Say goodbye to the Activity life cycle and welcome the Compose life cycle

Activity Lifecycle VS Compose Lifecycle

As an Android application developer, you know that managing the Activity lifecycle is an important part of building high-quality applications.

At first, I was skeptical about Compose Lifecycle. I've been using Activity Lifecycle for years and it has worked fine for me. But after trying out Compose, I was blown away by how much easier it made my life as a developer

In Activity Lifecycle, there are many lifecycle methods that need to be managed, and it can quickly become complicated. But with Compose, the lifecycle is simpler, and the framework handles many of the details of managing UI components.

In this blog post, we'll explore the advantages of Compose Lifecycle and why developers should consider using it instead of traditional Activity Lifecycle.

Recall the life cycle of Activity

The life cycle of an Activity in Android is the series of states that an Activity can go through when it is created, used, and destroyed. The Android system manages the life cycle of an Activity and calls different methods on the Activity at different points in its life cycle.

The life cycle of Activity is as follows:

onCreate(): This method is called when the Activity is first created. This is where you should perform initialization work, such as setting up the UI, initializing variables, or reading data from the database.
onStart(): This method is called when the Activity is visible to the user, but before the user can interact with it.
onResume(): This method is called when the Activity is ready for user interaction. This is where you should start animations, play audio, or anything else that requires user interaction.
onPause(): This method is called when the Activity loses focus, such as when the user receives a phone call or opens another application. This is where you should pause animations, stop audio, or anything else that requires stopping user interaction.
onStop(): This method is called when the Activity is no longer visible to the user. You should release resources here, such as closing database connections or stopping background services.
onRestart(): This method is called when the Activity is being restarted, such as when the user presses the back button.
onDestroy(): This method is called when the Activity is destroyed, such as when the user navigates away from the Activity or when the system needs to release resources.

By understanding an activity's lifecycle, you can effectively manage its resources and ensure your app provides the best possible user experience.

Suppose you are building a photo gallery application that allows users to view and manage their photos. When users open the app, the first screen they see is the gallery screen, which shows all their photos. Here's what the Activity lifecycle does in this case:

onCreate(): In this method, you initialize UI components for the gallery screen, such as a RecyclerView or a ListView that display photos. You'll also initialize any data structures or variables needed to manage photos.
onStart(): This method is called when the gallery screen is visible to the user, but before the user can interact with it. In this method, you'll start any necessary background processes, such as loading thumbnails of photos from disk or over the network.
onResume(): This method is called when the gallery screen is ready for user interaction. In this method, you'll start any animations, such as a slideshow of photos, or enable touch gestures to allow users to swipe photos.
onPause(): This method is called when the gallery screen loses focus, such as when the user receives a phone call or opens another application. In this method, you pause any animations or background processes, such as stopping a slideshow or canceling any network requests.
onStop(): This method is called when the gallery screen is no longer visible to the user. In this method, you release any resources, such as closing database connections or stopping background services.
onRestart(): This method is called when the gallery screen restarts, for example when the user navigates back to the gallery screen after viewing a photo. In this method, you resume any background processes or animations that were paused in the onPause() method.
onDestroy(): This method is called when the gallery screen is destroyed, such as when the user navigates away from the app or when the system needs to release resources. In this method, you release any remaining resources, such as unregistering event listeners or freeing memory.

Jetpack Compose Lifecycle

Jetpack Compose is a new way to build UI components in Android applications. In traditional Android application development, UI components are created using XML and Java/Kotlin code. The lifecycle of these UI components is managed by the Android system through Activity or Fragment lifecycle callbacks.

In contrast, the Jetpack Compose UI toolkit uses a declarative programming model to define UI components. Developers define UI components using Composable functions , which are functions annotated with annotations. ComposableWhen a function is called , it describes the UI component it represents, rather than building it directly.

The Jetpack Compose UI toolkit manages the lifecycle of UI components in a similar way to the traditional Android lifecycle, with some differences.

