The Local Storage plugin can store data locally on the user's device. For example it can be used to store a personal best high-score. Note if you want a full-state save and load feature, see How to make savegames.
Local Storage works offline, since it stores data to the device itself. Different browsers use different storages, so data is not shared between different browsers on the same computer - each browser has its own unique storage. This is also separate to the browser cache, which is temporary storage to avoid needlessly re-downloading the same files over and over again. However the user can usually still choose to clear local storage data from their browser (with an option perhaps named something like "clear offline website data"). Non-browser export options like NW.js, Cordova or Windows Store apps are not affected by the user clearing any amount of data from any browser, they also use separate storage. Finally for security reasons browsers use separate storage per domain. For example all content on scirra.com shares the same storage, but content on facebook.com uses a different storage and cannot access any data saved from scirra.com.
To prevent abuse, most browsers implement a storage quota, which is a maximum amount of data that can be saved locally. On most modern browsers this is at least 50mb. If the quota is exceeded, the On error trigger will fire.
Using Local Storage
Local Storage uses a very simple storage system: values are stored under named keys, similar to how the Dictionary object works. For example the value 100 could be stored under a key named score.
Local Storage is asynchronous. This means reading and writing data does not complete immediately. The actions only start the process of reading or writing a value, and the game continues to run in the interim. This ensures that slow or busy storage systems do not impact the performance of the game. When the read or write is complete, a trigger fires (On item get or On item set) which indicates either the value is available to read (with the ItemValue expression) or that the value was successfully written.
For example here is a flow to read the value of the key "score":
- Use the action Get item "score"
- A moment later, On item "score" get triggers
- In this trigger, use the ItemValue expression to read the item
Here is a flow to set the new value of "score":
- Use the action Set item "score" to 100
- A moment later, On item "score" set triggers
- You may not need to do anything in this trigger, but it indicates the value has been successfully written. The Key and ItemValue expressions are still set in this trigger in case you need them.
Note that you must be careful to avoid "races" when using asynchronous storage. For example the Clear storage action may take a moment to complete before it fires On storage cleared. It is possible to write more values to storage in between, while Clear storage is still processing. This is like "racing" the Clear storage and Set item actions: the end result depends on what order they complete in, which is unpredictable. In this case it is more or less random what happens: the written keys may be cleared, or they may not be - you cannot rely on any specific result. Therefore you should be careful to avoid this case.
Simplifying usage with Dictionary
Although it improves performance, dealing with asynchronous reads and writes can sometimes be difficult. One simple way to conveniently have synchronous storage is to store an entire Dictionary object's contents to Local Storage, by saving its AsJSON string. Then you can load this content from Local Storage with the Load action. This means only saving and loading the dictionary contents is asynchronous, and the rest of the time you can use the Dictionary object's features to synchronously access data, such as simply using its Get expression to immediately read a value. However you must remember to save the Dictionary again at some point before the user quits the game.
Local Storage conditions
- On any item get
- Triggered after any Get item action completes.
- On any item removed
- Triggered after any Remove item action completes.
- On any item set
- Triggered after any Set item action completes.
- On item exists
- Triggered after the Check item exists action completes if the key checked does indeed exist. In this trigger the ItemValue expression is also set to the value of this key, so there is no need to use another Get item action to read it.
- On item get
- Triggered after a Get item action completes for a given key. The ItemValue expression is set to the value of the key.
- On item missing
- Triggered after the Check item exists action completes if the key checked does not exist.
- On item removed
- Triggered after the Remove item action completes for a given key.
- On item set
- Triggered after the Set item action completes for a given key. This indicates the data is now in storage.
- Compare key
- Compare the current value of the Key expression, which is the name of the current key in a trigger. This can be useful in the On any item... triggers.
- Compare value
- Compare the current value of the ItemValue expression, which is set to the item value when getting an item or in On item exists.
- On all key names loaded
- Triggered after the Get all key names action completes. In this trigger the KeyCount and KeyAt expressions give the list of all the key names.
- On error
- Triggered at any time while using Local Storage if an error occurs, such as if a write failed, or the maximum storage quota was exceeded. The ErrorMessage expression is set to the error message if available.
- On storage cleared
- Triggered after the Clear storage action completes and storage is now empty.
- Is processing gets
- True if any Get item actions are still processing, i.e. any On item get trigger is yet to fire for a Get item action.
- Is processing sets
- True if any Set item actions are still processing, i.e. any On item set trigger is yet to fire for a Set item action.
- On all gets complete
- Triggered when all outstanding Get item actions are completed, i.e. when Is processing gets first becomes false. In other words if 10 Get item actions are all used at the same time, On all gets complete triggers when all 10 items have been read and fired their On item get triggers.
- On all sets complete
- Triggered when all outstanding Set item actions are completed, i.e. when Is processing sets first becomes false. In other words if 10 Set item actions are all used at the same time, On all sets complete triggers when all 10 items have been written and fired their On item set triggers.
Local Storage actions
- Check item exists
- Check if a key exists in storage. This triggers either On item exists if the key exists, or On item missing if the key does not exist. If the item exists, the ItemValue expression is set to the key value in the On item exists trigger, so there is no need to use a subsequent Get item action to read the value.
- Get item
- Read the value of a key in storage. This triggers On item get when the value has been read, where the ItemValue expression is set to the value of the key.
- Remove item
- Remove (delete) a key from storage. This triggers On item removed when the key has been removed.
- Set item
- Set the value of a key in storage. This triggers On item set when the value has been written.
- Clear storage
- Remove (delete) all items from storage, reverting it back to the empty state. This triggers On storage cleared when completed.
- Get all key names
- Retrieve a list of all the key names that currently exist in storage. This triggers On all key names loaded when the list has been loaded, where the KeyCount and KeyAt expressions can be used to access the list.
Local Storage expressions
- The value for a key that has been read or written in any Local Storage trigger, such as On item get or On item exists. This can be either a string or a number. This returns 0 if used outside of a Local Storage trigger.
- The name of the key that was modified in any Local Storage trigger, such as On item get, On item set or On any item set. This returns an empty string if used outside of a Local Storage trigger.
- In On error, the text of the error message if any is available.
- In On all key names loaded, the name of the key at the given zero-based index in the list.
- In On all key names loaded, the number of key names in the list.