com.smartgwt.client.util

Class Offline

java.lang.Object

com.smartgwt.client.util.Offline

public class Offline
extends java.lang.Object

The Offline class provides features for storing and retrieving values in persistent browser storage. Because this storage is both persistent (ie, it remains available after the browser is closed and re-opened) and local to the browser, it allows applications to operate when the client is not connected to a network, albeit with some obvious limitations.

As well as providing straightforward APIs for storing, retrieving and removing values, Offline support is integrated into the normal request/response cycle. You can configure the framework so that server responses are automatically cached in Offline storage. Then, at some future point when the client is offline, responses for known requests are returned from the Offline cache.

Smart GWT makes use of various underlying storage techniques, depending on what the browser supports, but the API to a Smart GWT program is the same regardless of the underlying storage - simple key/value pairs. Currently, we provide Offline support for all HTML5-compliant browsers, plus earlier versions of Internet Explorer (6 and 7). The amount of storage available is dictated by the browser, and varies from approximately 500KB to approximately 5MB.

Constructor Summary

Constructors

Constructor and Description
Offline()

Method Summary

Methods

Modifier and Type	Method and Description
static java.lang.Object	get(java.lang.String key) Returns the value mapped to the passed-in key from browser-local storage, or null if no such key exists.
static void	goOffline() Explicitly sets this session into offline mode.
static void	goOnline() Explicitly sets this session into online mode.
static java.lang.Boolean	isOffline() Returns true if the current browser session is offline (ie, not connected to a network).
static void	put(java.lang.String key, java.lang.Object value) Stores the passed-in value in browser-local storage, mapped to the passed-in key.
static void	put(java.lang.String key, java.lang.Object value, boolean recycleEntries) Stores the passed-in value in browser-local storage, mapped to the passed-in key.
static void	remove(java.lang.String key) Removes the key/value pair mapped by the passed-in key from browser-local storage
static void	useNativeOfflineDetection() Tells the Offline system to query the browser for the current online/offline state.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Offline

public Offline()

Method Detail

goOffline

public static void goOffline()

Explicitly sets this session into offline mode. This setting will override whatever state the browser reports. This allows users to manually set an application into offline or online state.

See Also:

goOnline(), useNativeOfflineDetection()

goOnline

public static void goOnline()

Explicitly sets this session into online mode. This setting will override whatever state the browser reports. This allows users to manually set an application into offline or online state.

See Also:

goOffline(), useNativeOfflineDetection()

isOffline

public static java.lang.Boolean isOffline()

Returns true if the current browser session is offline (ie, not connected to a network). If an online/offline state has been set explicitly (see Offline.goOffline() and Offline.goOnline()), the explicitly-set state will be returned. Otherwise, the offline state as reported by the browser will be returned. See useNativeOfflineDetection for important notes on browser detection of offline state.

Returns:

true if the current browser session is offline

See Also:

goOffline(), goOnline(), useNativeOfflineDetection()

remove

public static void remove(java.lang.String key)

Removes the key/value pair mapped by the passed-in key from browser-local storage

Parameters:

key - The key to remove

See Also:

put(java.lang.String, java.lang.Object), get(java.lang.String)

useNativeOfflineDetection

public static void useNativeOfflineDetection()

Tells the Offline system to query the browser for the current online/offline state. Calling this method switches off the explicit offline mode setting switched on by calling Offline.goOnline() or Offline.goOffline().

It is important to note that browsers vary quite considerably in their ability to detect that they are offline. Many older browsers simply can't do it; HTML5 browsers expose the navigator.onLine property, but each browser's implementation is different. Some browsers have a manual "Work Offline" mode which allows the user to make the decision, and Smart GWT provides an equivalent mechanism with the goOffline and goOnline methods. Generally speaking, these methods are more reliable than allowing the browser to decide whether your application is offline.

See Also:

goOnline(), goOffline()

get

public static java.lang.Object get(java.lang.String key)

Returns the value mapped to the passed-in key from browser-local storage, or null if no such key exists.

Parameters:

key - The key to retrieve a value for

Returns:

The value mapped to the passed in key, or null if no such key exists

See Also:

put(java.lang.String, java.lang.Object)

put

public static void put(java.lang.String key,
       java.lang.Object value)

Stores the passed-in value in browser-local storage, mapped to the passed-in key. If there is no room left to store the value, we discard values from the offline store, oldest first, until there is room to store the value. If you don't want this behavior, explicitly pass false in the recycleEntries parameter.

Note that limitations in the underlying storage engines mean that only primitive values - Strings, numbers and booleans - can be stored. If you wish to store an Array or Object, you will have to serialize it to JSON first, and then eval it after retrieval to turn it back into an object.

Note: This method throws an exception if it could not store the value (either because storage is full and recycleEntries was false, or because the value to store is simply too large)

Parameters:

key - The key to use when storing the value

value - The value to store

put

public static void put(java.lang.String key,
       java.lang.Object value,
       boolean recycleEntries)

Stores the passed-in value in browser-local storage, mapped to the passed-in key. If there is no room left to store the value, we discard values from the offline store, oldest first, until there is room to store the value. If you don't want this behavior, explicitly pass false in the recycleEntries parameter.

Note that limitations in the underlying storage engines mean that only primitive values - Strings, numbers and booleans - can be stored. If you wish to store an Array or Object, you will have to serialize it to JSON first, and then eval it after retrieval to turn it back into an object.

Note: This method throws an exception if it could not store the value (either because storage is full and recycleEntries was false, or because the value to store is simply too large)

Parameters:

key - The key to use when storing the value

value - The value to store

recycleEntries - If false, suppresses the default behavior of repeatedly discarding the oldest entry if there is insufficient space to store the value

See Also:

get(java.lang.String)