Android MVVM LiveData Data Binding 在Android中的应用

我们已经使用Data Binding实现了MVVM，并在单独的教程中介绍了LiveData和Data Binding。今天，我们将在我们的MVVM Android应用程序中使用LiveData与Data Binding。我们将看到LiveData如何使得从ViewModel更新UI变得容易。

MVVM （Model-View-ViewModel）LiveData（实时数据）Data Binding（数据绑定）

到目前为止，我们一直在使用数据绑定来从ViewModel更新视图。LiveData是一个方便的数据容器，作为要传递的数据的容器。LiveData最好的是它具有生命周期感知功能。所以如果您在后台，界面就不会尝试更新。这样可以避免很多运行时崩溃。我们将使用MutableLiveData类，因为它提供了公共方法setValue()和getValue()。让我们使用上述概念创建一个简单的登录应用程序。首先，我们将使用LiveData以及双向数据绑定，然后彻底将数据绑定的可观察对象重构为LiveData。

开始使用

在您应用程序的build.gradle中添加以下依赖项。

android {
    ...

    dataBinding {
        enabled = true
    }
    ...
}

dependencies {
    ...
    implementation 'android.arch.lifecycle:extensions:1.1.1'
    implementation 'com.android.support:design:28.0.0-beta01'
    ...
}

项目结构

LoginViewModelOld文件中将包含旧的代码，而LoginViewModel文件将包含重构后的代码。

模型

我们在User.java类中定义了我们的模型。

package com.Olivia.androidmvvmdatabindinglivedata;


import android.util.Patterns;

public class User {

    private String mEmail;
    private String mPassword;


    public User(String email, String password) {
        mEmail = email;
        mPassword = password;
    }

    public String getEmail() {
        if (mEmail == null) {
            return "";
        }
        return mEmail;
    }


    public String getPassword() {

        if (mPassword == null) {
            return "";
        }
        return mPassword;
    }

    public boolean isEmailValid() {
        return Patterns.EMAIL_ADDRESS.matcher(getEmail()).matches();
    }


    public boolean isPasswordLengthGreaterThan5() {
        return getPassword().length() > 5;
    }

}

布局

以下是activity_main.xml的代码:

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

    <data>

        <variable
            name="loginViewModel"
            type="com.Olivia.androidmvvmdatabindinglivedata.LoginViewModel" />
    </data>


    <ScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:layout_margin="8dp"
            android:orientation="vertical">

            <android.support.design.widget.TextInputLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                app:error="@{loginViewModel.errorEmail}"
                app:errorEnabled="true">

                <EditText
                    android:id="@+id/inEmail"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:hint="Email"
                    android:inputType="textEmailAddress"
                    android:padding="8dp"
                    android:text="@={loginViewModel.email}" />

            </android.support.design.widget.TextInputLayout>

            <android.support.design.widget.TextInputLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                app:error="@{loginViewModel.errorPassword}"
                app:errorEnabled="true">

                <EditText
                    android:id="@+id/inPassword"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:hint="Password"
                    android:inputType="textPassword"
                    android:padding="8dp"
                    android:text="@={loginViewModel.password}" />

            </android.support.design.widget.TextInputLayout>


            <Button
                android:id="@+id/button"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="8dp"
                android:onClick="@{()-> loginViewModel.onLoginClicked()}"
                android:text="LOGIN" />


            <ProgressBar
                android:id="@+id/progressBar"
                style="?android:attr/progressBarStyleLarge"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center_horizontal"
                android:layout_marginTop="8dp"
                android:visibility="@{loginViewModel.busy}" />


        </LinearLayout>

    </ScrollView>

</layout>

进度条将被显示以模拟登录功能。

视图模型

下面是LoginViewModel.java的代码:

package com.Olivia.androidmvvmdatabindinglivedata;

import android.arch.lifecycle.LiveData;
import android.arch.lifecycle.MutableLiveData;
import android.databinding.BaseObservable;
import android.databinding.Bindable;
import android.databinding.ObservableField;
import android.os.Handler;
import android.support.annotation.NonNull;


public class LoginViewModel extends BaseObservable {
    private String email;
    private String password;
    private int busy = 8;
    public final ObservableField<String> errorPassword = new ObservableField<>();
    public final ObservableField<String> errorEmail = new ObservableField<>();

    public LoginViewModel() {
    }

    private MutableLiveData<User> userMutableLiveData;

    LiveData<User> getUser() {
        if (userMutableLiveData == null) {
            userMutableLiveData = new MutableLiveData<>();
        }

        return userMutableLiveData;
    }

    @Bindable
    @NonNull
    public String getEmail() {
        return this.email;
    }

    public void setEmail(@NonNull String email) {
        this.email = email;
        notifyPropertyChanged(BR.email);
    }

    @Bindable
    @NonNull
    public String getPassword() {
        return this.password;
    }

    public void setPassword(@NonNull String password) {
        this.password = password;
        notifyPropertyChanged(BR.password);
    }

    @Bindable
    public int getBusy() {
        return this.busy;
    }

    public void setBusy(int busy) {
        this.busy = busy;
        notifyPropertyChanged(BR.busy);
    }


    public void onLoginClicked() {

        setBusy(0); //View.VISIBLE
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {


