Android App Components Activities, Fragments, and Intents

Activities :
=========
Activity is one of the most important component for any android app

Activities are the User Interface (UI) screens which user see.
It is similar to the main() function in different programming languages.

Its is the main entry point for user interaction.
You can have multiple activities in your app.
All your activities must be declared in the manifest file, with their attributes.

Every activity has different functions throughout its life, onCreate(), onStart(), onResume(), onPause(), onStop(), onRestart(), onDestroy().

To create activity we can use two methods either we create class file and xml file and include the class in manifest file. ---- method 1 

or we can create activity using empty activity like this.. --- method 2

note:- for using method 1 we must include our class file in manifest file  otherwise it will show error in logcat.

okay now let us see the activity life cycle.
let us program the life cycle of activity in our project.

okay function is coded let us see the result.

when our app is launched the method which were invoked are:-

- on create
- 0n start
- on resume

okay now when we press back button the method which were invoked are:-

-on pause
-on stop
-on destroy
 okay now let us start to learn fragments


Android Activity Lifecycle is controlled by 7 techniques for android.app.Activity class. The android Activity is the subclass of ContextThemeWrapper class.







You should always call up to your superclass when implementing these methods.


public class Activity extends ApplicationContext {

     protected void onCreate(Bundle savedInstanceState);

     protected void onStart();

     protected void onRestart();

     protected void onResume();

     protected void onPause();

     protected void onStop();

     protected void onDestroy();

 }

Activity Lifecycle

•  Lifetime:
  • onCreate() to
  •  onDestroy()

•  Visible when:
  •  onStart() to
  •  onStop()

•  Foreground
  •  onResume to
  •  onPause()

 Android Activity Lifecycle Example

package com.example.activitylifecycle;  
import android.os.Bundle;  
import android.app.Activity;  
import android.util.Log;  
import android.view.Menu;  
public class MainActivity extends Activity {  
    @Override  
    protected void onCreate(Bundle savedInstanceState) {  
        super.onCreate(savedInstanceState);  
        setContentView(R.layout.activity_main);  
        Log.d("lifecycle","onCreate invoked");  
    }  
    @Override  
    protected void onStart() {  
        super.onStart();  
         Log.d("lifecycle","onStart invoked");  
    }  
    @Override  
    protected void onResume() {  
        super.onResume();  
         Log.d("lifecycle","onResume invoked");  
    }  
    @Override  
    protected void onPause() {  
        super.onPause();  
         Log.d("lifecycle","onPause invoked");  
    }  
    @Override  
    protected void onStop() {  
        super.onStop();  
         Log.d("lifecycle","onStop invoked");  
    }  
       @Override  
    protected void onRestart() {  
        super.onRestart();  
         Log.d("lifecycle","onRestart invoked");  
    }     
    @Override  
    protected void onDestroy() {  
        super.onDestroy();  
         Log.d("lifecycle","onDestroy invoked");  
    }  

}



Fragments :
==========

In most of the applications these days, fragments are largely used.
As there are a lot of android devices with different resolutions, its a bit tough to handle all of those, that’s where fragments come handy. We can combine 2 or more fragments and show them in an activity.

A Fragment is a component that is used by an activity.
Even though it is used by an activity, it has its own lifecycle.







Intents :
===========
Intent is one of the most important and most used app component of an android application.

Using Intents, you call to other app components or to other activity or also call other applications on your phone.

Intents are two types:

Explicit Intents where you call another activity or something with a class name. For instance, you can call another activity when some action happened in one activity. So you here explicitly specifies which activity to call.


Implicit Intents where we do not specify a class name but specify some sort of action, which can be handled by some other inbuilt apps or some other apps. For instance, you may want to open a camera, showing a map, sending emails etc. Here you don’t directly call camera app or map app, you will just specify the action.

Intent i = new Intent(FirstActivity.this, SecondActivity.class) ;

startActivity(i);