Local Storage

Local Storage
Ivano Malavolta
ivano.malavolta@univaq.it
http://www.di.univaq.it/malavolta

Roadmap

• Introduction
• Web Storage
• WebSQL
• IndexedDB
• File System Access
• Final Considerations

Introduction

There are 4 ways to store data locally

• Web storage
– Local Storage
– Session Storage
• WebSQL
• Indexed DB
• File System Access

Web Storage

LocalStorage
stores data in key/value pairs
persists across browser sessions

SessionStorage
stores data in key/value pairs
data is erased when a browser session ends

Introduction

WebSQL Database
relational DB
support for tables creation, insert, update, …
transactional

Its evolution is called IndexedDB but it is actually
IndexedDB,
not supported by most mobile browsers

Introduction

File System Access
you can access files locally to your app
supports main FS operations
– files creation, move, delete, rename, creation, etc.
it is not transactional

Introduction
Mobile browsers compatibility matrix

Web Storage

It is based on a single persistent object called
localStorage

You can set values by calling
window.localStorage.setItem(“name”, “Ivano”);

You can get values back by calling
var name = window.localStorage.getItem(“name”);

Supported Methods
localStorage.key(0)
Returns the name of the key at the position specified

getItem(“key”)
Returns the item identified by it's key

setItem(“key”, “value”)
Saves and item at the key provided

removeItem(“hey”)
Removes the item identified by it's key

clear()
Removes all of the key value pairs

Complex Objects

Current implementations support only string-to-string
mappings

you can store only strings
keys can be only strings

You can use JSON serialization if you need to store
complex data structures

Example of JSON Serialization
// simple class declaration
function Person(name, surname) {
this.name = name;
this.surname = surname;
}

// object creation
var user = new Person(‘Ivano’, ‘Malavolta’);

// object serialization
window.localStorage.setItem(“user”, JSON.stringify(user));

// object retrieval
var current =
JSON.parse(window.localStorage.getItem(“user”));

Checking Existence

You can simply check if the needed element is == null

if (window.localStorage.getItem(“user”)) {
// there is an object in user
} else {
// the user key does not have any value
}

Selecting elements
In this case you have to manually iterate on elements

var users = [...]; // array of Person objects
window.localStorage.setItem(“users”,
JSON.stringify(users));

var allUsers =
JSON.parse(window.localStorage.getItem(“users”));
var ivanos = [];
for(var i=0; i<allUsers.length; i++) {
if(allUsers[i].name == ‘Ivano’)
ivanos.push(allUsers[i]);
}

Counting Elements

Also in this case, we have to do it manually

var usersNumber =
JSON.parse(window.localStorage.getItem(“users“)).length;

Session Storage

Session Storage provides the same interface as Local
Storage
you can call the same methods

but
Session Storage is cleared between app launches

WebSQL
It provides you a structured SQL relational database

You have to setup a DB schema

You can then perform classical SQL queries

tx.executeSql("SELECT * FROM User“, [],
function(tx, result) {
// callback code
});

Opening a DB

Done via a dedicated function

var db =
openDatabase(‘Test', ‘1.0', ‘Test DB', 100000);

It creates a new SQLLite DB and returns a new
Database object

The Database object will be used to manipulate the data

Opening a DB: syntax
openDatabase(name, version, displayname, size);

DB name
the name of the DB

DB version
the version of the DB

DB Display name
the display name of the DB

DB Size
the size of the DB in bytes

Database

It allows to manipulate the data via 2 methods:

changeVersion
atomically verify the version number and change it

db.changeVersion("1.0", "1.1");

transaction
performs a DB transaction

Transactions
It allow you to execute SQL statements against the DB

db.transaction(queries, error, success);

3 callback functions:

queries : contains the queries to be performed
error : executed if the transaction results in an error
success : executed if the transaction terminates correctly

Transaction Example

http://bit.ly/JlUJde

executeSql
It is the method that performs a SQL statement

The user can build up a database transaction by calling
the executeSql method multiple times

function populateDB(tx) {
tx.executeSql('DROP TABLE IF EXISTS USER');
tx.executeSql('CREATE TABLE IF NOT EXISTS USER (id
unique, name, surname)');

tx.executeSql('INSERT INTO USER(id, name, surname)
VALUES (1, ?, ?)‘, [“Ivano“, “Malavolta“],
success, error);
}