                User user = new User(getEmail(), getPassword());

                if (!user.isEmailValid()) {
                    errorEmail.set("Enter a valid email address");
                } else {
                    errorEmail.set(null);
                }

                if (!user.isPasswordLengthGreaterThan5())
                    errorPassword.set("Password Length should be greater than 5");
                else {
                    errorPassword.set(null);
                }

                userMutableLiveData.setValue(user);
                setBusy(8); //8 == View.GONE

            }
        }, 5000);
    }
}

ObservableField是一个对象包装器，使其可观察。在上述代码中，我们将User封装在LiveData中。每当用户对象发生变化时，它将在MainActivity中被观察，并采取相应的动作。当按钮被点击时，我们将ProgressBar设置为可见。View.VISIBLE = 0。View.GONE == 8 在5秒的延迟后，将验证电子邮件和密码，并更新TextInputLayout绑定项。

ObservableField 不具备生命周期感知能力。（ObservableField 不会感知生命周期。）

以下是MainActivity.java类的代码:

package com.Olivia.androidmvvmdatabindinglivedata;

import android.arch.lifecycle.Observer;
import android.databinding.DataBindingUtil;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Toast;

import com.Olivia.androidmvvmdatabindinglivedata.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);


        ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        LoginViewModel loginViewModel = new LoginViewModel();
        binding.setLoginViewModel(loginViewModel);

        loginViewModel.getUser().observe(this, new Observer() {
            @Override
            public void onChanged(@Nullable User user) {
                if (user.getEmail().length() > 0 || user.getPassword().length() > 0)
                    Toast.makeText(getApplicationContext(), "email : " + user.getEmail() + " password " + user.getPassword(), Toast.LENGTH_SHORT).show();
            }
        });

    }
}

在上面的代码中，observe方法会查找包含在MutableLiveData中的User对象的更改。它会显示一个带有用户名和密码的toast。现在，让我们完全用LiveData替换ObservableField。

将ObservableField重构为LiveData。

以下是新的LoginViewModel.java类的代码：

package com.Olivia.androidmvvmdatabindinglivedata;

import android.arch.lifecycle.LiveData;
import android.arch.lifecycle.MutableLiveData;
import android.arch.lifecycle.ViewModel;
import android.os.Handler;


public class LoginViewModel extends ViewModel {


    public MutableLiveData<String> errorPassword = new MutableLiveData<>();
    public MutableLiveData<String> errorEmail = new MutableLiveData<>();

    public MutableLiveData<String> email = new MutableLiveData<>();
    public MutableLiveData<String> password = new MutableLiveData<>();
    public MutableLiveData<Integer> busy;

    public MutableLiveData<Integer> getBusy() {

        if (busy == null) {
            busy = new MutableLiveData<>();
            busy.setValue(8);
        }

        return busy;
    }


    public LoginViewModel() {

    }

    private MutableLiveData<User> userMutableLiveData;

    LiveData<User> getUser() {
        if (userMutableLiveData == null) {
            userMutableLiveData = new MutableLiveData<>();
        }

        return userMutableLiveData;
    }


    public void onLoginClicked() {

        getBusy().setValue(0); //View.VISIBLE
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {


                User user = new User(email.getValue(), password.getValue());

                if (!user.isEmailValid()) {
                    errorEmail.setValue("Enter a valid email address");
                } else {
                    errorEmail.setValue(null);
                }

                if (!user.isPasswordLengthGreaterThan5())
                    errorPassword.setValue("Password Length should be greater than 5");
                else {
                    errorPassword.setValue(null);
                }

                userMutableLiveData.setValue(user);
                busy.setValue(8); //8 == View.GONE

            }
        }, 3000);
    }
}

上面的类现在扩展了ViewModel，因为我们不再需要BaseObservable。现在，我们已经将ObservableFields更改为MutableLiveData。由于数据绑定的帮助，MutableLiveData中的更改会自动更新到布局中。我们的MainActivity.java类现在已更新为:

package com.Olivia.androidmvvmdatabindinglivedata;

import android.arch.lifecycle.Observer;
import android.arch.lifecycle.ViewModelProviders;
import android.databinding.DataBindingUtil;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Toast;

import com.Olivia.androidmvvmdatabindinglivedata.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);


        ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        LoginViewModel loginViewModel = ViewModelProviders.of(this).get(LoginViewModel.class);
        binding.setLoginViewModel(loginViewModel);
        binding.setLifecycleOwner(this);
        

        loginViewModel.getUser().observe(this, new Observer() {
            @Override
            public void onChanged(@Nullable User user) {
                if (user.getEmail().length() > 0 || user.getPassword().length() > 0)
                    Toast.makeText(getApplicationContext(), "email : " + user.getEmail() + " password " + user.getPassword(), Toast.LENGTH_SHORT).show();
            }
        });

    }
}

ViewModelProviders.of也可以用于创建ViewModel实例，就像上面那样。这个方法只实例化ViewModel一次。每次后续调用都会重用这个实例。LifecycleOwner是一个我们的Activity可以绑定到的接口。上述应用程序的输出如下所示：正如你所看到的，当用户离开应用程序时，Toast消息不会显示。因为LiveData是感知生命周期的。当你再次打开应用程序时，Toast将会显示。这就结束了本教程。你可以从下面的链接下载这个项目：

安卓MVVM数据绑定LiveData

Github 项目链接