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Understanding ArkUI Component Lifecycle: From Creation to Destruction

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In this article we will learn about HarmonyOS Next lifecycle

Introduction

HarmonyOS’s ArkUI framework offers a declarative UI paradigm for building modern, responsive applications. To develop efficient and well-performing applications in this environment, developers must thoroughly understand the lifecycle of custom components — from their initial rendering to their eventual removal from the view hierarchy.

Description

Component lifecycle refers to the sequence of phases a UI component goes through from its creation until its destruction. In ArkUI, custom components follow a specific lifecycle with defined callback methods that allow developers to execute code at precise moments during a component’s existence. Understanding these lifecycle events enables proper resource management, state handling, and performance optimization.

The ArkUI component lifecycle consists of several key phases including initialization, mounting to the display, updating when data changes, and unmounting when removed from the view. Developers need to know how to leverage these lifecycle callbacks to implement appropriate behaviors at each stage.

ArkUI provides several lifecycle callback methods for custom components that developers can implement:

aboutToAppear()

  • When it’s called: After the component is created but before its first rendering
  • Purpose: Initialize component state, subscribe to data sources, or prepare resources
  • Example: 
aboutToAppear(): void {
  console.log('Component is about to appear');
  this.timer = setInterval(() => {
    this.count++;
  }, 1000);
}
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aboutToDisappear()

  • When it’s called: Before the component is destroyed or removed from the display
  • Purpose: Clean up resources, unsubscribe from data sources, cancel timers
  • Example: 
aboutToDisappear(): void {
  console.log('Component is about to disappear');
  clearInterval(this.timer);
}
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onPageShow()

  • When it’s called: When a page containing the component becomes visible
  • Purpose: Resume activities or update data when the page becomes active
  • Example: 
onPageShow(): void {
  console.log('Page containing the component is shown');
  this.updateData();
}
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onPageHide()

  • When it’s called: When a page containing the component is hidden
  • Purpose: Pause activities or save state when the page becomes inactive
  • Example: 
onPageHide(): void {
  console.log('Page containing the component is hidden');
  this.saveCurrentState();
}
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onBackPress()

  • When it’s called: When the user presses the back button
  • Purpose: Customize back navigation behavior
  • Return value: Return true to indicate the component handled the event; false to let the default system behavior proceed
  • Example: 
onBackPress(): boolean {
  if (this.hasUnsavedChanges) {
    this.showConfirmDialog();
    return true; // Component handled the back press
  }
  return false; // Let system handle the back press
}
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Key Takeaways

  1. Initialization Phase: Use aboutToAppear() to set up your component, initialize variables, and start processes.
  2. Cleanup Phase: Always implement aboutToDisappear() to clean up resources, prevent memory leaks, and cancel any subscriptions or timers.
  3. Page Visibility: Leverage onPageShow() and onPageHide() to optimize resource usage when your component is not visible.
  4. Back Navigation: Customize back button behavior with onBackPress() when needed, but return false for standard behavior.
  5. Performance Considerations: Avoid heavy operations in lifecycle callbacks, especially aboutToAppear(), to ensure smooth UI rendering
  6. Execution Order: Remember that lifecycle methods execute in a specific order:
  • Creation: aboutToAppear()
  • Page visibility changes: onPageShow() and onPageHide()
  • Destruction: aboutToDisappear()
  • Destruction: aboutToDisappear()

Conclusion

That wraps up our deep dive into the HarmonyOS Lifecycle. With this knowledge, you’re now better equipped to build responsive, efficient, and user-friendly HMOS applications. Mastering these lifecycle events is key to managing resources wisely, maintaining a seamless user experience, and reacting adeptly to system changes.

References

Custom Component Lifecycle

https://forums.developer.huawei.com/forumPortal/en/topic/0203189979925825041

Written by Mücahid Kıncır

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