The Jetpack Compose lifecycle consists of three main phases:

composition (creation)
Reorganization (Update)
deal with. (Destroyed)
Composite : When a UI component is first created, it enters the compositing phase. At this stage, the Compose runtime calls the Composable functions that define the UI components. Composable functions describe the structure and behavior of UI components.

Reorganization : A UI component can be reorganized whenever the data it depends on changes. During the recompose phase, the Compose runtime checks to see if the UI components need to be updated, and if so, it calls the Composable function again. Composable functions can update UI components by modifying their properties or structure.

Disposal : When a UI component is no longer needed, it enters the disposition phase. During this phase, the Compose runtime removes the UI component from the UI hierarchy and invokes any cleanup logic registered for that component.

The Jetpack Compose framework in Android provides a new life cycle for UI components, which is different from the Activity life cycle. Here are some key differences between the two:

Initialization: In the Activity life cycle, UI components are usually initialized in the onCreate() method. Instead, in the Compose lifecycle, UI components are initialized in a method setContent()that is called when the Composable is first inflated.
Update: During the Activity lifecycle, UI components are updated based on events or changes in application state. In contrast, in the Compose lifecycle, UI components are recomposed whenever input data or state changes. This means that composables are more reactive and can be updated more frequently and efficiently than traditional UI components.
Separation of concerns: The Compose lifecycle separates the concerns of UI composition and UI logic, whereas in the Activity lifecycle these concerns are often mixed together. This can make code in Compose cleaner and more maintainable, as UI logic can be separated into different composables and reused across different screens or components.
Simplicity: The Compose lifecycle is simpler and more streamlined than the Activity lifecycle. In Compose, there are fewer lifecycle methods to manage, and the framework takes care of many of the details of UI components. This makes it easier to develop and maintain Compose-based UIs.
Easier testing: Finally, Compose makes testing easier because it provides better support for unit testing. With Compose, developers can test individual UI components in isolation, which makes finding and fixing bugs much easier.

Lifecycle example

A real-world example of a Compose lifecycle might be a weather application that displays the current temperature, weather conditions, and other relevant information. In this case, the composable function will create UI elements based on current weather data such as temperature, humidity, and wind speed.

As the weather changes, the Compose framework will update UI elements to reflect the new data. For example, if the temperature drops, Composable the function and the UI element is updated to show the new temperature.

Finally, when the user closes the application, Composablethe function is no longer needed and the UI elements are destroyed.

Here's an example of a simple composable function that displays a text message and updates it when a button is clicked:

@Composable 
fun  ComposeLifecycleExample () {
    
     
    var message by remember {
    
     mutableStateOf( "Hello, World!" ) }     Column {
    
     
        Text(text = message) 
        Button(onClick = {
    
     message = "Button clicked!" }) {
    
     
            Text(text = "点击我" ) 
        } 
    } 
}

In this example, the Composable function creates a ColumnText element that contains a Text element and a ButtonText element. The messagevariable is mutableStateOfdeclared with a function that allows the Composable function to update the value of the UI messageand trigger a reorganization of the UI.

When the Button is clicked, onClickthe callback is triggered and the value of is updated messageto "Button clicked!". As a result, the Text element is reassembled into the new message text.

This is a simplified example, but it demonstrates the basic concept of how the Compose lifecycle works. When the Composable function is called for the first time, an initial message value of "Hello, World!" is displayed. When the button is clicked, the message value is updated and the UI is reassembled with the new message text

in conclusion

All in all, Compose Lifecycle is a powerful tool that can make Android application development easier and more efficient. By simplifying the lifecycle, making the UI more reactive and efficient, and separating the concerns of UI composition and UI logic, Compose allows developers to focus on writing high-quality code that is easier to test and maintain. So, if you're an Android app developer, consider giving Compose Lifecycle a try and see how it can benefit your projects.