Result Sets
When the executeSql method is called, it will invoke it's
callback with a SQLResultSet parameter

It has 3 properties:
insertId
the ID of the row that has been inserted
rowsAffected
the number of rows changed by the SQL statement
rows
the data returned from a SQL select statement
rows is an object of type SQLResultSetList

Results Sets Example
...
tx.executeSql('INSERT INTO USER(id, name,surname)
VALUES (1, ?, ?)‘, [“Ivano“, “Malavolta“],
success, error);
}

function success(tx, results) {
var affected = results.rowsAffected(); // 1
}

function error(err) {
// code for managing the error
}

Result Set Lists
It contains the data returned from a SQL Select statement

length
the number of rows returned by the SQL query

item(index)
returns the row at the specified index represented by a
JavaScript object

Result Set List Example
...
tx.executeSql(‘SELECT * FROM USER‘, [],
success, error);
}

function success(tx, results) {
var size = results.rows.length;
for (var i=0; i<size; i++){
console.log(results.rows.item(i).name);
}
}

Errors

It contains information about an occurred error

code
A predefined error code
es. UNKNOWN_ERR, DATABASE_ERR, TOO_LARGE_ERR,
QUOTA_ERR, SYNTAX_ERR, TIMEOUT_ERR

message
A description of the error

Error Code Example
...
tx.executeSql(‘SELECT * FROM USER‘,[],
success, error);
}

function error(err) {
console.log(err.code);
}

Indexed DB
It tries to combine Web Storage and WebSQL

You can save data as key/value pairs
key/value

You can define multiple DBs

Good Performance
data is indexed
asynchronous it does not block the UI

Indexed DB

An Indexed DB is a collection of object stores

You can drop objects into the stores

You can see stores as a big SQL table with only
key/value
key/value pairs

you don’t need to define a schema upfront

File System Access

It allows you to read, write and navigate file system
hierarchies

It is fundamental for managing and storing large files
client-
and binary content on the client-side

File System Access Workflow

1. Request file system access
– persistent or temporary file system

2. then you can perform CRUD operations for both
files and folders:
– Create
– Read
– Update
– Delete

Request File System
requestFileSystem(type, size, successCb, [errorCb])

type
LocalFileSystem.TEMPORARY
LocalFileSystem .PERSISTENT
size
size in bytes the app will require for storage.
successCb
success callback, its argument is a FileSystem object
ErrorCb
error callback, its argument is a FileError object

Temporary VS Persistent

Temporary
the files stored by the app can be deleted at the
browser’s discretion

no guarantee of persistence

Persistent
cannot be deleted by the browser without
authorization by the app

Local File System Example
window.requestFileSystem(
LocalFileSystem.PERSISTENT,
0,
onSuccess,
onError);

function onSuccess(fileSystem) {
console.log(fileSystem.name);
}

File System
The FileSystem object has 2 properties:

name
the name of the file system
it is unique across the list of exposed file systems

root
the DirectoryEntry object representing the root
folder of the file system

Resolving a File URI

resolveLocalFileSystemURI
retrieve a DirectoryEntry or FileEntry using a URI

window.resolveLocalFileSystemURI(
"file:///userImg.png", onSuccess, onError);

function onSuccess(fileEntry) {
console.log(fileEntry.name);
}

Entities

FileEntry
The real objects
DirectoryEntry

File Descriptor

FileReader
FileWriter Writing & Reading objects
DirectoryReader

File Entry
It represents a file on a file system

isFile (boolean)
Always true

isDirectory (boolean)
Always false

name (DOMString)
the name of the FileEntry, excluding the path

fullPath (DOMString)
the full absolute path from the root

File Entry Methods
getMetadata(success,
getMetadata(success, fail)
Look up metadata about a file

moveTo(parentEntry, newName, success, fail)
moveTo( parentEntry, newName,
Move a file to a different location on the file system

copyTo(parentEntry, newName, success, fail)
copyTo(parentEntry, newName,
Copy a file to a different location on the file system

toURI()
toURI()
Return a URI that can be used to locate a file

File Entry Methods
remove(success, fail)
Delete a file

getParent(success,
getParent(success, fail)
Look up the parent directory

createWriter(success,
createWriter(success, fail)
Creates a FileWriter object that can be used to write
to a file

file(success, fail)
Creates a File object containing file properties

File
It contains attributes of a single file

name (DOMString)
The name of the file
fullPath (DOMString)
The full path of the file including the file name
type (DOMString)
The mime type of the file
lastModifiedDate (Date)
The last time the file was modified
size (long)
The size of the file in bytes

