
for (...) {
  this.metadata_[path] = metadata[path];
  this.metadata_[path].modificationTime = new Date(...);
}

Also "item" is not a helpful naming. How about "path"?

[cmihail, 2014/07/01 00:07:05]

Done.

+    metadata[item].modificationTime =
+        new Date(metadata[item].modificationTime);
+  }
+  this.metadata_ = metadata;
+
+  // A map with currently opened files. As key it has requestId of
+  // openFileRequested and as a value the file path.
+  this.openedFiles_ = openedFiles ? openedFiles : {};
+
+  // Mount device and save device information on local storage in order to be

[mtomasz, 2014/06/30 08:30:31]

Mount device -> Mount the volume and save its information in local .. 

[cmihail, 2014/07/01 00:07:05]

Done.

+  // able to recover metadata in case of restarts, system crashes, etc.
+  if (mount) {
+    chrome.fileSystemProvider.mount(
+        {fileSystemId: entry.name, displayName: entry.name},
+        function() { saveState(); },
+        function() { console.error('Failed to mount.'); });
+
+  }
+};
+
+function onUnmountRequested(options, onSuccess, onError) {
+  if (Object.keys(volumes[options.fileSystemId].openedFiles_).length != 0) {
+    onError('IN_USE');
+    return;
+  }
+
+  chrome.fileSystemProvider.unmount(
+      {fileSystemId: options.fileSystemId},
+      function() {
+        delete volumes[options.fileSystemId];
+        saveState(); // Remove volume from local storage state.
+        onSuccess();
+      },
+      function() {
+        onError('FAILED');
+      });
+};
+
+function onGetMetadataRequested(options, onSuccess, onError) {
+  restoreState(options.fileSystemId, function () {
+    var entryMetadata =
+        volumes[options.fileSystemId].metadata_[options.entryPath];
+    if (!entryMetadata)
+      error('NOT_FOUND');
+    else
+      onSuccess(entryMetadata);
+  }, onError);
+};
+
+function onReadDirectoryRequested(options, onSuccess, onError) {
+  restoreState(options.fileSystemId, function () {
+    var directoryMetadata =
+        volumes[options.fileSystemId].metadata_[options.directoryPath];
+    if (!directoryMetadata) {
+      onError('NOT_FOUND');
+      return;
+    }
+    if (!directoryMetadata.isDirectory) {
+      onError('NOT_A_DIRECTORY');
+      return;
+    }
+
+    // Retrieve directory contents from metadata.
+    var entries = [];
+    for (var entry in volumes[options.fileSystemId].metadata_) {
+      // Do not add itself on the list.
+      if (entry == options.directoryPath)
+        continue;
+      // Check if the entry is a child of the requested directory.
+      if (entry.indexOf(options.directoryPath) != 0)
+        continue;
+      // Restrict to direct children only.
+      if (entry.substring(options.directoryPath.length + 1).indexOf('/') != -1)
+        continue;
+
+      entries.push(volumes[options.fileSystemId].metadata_[entry]);
+    }
+    onSuccess(entries, false /* Last call. */);
+  }, onError);
+};
+
+function onOpenFileRequested(options, onSuccess, onError) {
+  restoreState(options.fileSystemId, function () {
+    if (options.mode != 'READ' || options.create) {
+      onError('INVALID_OPERATION');
+    } else {
+      volumes[options.fileSystemId].openedFiles_[options.requestId] =
+          options.filePath;
+      onSuccess();
+    }
+  }, onError);
+};
+
+function onCloseFileRequested(options, onSuccess, onError) {
+  restoreState(options.fileSystemId, function () {
+    if (!volumes[options.fileSystemId].openedFiles_[options.openRequestId]) {
+      onError('INVALID_OPERATION');
+    } else {
+      delete volumes[options.fileSystemId].openedFiles_[options.openRequestId];
+      onSuccess();
+    }
+  }, onError);
+};
+
+function onReadFileRequested(options, onSuccess, onError) {
+  restoreState(options.fileSystemId, function () {
+    var filePath =
+        volumes[options.fileSystemId].openedFiles_[options.openRequestId];
+    if (!filePath) {
+      onError('INVALID_OPERATION');
+      return;
+    }
+
+    var contents = volumes[options.fileSystemId].metadata_[filePath].contents;

[mtomasz, 2014/06/30 08:30:31]

nit: metadata_ is private here. Let's make it public, so we don't access a
private member (metadata_ -> metadata).

[cmihail, 2014/07/01 00:07:05]

Done. Also modified the other variables.

+
+    // Write the contents as ASCII text.
+    var buffer = new ArrayBuffer(options.length);
+    var bufferView = new Uint8Array(buffer);
+    for (var i = 0; i < options.length; i++) {
+      bufferView[i] = contents.charCodeAt(i);
+    }
+
+    onSuccess(buffer, false /* Last call. */);
+  }, onError);
+};
+
+// Save state in case of restart, extension suspend, crashes, etc.

[mtomasz, 2014/06/30 08:30:30]

nit: restart -> restarts
nit: extension suspend -> event page suspends

[cmihail, 2014/07/01 00:07:05]

Done.

