Android app liveData viewModel

viewModel

• what?
  • It is used to store the UI-related data
  • each fragment owns itself viewModel
  • fragment will be created several times when the configurations are changed. However, viewModel just is created for once.
• How to
  • restore app state
  • add dependency in app level build.gradle implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0'
  • init viewModel in related fragment by ViewModelProvider, extract existing UI data
• recommended layer architecture
  • UI layer
    • UI element, for Views and Jetpack Compose
    • state holder, like viewModel classes, which can hold data, expose it to the UI, and handle logic.
  • Domain layer: for writing some complex business logic
  • Data layer
    • repository,
    • data sources, a file, network resource, or a local database

liveData

• What?
  • liveData is a observable data holder
    • detect wrapped data changes
    • is wrapper which can be used with any data
    • can be associated with a liecycleOwner(activity and fragment), only updates the observer in active state (STARTED and RESUMED)
    • mutableLive can allow user to edit its value rather than liveData is ReadOnly
      • liveData type value is exposed to other classes, which can restrict the using scope

data binding with viewModel and liveData

• how to integrate with viewModel
  • why?
    • it can unwrap the architecture dependency
      • Views is defined by XML layout
      • data of views is saved by viewModel
      • there is a UI controller between both of them.
    • not need to pass data to UI controller
  • operations
    • viewModel passes data to View XML by data binding directly
    • add data->variable->binding viewModel with view XML
• how to integrate with liveData
  • bind lifeCycle in Fragment class
  • set value in XML by viewModel directly
    liveData & viewModel