LocalStorage

Local storage is a mechanism to store data in the browser. It allows the game to save data locally.

In Messenger, local storage content is defined by a String. When users need to save something to local storage, they need to first serialize it (for example, use Json).

Users can read or write to local storage by editing globalData.userData and the data type in local storage is defined in Lib/UserData.elm. However, not all things in UserData are needed to store to local storage.

Users will need to implement initGlobalData and saveGlobalData functions. They act as a decoder and encoder for global data.

initGlobalData is called when the game starts. Its type is:

initGlobalData : String -> GlobalDataInit UserData

The string it needs is the real local storage data. Users will use the local storage data to initialize writable global data and initial volume.

GlobalDataInit contains camera, volume, extraHTML, canvasAttributes, and userData. Runtime fields such as timestamps, pressed keys, current scene, loaded resources, and mouse position are initialized and maintained by Messenger.

See Global Data and Getters for the exact writable fields and read-only getters.

saveGlobalData is called when the user wants to save the global data (emitted by a SOMSaveGlobalData message). Users may encode some part of global data.

saveGlobalData : Runtime -> GlobalData UserData -> String

saveGlobalData receives the current read-only Runtime and the full writable GlobalData. If you need engine-owned values while saving, read them from runtime with getters.

tip

Also see tetris example to learn how to use localStorage in Messenger. The userdata is defined at UserData.elm.