Android Back Stack, Tasks, and Launch Modes

In Android, managing screen navigation and app flows effectively depends on understanding the backstack, tasks, and launch modes. Here’s a breakdown to help you use these concepts efficiently.

Back Stack:

The back stack holds activities in the order they’re opened. When the user presses the back button, the topmost activity is removed, and the previous one is shown.

Example: In a shopping app, if the user navigates Home > Product Details > Cart, the back stack looks like:

Home => Product Details => Cart        

Pressing back removes Cart, returning to Product Details.

Note: The back stack reflects only the current task. Switching to another app doesn’t affect the order until the user returns.

Tasks:

A task is a collection of screens (activities) that work together to accomplish a user goal. Each task has its own back stack and can hold activities from multiple apps.

Example: A task in a shopping app may have Home > Product Details > Cart. If the user clicks a link to a browser (Task B), Task A’s back stack pauses until the user returns, where they left off at Cart.

Task A => [Home , Product Details , Cart
Task B => [Browser]        

Launch Modes:

Launch modes define how activities are created and managed within a task. Launch Modes are Controlling Activity Creation and Navigation.

Let's see the launch mode types:

1- standard: Every call creates a new instance.

Example: Navigating Home > Profile > Home results in two Home instances:

Home => Profile => Home        

2- singleTop: Reuses the existing instance if it’s already at the top.

Example: If Notifications is on top and singleTop is enabled, clicking Notifications again won’t create a new instance, preserving its state.

?  Home => Profile => Notifications => Notifications
?  Home => Profile => Notifications        

Note: Use singleTop to prevent duplicate activities only if the target is already on top.

3- singleTask: Reuses an existing instance, bringing it to the front and clearing anything above it so, it brings the entire task to the foreground instead of creating a new instance.

Example: If Notifications exists deep in the stack, clicking it again will bring it to the front and remove any activities above it

Before Navigate:
Home => Notifications =>  Profile => Settings
After Navigate:
Home => Notifications        

Use singleTask to reuse an activity in the current task and clear activities on top.

4- singleInstance: Launches in a separate task, ensuring it’s the only activity there. It frequently used in places need more security like payment activity

Mastering these principles helps ensure a smooth, predictable user experience, optimizing both performance and navigation flow in your app and offering flexibility for managing complex navigation and user workflows in Android. Each choice impacts how your app behaves and feels, so understanding these principles ensures better user experiences.