Null Safety

!! Not-null Assertion //use it when you are sure the value is notnull

//throws nullpointerexception if the value is found to be null

?.let{} safe call with let

?. Safe call operator

Val name: String = null

By default kotlin not support nullable.

If we want to assign null we can use -> Val name: String? = null

?.let{} safe call with let

//1. returns the length if the name is not null else return null

//2. Name?.let{//It Executes the block only if the name is notnull

Println("")

}

?: Elvis

// when we have a nullable reference name, we can say "is name is not null",use it ,

//otherwise use some non nullable value

3. Val nameLen=if(name!=null)

Name.length

Else

-1

Or

Val leng = name?.length?: -1

Difference between DP , SP ,DPI and Pixel

DPI- Dots per Inch

DP - Density independent pixels—>it scales as per device screen density—>used to define layouts or views dimension—>pixel unit I.e independent of density of the device screen

Px- Pixel

SP: scale independent pixel—>used only for textview size—>it scales as per device screen density and user setting preferences.

px= dp*(dpi/160)

1dp equals to how many pixel for mdpi

Here mdpi = 160 pixel

         hdpi=  240 pixel

        Xhdpi = 320 pixel

xxhdpi=480 pixel

Px = 1*160/160

Px =1dp

For hdpi=1px=1*240/160=1.5px—>scale factor

Screen size measured in diagonally- 5.0’ screen

Screen resolution 720*1280 pixels

Screen density measured in DPI(Dots per Inch)

        mdpi = 160 pixel

         hdpi=  240 pixel

        Xhdpi = 320 pixel

xxhdpi=480 pixel

MVP Design Pattern

The mvp pattern separates the data model from view through presenter

This arch improves the testability of apps

0-package

MainActivity implements SigninView{

Private SigninPresenter signinPresenter;

 Oncreate()

signinPresenter= new SigninPresenter(MainActivity.this)

signinPresenter.signIn(“raja”,”raja”);

void showValidationError()

Log.e("validation","validation");

void signinsuccess()

Log.e("success","success");

void signingError();

Log.e("failure","failure");

}

--------------------------------

View-package

1.Interface SigninView{

void showValidationError();

void signinsuccess();

void signingError();

}

Presenter-package

2.interface SigninPresenter{

void signIn(String username, String password);
}

Model-package

3.class SigninPresenterImpl implements  SigninPresenter{

private SigninView signinView;

4.SigninPresenterImpl(SigninView signinView)

this.signinView=signinView;

@Override

5.void signIn(String username, String password){

if(TextUtils.isEmpty("username") || TextUtils.isEmpty("password")){

    signinView.showValidationError()

}else{

    if(userName.equalIqnorecase("raja") && password.equalIqnorecase("raja")){

        signinView.signinsuccess()

    } else{

        signinView.signinerror()

    }

}

}

mvvm sample

MVVM sample


android{

dataBinding {
enabled = true
}
}

usermodel:

public class User {

    private String username;
    private String password;
    public String usernamehint;
    public String passwordhint;


    public User() {
    }

    public User(String usernamehint, String passwordhint) {
        this.usernamehint = usernamehint;
        this.passwordhint = passwordhint;
    }
}

viewmodel:

public class UserModel extends BaseObservable {

    private String username;
    private String password;
    public String usernamehint;
    public String passwordhint;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
        notifyPropertyChanged(R.id.email);
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
        notifyPropertyChanged(R.id.pass);
    }

    public String getUsernamehint() {
        return usernamehint;
    }

    public void setUsernamehint(String usernamehint) {
        this.usernamehint = usernamehint;
    }

    public String getPasswordhint() {
        return passwordhint;
    }

    public void setPasswordhint(String passwordhint) {
        this.passwordhint = passwordhint;
    }

    public UserModel(User user) {
        this.usernamehint =user.usernamehint;
        this.passwordhint =user.passwordhint;
    }
}



interface:

