Android App Activities

Activity Basics

• An app consists of activities, layouts, and resources, with the main activity serving as the app's primary interface.

Process Management

• By default, each app operates in its own process, enhancing security and stability.
• The system can terminate processes to free memory if resources are strained.

Activity Launch

• When an activity starts, Android checks for an existing process for the app.
• If a process is found, the activity runs within it; otherwise, a new process is created.
• The onCreate() method is always called when an activity is created.

UI Interaction

• Activities interact with the user interface using setContentView(View).

Activity Functions

• An activity provides the window for app UI rendering.
• Generally, a single activity corresponds to one screen.

Activity Registration

• Activities must be registered in the app's manifest and managed with appropriate lifecycles.

Lifecycle States

• Lifecycle states and callbacks are specific to each activity, enabling distinct behaviors at different lifecycle stages.

Activity Stack

• The runtime system maintains an Activity Stack for each running application.
• When an application is launched, the first activity is placed on the stack.
• Launching another activity places it on the stack, pushing the previous activity down.

Android Activity Lifecycle Methods

• onCreate(): Called when the activity is first created, providing a Bundle with the activity's saved state and calls onStart().
• onRestart(): Called when an activity is stopped but is just about to be started again, calling onStart().
• onStart(): Called when the activity becomes visible, calling onResume() or onStop().
• onResume(): Called when the activity is in the foreground, calling onPause().
• onPause(): Called when the activity is no longer in the foreground, typically when another activity is resuming, calling onResume() or onStop().
• onStop(): Called when the activity is no longer visible, calling onRestart() or onDestroy().
• onDestroy(): Called when the activity is about to be destroyed or is finishing, with no subsequent method calls.

Activity Pauses

• onPause() is called when an event interrupts app execution, like a phone call, navigation to another activity, or the screen turning off.
• In Android 7.0 (API level 24) or higher, onPause() is called when multiple apps run in multi-window mode.
• When a new, semi-transparent activity (like a dialog) opens, the activity remains paused as long as it is partially visible but not in focus.

Activity Stops

• onStop() is called when a newly launched activity covers the entire screen, or the user navigates to the device's home screen.

Activity Destroys

• onDestroy() is called when the user completely dismisses the activity or the system temporarily destroys the activity due to a configuration change.
• Examples of configuration changes are device rotation or language change.

Implementing a Stop Watch Activity

• The layout can be implemented using Linear Layout, Relative Layout, or Constraint Layout.

Handler

• A Handler is used to schedule code to run in the future.
• The code to be scheduled is wrapped in a Runnable object.

Handler use

• Handlers use the Post() and postDelayed() methods to specify when the code should run.
• The post() method posts code to be run as soon as possible.
• postDelayed() posts code to be run in the future.

Context Interface

• Context serves as an interface to global application information, providing access to resources, classes, and operations

Activity Class

• The Activity class implements default lifecycle methods and defines methods like findViewById(Int) and setContentView(View).

Activity States

• The main state of an activity is when it's running or active, in the foreground, has focus, and is interactive.
• The onCreate() method is immediately called after launching, so normal setups such as calling setContentView() can occur.
• The onDestroy() method is the final call before an activity is destroyed, whether it's told to finish, due to device configuration, or to save space.

Save Activity State

• Device configuration changes cause the loss of local variables.
• The onSaveInstanceState() method is implemented to store values before the activity is destroyed.

onSaveInstanceState()

• The onSaveInstanceState() method takes a Bundle to store data.

Lifecycle Visibility Methods

• The additional lifecycle methods: onStart(), onStop(), and onRestart().
• Inherit from Android Activity class like onCreate() and onDestroy().

Visibility Methods

• onStart() is called when your activity becomes visible.
• onStop() is called your activity has stopped being visible.
• onRestart() is called after your activity has been made invisible but before it gets made visible again.

Implement onStop():

• A new variable records the stopwatch state before onStop().
• This determines if running is reset when the activity is visible again.

Implement onPause()

• When an activity is visible but out of focus, it is Paused.
• This occurs when another activity appears on top of your activity that isn’t full-size or that’s transparent.
• Properly using the Activity Life cycle to avoid consuming unnecessary resources.

App Performance

• The Activity Life cycle avoids losing the user's progress and prevents crashing.
• Avoid the user’s progress when the screen rotates between landscape and portrait orientation