Android data observer mode and LiveData life cycle
Android data observer mode and LiveData life cycle
introduce
In Android development, a common design mode is an observer mode.Observer mode allows one object (referred to as an observer) to subscribe to the state changes in another object (called the observer) and receive notifications in state changes.LiveData is a response programming tool introduced in the Android Jetpack component library. It combines the observer mode and life cycle perception to provide response and life cycle security.
This article will discuss the observer mode in Android and the life cycle of LiveData, and provide Java code examples to illustrate their applications.
Part 1: Observer mode
Observer mode is a behavioral design model. By establishing a one -to -many relationship, it allows multiple observer objects to monitor and receive the state changes in the object of the observed object at the same time.
In Android, the observer mode is often used to achieve data observation and update.A common scenario is that when a data source changes, all observers need to be notified to update the data display.Here are several important roles in the observer mode:
-Oservable: The observer is an observed object that is responsible for maintaining a list of observer.When the state of the observer changes, all observers will be notified.
-Catisher (Observer): Observer is an interface or abstract class that defines the operation of the observer, including receiving the notification of the observer, updating status, etc.
-Sapient Observer: Specific observers realize the observer interface, which receives notifications by registering to the observer and executes the corresponding logic.
The following is a simple sample code to demonstrate the usage of the observer mode:
interface Observer {
void update(String data);
}
class Observable {
private List<Observer> observers = new ArrayList<>();
void addObserver(Observer observer) {
observers.add(observer);
}
void removeObserver(Observer observer) {
observers.remove(observer);
}
void notifyObservers(String data) {
for (Observer observer : observers) {
observer.update(data);
}
}
}
class ConcreteObserver implements Observer {
@Override
void update(String data) {
// Execute the logic of the observer
}
}
// When using an observer mode, instantiated observer and observer
Observable observable = new Observable();
Observer observer1 = new ConcreteObserver();
Observer observer2 = new ConcreteObserver();
// Register the observer to the observer
observable.addObserver(observer1);
observable.addObserver(observer2);
// When the data changes, notify the observer update
observable.notifyObservers("New data");
Part 2: Livedata Life cycle
LiveData is a response programming tool in the Android Jetpack library. It has a life cycle perception and can automatically handle life cycle management so that data updates are correctly matched with UI.
LiveData provides a implementation of an observer mode that allows observer (usually Activity or Fragment) to subscribe to the LiveData object and receive notifications when data changes.Livedata objects will notice the life cycle of the observer and distribute the latest data when the observer is in activity.When an observer is in a non -active state, LiveData automatically stops distributing data to avoid memory leakage.In this way, LiveData ensures that observer only receives valid data for it.
Here are several core concepts of LiveData:
-Livedata: LiveData is an abstract class that is used for packaging data and provides the implementation of the observer mode.The LiveData object can notify the data update when the observer is in an active state.
-OBSERVER: Observer is an interface that is used to receive data update for receiving LiveData.In Android, the implementation class of the Oolver interface is usually used to observe the data of LiveData.
-Lifecycleowner: LifecycleOWner is an interface that indicates that the life cycle object (such as Activity or Fragment).LiveData will automatically distribute data based on the life cycle of LifeCycleowner.Observer can be registered to LifecycleOWner through LiveData's Observation method.
The following is a simple sample code to demonstrate the usage of LiveData:
public class MyActivity extends AppCompatActivity {
private MyViewModel myViewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Initialize ViewModel
myViewModel = new ViewModelProvider(this).get(MyViewModel.class);
// Use Observe method to register Observer to livedata
myViewModel.getData().observe(this, new Observer<String>() {
@Override
public void onChanged(String newData) {
// When the livedata data is updated, execute this callback
// Update UI or execute other logic
}
});
}
}
public class MyViewModel extends ViewModel {
private MutableLiveData<String> data;
public LiveData<String> getData() {
if (data == null) {
data = new MutableLiveData<>();
// Load the data and assign it to LiveData
loadData();
}
return data;
}
private void loadData() {
// Load data in the background thread
// After loading, you can use the SetValue or PostValue method to distribute the value of LiveData
data.setValue("New data");
}
}
In the above example, Activity has registered an Observer to observe the LiveData object in ViewModel.When the value of LiveData changes, Observer's onChanged callback will be triggered, and you can update the UI or execute other logic here.
Summarize
Through the observer mode and the life cycle perception of Livedata, we can achieve data observation and update well.Observer mode provides us with a simple and effective way to handle changes in state, while LiveData provides us with a data observation mechanism for life cycle perception.The combination of these modes and tools brings better maintenance and code quality for Android development.