public interface UserLogin {

    void onClickLogin();
}

activity:

private ActivityMainBinding activityMainBinding;
    @Override    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.activity_main);        activityMainBinding = DataBindingUtil.setContentView(this,R.layout.activity_main);
        UserModel um =new UserModel(new User("User Name","Password"));
        activityMainBinding.setLogin(um);

        activityMainBinding.setOnClickButtion(new UserLogin() {
            @Override            public void onClickLogin() {
                Toast.makeText(getApplicationContext(),""+activityMainBinding.getLogin().getUsername(),Toast.LENGTH_LONG).show();
            }
        });


    }

layout:

<?xml version="1.0" encoding="utf-8"?><layout  xmlns:android="http://schemas.android.com/apk/res/android"         xmlns:tools="http://schemas.android.com/tools"         xmlns:app="http://schemas.android.com/apk/res-auto">


<data>
    <variable            name="Login"            type="mvvm.mvvmsample.viewmodel.UserModel">
    </variable>
    <variable            name="onClickButtion"            type="mvvm.mvvmsample.commands.UserLogin">
    </variable>

</data>

<LinearLayout        android:layout_width="match_parent"        android:layout_height="match_parent"        android:orientation="vertical"        android:gravity="center"        android:layout_margin="5dp"        android:padding="5dp"        tools:context=".MainActivity_">

    <EditText            android:id="@+id/email"            android:layout_width="match_parent"            android:layout_height="wrap_content"            android:gravity="center"            android:text="@={Login.username}"            android:hint="@{Login.usernamehint}"
           />
    <EditText            android:id="@+id/pass"            android:layout_width="match_parent"            android:layout_height="wrap_content"            android:gravity="center"            android:text="@={Login.password}"            android:hint="@{Login.passwordhint}"
    />
    <Button            android:layout_width="match_parent"            android:layout_height="wrap_content"            android:background="@color/colorAccent"            android:text="Login"            android:onClick="@{(v)-> onClickButtion.onClickLogin()}"    />

</LinearLayout>

Retrofit Call In Android:Step by Step Implementation

Retrofit Call In Android:

url: https://jsonplaceholder.typicode.com/posts

1.manifest

<uses-permission android:name="android.permission.INTERNET"/>

2.gradle

dependencies {

implementation 'com.squareup.retrofit2:retrofit:2.5.0'implementation 'com.squareup.retrofit2:converter-gson:2.5.0'}


3.model


public class Post {

    private int userId;
    private int id;
    private String title;
    @SerializedName("body")
    private String textBody;

    public int getUserId() {
        return userId;
    }

    public int getId() {
        return id;
    }

    public String getTitle() {
        return title;
    }

    public String getTextBody() {
        return textBody;
    }
}

4.interface for 
import java.util.List;

import retrofit2.Call;
import retrofit2.http.GET;

public interface JsonPlaceHolderApi {
    @GET("posts")
Call<List<Post>> getPosts();
}

5. mServiceCall()---onCreate()
textview = (TextView) findViewById(R.id.text_result);

private void mServiceCall() {

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://jsonplaceholder.typicode.com/")
            .addConverterFactory(GsonConverterFactory.create()).build();

    JsonPlaceHolderApi jsonPlaceHolderApi= retrofit.create(JsonPlaceHolderApi.class);
    Call<List<Post>> call=jsonPlaceHolderApi.getPosts();
    call.enqueue(new Callback<List<Post>>() {
        @Override        public void onResponse(Call<List<Post>> call, Response<List<Post>> response) {
            if(!response.isSuccessful()){
                textview.setText("code:"+response.code());
            }
            List<Post> posts=response.body();
            for(Post post:posts){
                String content ="";
                content+="ID:"+post.getId()+"\n";
                content+="Text Body:"+post.getTextBody();
                textview.append(content);


            }
        }

        @Override        public void onFailure(Call<List<Post>> call, Throwable t) {

        }
    });
}