Directory Entry
It represents a directory on a file system

It has the same properties of FileEntry

Directory Entry Methods
getMetadata(success,
getMetadata(success, fail)
Look up metadata about a directory

moveTo(parentEntry, newName,
moveTo(parentEntry , newName, success, fail)
Move a directory to a different location on the file system

copyTo(parentEntry, newName,
copyTo(parentEntry, newName, success, fail)
Copy a directory to a different location on the file system

toURI()
toURI()
Return a URI that can be used to locate a directory


getParent(success, fail)
getParent(success,
Look up the parent directory

createReader()
createReader()
Creates a DirectoryReader object that can be used to read a
directory

getDirectory(path, options, success, fail)
getDirectory(path,
Create or look up a directory
options:
create (true | false)
exclusive (true | false)

getFile(path, options, success, fail)
getFile(path,
Create or look up a file within the directory
options:
create (true | false)
exclusive (true | false)

removeRecursively(success, fail)
removeRecursively(success,
Delete a directory and all of its contents

File Reader
It is used to read the contents of a file

Files can be read as text or as a base64 data encoded
string

You can also abort() e file reading activity

You can register your own event listeners to receive
the following events:
loadstart, progress, load, loadend, error, abort

File Reader Example
entry.file(win, fail);

function win(file) {
var reader = new FileReader();
reader.onloadend = function(evt) {
console.log(evt.target.result);
};
reader.readAsText(file);
// reader.abort();
};

function fail(evt) {
console.log(error.code);
};

File Writer

It is used to write to a file

The data to be written must be UTF-8 encoded
UTF-

You can register your own event listeners to receive
the following events:

writestart, progress, write, writeend, error, abort

File Writer
A FileWriter is created for a single file

You can use it to write to a file multiple times
the FileWriter maintains the file's position and length
attributes, so you can seek and write anywhere in the file

By default, the FileWriter writes to the beginning of the file
(will overwrite existing data)

Set the optional append boolean to true in
the FileWriter's constructor to begin writing to the end
of the file

File Writer Methods

abort()
Aborts writing file
seek(byte)
Moves the file pointer to the byte specified.
truncate(length)
Shortens the file to the length specified.
write(data)
Writes data to the file

File Writer Example
entry.createWriter(win, fail);

function win(writer) {
writer.onwrite = function(evt) {
console.log(“ok");
};
writer.write(“Ivano Malavolta");
};

function fail(evt) {
// error management
};

Directory Reader

It is an object that lists files and directories in a
directory

it has only one method:

readEntries(success,
readEntries(success, fail)
Read the entries of the directory

Directory Reader Example
var directoryReader = dirEntry.createReader();
directoryReader.readEntries(success, fail);

function success(entries) {
var i;
for (i=0; i<entries.length; i++) {
console.log(entries[i].name);
}
}

function fail(error) {
console.log(error.code);
}

A Final Example:
Looking for a file and reading it
window.requestFileSystem(window.PERSISTENT, 0, initFS,
error);

function initFS(fs) {
fs.root.getFile(‘log.txt', {}, win, error);
}

function win(fileEntry) {
fileEntry.file(read, error);
}

function read(file) {
var reader = new FileReader();
reader.onloadend = function(e) {
console.log(this.result);
};
reader.readAsText(file);
}
function error(err) { console.log(err);}

Final Considerations
You will likely use all of them in combination

Use the right API for the right job

Web Storage
• it is not transactional race conditions
• very simple API, no schema
• only String data performance issues for complex data
due to JSON serialization
• session storage will be cleared after the app is closed
• 5Mb quota


WebSQL
• SQL-based fast and efficient
• transactional more robust
• asynchronous does not block the UI
• rigid data structure data integrity vs agility
• 5Mb quota


IndexedDB
• simple data model easy to use
• transactional more robust
• good search performance indexed data
• data is unstructured integrity problems
• 5Mb quota


File System
• not transactional
• indexing and search are not built-in
you have to implement your lookup functions
• unlimited storage
– useful for images, videos, documents, etc.

References

http://docs.phonegap.com/en/1.7.0/

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