Live Data

基本介绍

LiveData 是一个可观察的数据持有类。与普通的可观察对象不同，LiveData 具备生命周期感知能力，也就是说它会遵循应用中其他组件（如 Activity、Fragment 或 Service）的生命周期。

这种生命周期感知能够确保 LiveData 只会通知处于活跃生命周期状态的观察者进行更新。

如果生命周期状态是 STARTED 或 RESUMED，LiveData 会将观察者视为“活跃状态”。只有活跃的观察者才会收到 LiveData 的更新通知。

代码示例

1. 开启 DataBinding

plugins {
    alias(libs.plugins.android.application)
}

android {
    namespace = "com.stewednoodles.viewmodelapp"
    compileSdk {
        version = release(36)
    }

    defaultConfig {
        applicationId = "com.stewednoodles.viewmodelapp"
        minSdk = 24
        targetSdk = 36
        versionCode = 1
        versionName = "1.0"

        testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            isMinifyEnabled = false
            proguardFiles(
                getDefaultProguardFile("proguard-android-optimize.txt"),
                "proguard-rules.pro"
            )
        }
    }
    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_11
        targetCompatibility = JavaVersion.VERSION_11
    }

    buildFeatures {
        dataBinding = true
    }
}

dependencies {
    implementation(libs.appcompat)
    implementation(libs.material)
    implementation(libs.activity)
    implementation(libs.constraintlayout)
    testImplementation(libs.junit)
    androidTestImplementation(libs.ext.junit)
    androidTestImplementation(libs.espresso.core)
}

2. 页面布局

页面布局的样式，如下所示

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

    <data>

        <variable
            name="viewModel"
            type="com.stewednoodles.viewmodelapp.MyViewModel" />
    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:id="@+id/main"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

        <TextView
            android:id="@+id/textView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="View Model App"
            android:textSize="32sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.149" />

        <TextView
            android:id="@+id/textView2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="88dp"
            android:text="0"
            android:textSize="48sp"
            android:textStyle="bold"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/textView" />

        <Button
            android:onClick="@{viewModel::incrementCounter}"
            android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="132dp"
            android:text="Increase Counter"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/textView2" />

    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

3. 创建 ViewModel

ViewModel 的代码，如下所示

package com.stewednoodles.viewmodelapp;

import android.view.View;

import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

public class MyViewModel extends ViewModel {
    private MutableLiveData<Integer> counter = new MutableLiveData<>();
    public LiveData<Integer> getCounter() {
        return counter;
    }

    public void incrementCounter(View view) {
        int currentValue = counter.getValue() != null ? counter.getValue() : 0;
        counter.setValue(currentValue + 1);
    }
}

3. MainActivity

MainActivity 的代码，如下所示

package com.stewednoodles.viewmodelapp;

import android.os.Bundle;
import android.view.View;

import androidx.activity.EdgeToEdge;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.graphics.Insets;
import androidx.core.view.ViewCompat;
import androidx.core.view.WindowInsetsCompat;
import androidx.databinding.DataBindingUtil;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;

import com.stewednoodles.viewmodelapp.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {
    int counter = 0;
    ActivityMainBinding mainBinding;
    MyViewModel viewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        EdgeToEdge.enable(this);
        setContentView(R.layout.activity_main);
        ViewCompat.setOnApplyWindowInsetsListener(findViewById(R.id.main), (v, insets) -> {
            Insets systemBars = insets.getInsets(WindowInsetsCompat.Type.systemBars());
            v.setPadding(systemBars.left, systemBars.top, systemBars.right, systemBars.bottom);
            return insets;
        });
        viewModel = new ViewModelProvider(this).get(MyViewModel.class);

        mainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        mainBinding.setViewModel(viewModel);

        // 观察 LiveData 数据
        viewModel.getCounter().observe(this, new Observer<Integer>() {
            @Override
            public void onChanged(Integer integer) {
                mainBinding.textView2.setText(String.valueOf(integer));

            }
        });
    }
}

4. 运行效果

运行效果，如下所示