+function saveState() {
+  var state = {};
+  for (var volumeId in volumes) {
+    var entryId = chrome.fileSystem.retainEntry(volumes[volumeId].entry_);

[cmihail, 2014/06/30 08:01:44]

As a mention: retainEntry executes quite often. I haven't find in the API a
method that tests if an Entry is already saved. I saw the isRestorable method,
but it's using the entryId not an Entry object.

As for the program, based on the retainEntry specification, there shouldn't be
any problems with the current code as only the most 500 recent entries are
stored.

[mtomasz, 2014/06/30 13:35:32]

That's a good point. I took a look at the C++ code and it seems that a file once
remembered, will not use more quota than 1. So we can keep it as it is.

+    state[volumeId] = {
+      entryId: entryId,
+      openedFiles: volumes[volumeId].openedFiles_
+    };
+  }
+  chrome.storage.local.set({state: state});
+}
+
+// Restore state. In this case the file system is already mounted and
+// we only need to obtain the metadata, which is done lazily.
+function restoreState(fileSystemId, onSuccess, onError) {
+  chrome.storage.local.get(['state'], function(result) {
+    // Check if metadata for the given file system is alread in memory.
+    if (volumes[fileSystemId]) {
+      onSuccess();
+      return;
+    }
+
+    chrome.fileSystem.restoreEntry(
+        result.state[fileSystemId].entryId,
+        function(entry) {
+          readMetadataFromFile(entry, false /* File system is mounted. */,
+              onSuccess, onError, result.state[fileSystemId].openedFiles);
+        });
+  });
+}
+
+// openedFiles is an optional parameter.
+function readMetadataFromFile(entry, mount, onSuccess, onError, openedFiles) {

[mtomasz, 2014/06/30 08:30:30]

readMetadataFromFile shouldn't need |openedFiles|. How about not passing it
here, and instead creating the volume in the onSuccess callback?

Currently readMetadataFromFile does not only read the metadata, but also mounts
the volume. We could split it, and make the code easier to read.

[cmihail, 2014/07/01 00:07:05]

Done.

+  entry.file(function(file) {
+    var fileReader = new FileReader();
+    fileReader.onload = function(event) {
+      var metadata = JSON.parse(event.target.result);
+      volumes[entry.name] = new Volume(entry, mount, metadata, openedFiles);
+      onSuccess();
+    };
+
+    fileReader.onerror = function(event) {
+      onError('FAILED');
+    };
+
+    fileReader.readAsText(file);
+  });
+}
+
+// Event called on clicking the file with the extension mentioned
+// in the manifest file.

[mtomasz, 2014/06/30 08:30:30]

Not only file extension, but also if mime type is matching.

Note, that currently this doesn't work, but I'm working on it, and it will soon
be fixed.

[cmihail, 2014/07/01 00:07:06]

Done.

+chrome.app.runtime.onLaunched.addListener(function(event) {
+  event.items.forEach(function(item) {
+    readMetadataFromFile(item.entry, true /* Mount file system. */,
+        function() {},
+        function(error) { console.error(error); } );
+  });
+});
+
+// Event called on chromeos startup.

[mtomasz, 2014/06/30 08:30:31]

... -> Event called on a profile startup.

[cmihail, 2014/07/01 00:07:05]

Done.

+chrome.runtime.onStartup.addListener(function () {
+  chrome.storage.local.get(['state'], function(result) {
+    // Nothing to change.
+    if (!result.state)
+      return;
+
+    // Remove opened files from state. The file entry restore

[mtomasz, 2014/06/30 08:30:31]

I think we could clarify it a little bit, eg.:
... -> Remove files opened before the profile shutdown from the state.

[cmihail, 2014/07/01 00:07:05]

Done. I added local storage state. It seems a bit more clear to me.

+    // will be done by the restoreState function.
+    for (var volumeId in result.state) {
+      result.state[volumeId].openedFiles = {};
+    }
+    chrome.storage.local.set({state: result.state});
+  });
+});
+
+// Event called when the extension is idle for about 10+ seconds and it will

[mtomasz, 2014/06/30 08:30:31]

I think we can remove this comment. The idling period before suspending may
change in the future.

Instead we could add a comment like: Save the state before suspending the event
page, so we can resume it once new events arrive. WDYT?

[cmihail, 2014/07/01 00:07:05]

That sounds good to me.

+// get killed to free resources. The extension state should be saved.
+chrome.runtime.onSuspend.addListener(function() {
+  saveState();
+});
+
+chrome.fileSystemProvider.onUnmountRequested.addListener(
+    onUnmountRequested);
+chrome.fileSystemProvider.onGetMetadataRequested.addListener(
+    onGetMetadataRequested);
+chrome.fileSystemProvider.onReadDirectoryRequested.addListener(
+    onReadDirectoryRequested);
+chrome.fileSystemProvider.onOpenFileRequested.addListener(
+    onOpenFileRequested);
+chrome.fileSystemProvider.onCloseFileRequested.addListener(
+    onCloseFileRequested);
+chrome.fileSystemProvider.onReadFileRequested.addListener(
+    onReadFileRequested);