安卓开发之Jetpack：LiveData

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与ViewModel的关系：在ViewModel中的数据发生变化时通知页面

LiveData的优势：

  • 确保界面符合数据状态
  • 不会发生内存泄漏
  • 不会因Activity停止而导致崩溃
  • 不再需要手动处理生命周期
  • 数据始终保存最新状态
  • 适当的配置更新
  • 共享资源

LiveData的基本使用

基本用法就是创建一个ViewModel，然后使用observer

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MutableLiveData<String> mLiveData = mTestViewModel.getLiveData();
mLiveData.observe(this, new Observer<String>() {
    @Override
    public void onChanged(@Nullable String s) {
       //更新UI
    }
});

简单应用

还是直接上代码：
布局xml，使用TextView显示读秒：

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<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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"
    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="0"
        android:textSize="30sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

MyViewModel.java

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package com.example.livedata;

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

public class MyViewModel extends ViewModel {
    public MutableLiveData<Integer> getCurrentSecond() {
        if (currentSecond == null) {
            currentSecond = new MutableLiveData<>();
            currentSecond.setValue(0);
        }
        return currentSecond;
    }

    private MutableLiveData<Integer> currentSecond;

}

activity.java

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package com.example.livedata;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;
import android.os.Bundle;
import android.widget.TextView;
import java.util.Timer;
import java.util.TimerTask;

public class MainActivity extends AppCompatActivity {
    private TextView textView;
    private MyViewModel myViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.textView);
        myViewModel = new ViewModelProvider(this, new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(MyViewModel.class);
        textView.setText(String.valueOf(myViewModel.getCurrentSecond().getValue()));

        myViewModel.getCurrentSecond().observe(this, new Observer<Integer>() {
            @Override
            public void onChanged(Integer i) {
                textView.setText(String.valueOf(i));
            }
        });
        startTimer();
    }

    private void startTimer() {
        new Timer().schedule(new TimerTask() {
            @Override
            public void run() {
                // 非UI线程用postValue改变VIewModel，UI线程用setValue
                myViewModel.getCurrentSecond().postValue(myViewModel.getCurrentSecond().getValue() + 1);
            }
        }, 1000, 1000);
    }
}

ViewModel+LiveData实现Fragment间通信

需求：有两个fragment，里面都是一个SeekBar，然后让两个SeekBar的process保存一致。
先创建两个fragment

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<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".FirstFragment">

    <SeekBar
        android:id="@+id/seekBar"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:min="0"
        android:max="100"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

第二个fragment叫SecondFragment
添加到activity：

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<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/fragmentContainerView"
        android:name="com.example.livedata.FirstFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginTop="24dp"
        android:layout_marginBottom="24dp"
        app:layout_constraintBottom_toTopOf="@+id/guideline3"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <androidx.constraintlayout.widget.Guideline
        android:id="@+id/guideline3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_constraintGuide_percent="0.5" />

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/fragmentContainerView2"
        android:name="com.example.livedata.SecondFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginTop="24dp"
        android:layout_marginBottom="24dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/guideline3" />

</androidx.constraintlayout.widget.ConstraintLayout>

同样创建ViewModel：

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package com.example.livedata;

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

public class MyViewModel extends ViewModel {
    private MutableLiveData<Integer> progress;

    public MutableLiveData<Integer> getProgress() {
        if (progress == null) {
            progress = new MutableLiveData<>();
            progress.setValue(0);
        }
        return progress;
    }

}

fragment的逻辑：

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package com.example.livedata;

import android.os.Bundle;

import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.SeekBar;


public class FirstFragment extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View root = inflater.inflate(R.layout.fragment_first, container, false);        // 第二个fragment这改成fragment_second
        SeekBar seekBar = root.findViewById(R.id.seekBar);
        MyViewModel viewModel = new ViewModelProvider(getActivity(), new ViewModelProvider.AndroidViewModelFactory(getActivity().getApplication())).get(MyViewModel.class);
        viewModel.getProgress().observe(getActivity(), new Observer<Integer>() {
            @Override
            public void onChanged(Integer integer) {
                seekBar.setProgress(integer);
            }
        });

        seekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
            @Override
            public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
                viewModel.getProgress().setValue(progress);
            }

            @Override
            public void onStartTrackingTouch(SeekBar seekBar) {

            }

            @Override
            public void onStopTrackingTouch(SeekBar seekBar) {

            }
        });
        // Inflate the layout for this fragment
        return root;
    }
}

第二个Fragment也上面的基本一致（名字进行修改）
activity的java文件保持默认。