Andriod --- JetPack (二)：LifeCycle 的诞生

解耦系统组件与普通组件

比如一个普通控件 Chronometer 的使用与 Activity 的生命周期关联的太强，即普通控件 Chronometer 的业务逻辑必须在 Activity 的生命周期中处理，这样的耦合度就太高，所以我们使用LifeCycle 来解决

我们先来看没有使用 LifeCycle 之前的代码：
MainActivity.java

package com.example.lifecycle;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.os.SystemClock;
import android.widget.Chronometer;

public class MainActivity extends AppCompatActivity {
    private Chronometer chronometer;
    private Long elapseTime = 0l;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        chronometer = findViewById(R.id.chronometer);
    }

    // 活跃状态
    @Override
    protected void onResume() {
        super.onResume();
        chronometer.setbase(SystemClock.elapsedRealtime() - elapseTime); // 返回启动后的毫秒数，包括睡眠时间。
        chronometer.start();
    }

    // 转到后台
    @Override
    protected void onPause() {
        super.onPause();
        elapseTime = SystemClock.elapsedRealtime() - chronometer.getbase();
        chronometer.stop();
    }
}

activity_main.xml

很明显，Chronometer 的使用与 Activity 的生命周期关联的太强
我们来看一下修改之后的代码：

MainActivity2 .java

package com.example.lifecycle;

import android.os.Bundle;
import android.os.SystemClock;
import android.widget.Chronometer;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity2 extends AppCompatActivity {

    private MyChronometer chronometer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        chronometer = findViewById(R.id.chronometer);
        getLifecycle().addObserver(chronometer);
    }
}

自定义 MyChronometer 类继承 Chronometer 实现 LifecycleObserver
MyChronometer .java

package com.example.lifecycle;

import android.content.Context;
import android.os.SystemClock;
import android.util.AttributeSet;
import android.widget.Chronometer;

import androidx.lifecycle.Lifecycle;
import androidx.lifecycle.LifecycleObserver;
import androidx.lifecycle.OnLifecycleEvent;

public class MyChronometer extends Chronometer implements LifecycleObserver {
    private Long elapseTime = 0l;

    public MyChronometer(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    // 通过注解的方式拥有 Activity 生命周期所拥有的方法
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    public void startMeter() {
        setbase(SystemClock.elapsedRealtime() - elapseTime); // 返回启动后的毫秒数，包括睡眠时间。
        start();
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    public void stopMeter() {
        elapseTime = SystemClock.elapsedRealtime() - getbase();
        stop();
    }
}

通过注解的方式拥有 Activity 生命周期所拥有的方法，这样就在很大的程度上解耦了

代码测试的时候记得开启GPS权限

添加依赖：

  implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0-alpha02'

AndroidMainfest.xml

MainActivity3.java

package com.example.lifecycle;

import androidx.appcompat.app.AppCompatActivity;

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

public class MainActivity3 extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity3_main);
    }

    public void startGps(View view) {
        startService(new Intent(this, MyLocationService.class));
    }

    public void stopGps(View view) {
        stopService(new Intent(this, MyLocationService.class));
    }
}

activity3_main.xml

MyLocationService.java

package com.example.lifecycle;

import android.util.Log;

import androidx.lifecycle.LifecycleService;

public class MyLocationService extends LifecycleService {

    public MyLocationService() {
        Log.d("liang", "MyLocationService");
        MyLocationObserver myLocationObserver = new MyLocationObserver(this);
        getLifecycle().addObserver(myLocationObserver);
    }
}

MyLocationObserver.java

package com.example.lifecycle;

import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.util.Log;

import androidx.core.app.ActivityCompat;
import androidx.lifecycle.Lifecycle;
import androidx.lifecycle.LifecycleObserver;
import androidx.lifecycle.OnLifecycleEvent;

public class MyLocationObserver implements LifecycleObserver {
    private Context context;
    private LocationManager locationManager;
    private MyLocationListener myLocationListener;
    public MyLocationObserver(Context context) {
        this.context = context;
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    public void startGetLocation() {
        Log.d("liang", "startGetLocation");
        locationManager = (LocationManager) context.getSystemService(Context.LOCATION_SERVICE);
        myLocationListener = new MyLocationListener();
        if (ActivityCompat.checkSelfPermission(context, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(context, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 3000, 1, myLocationListener);
    }


    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    public void stopGetLocation() {
        Log.d("liang", "stopGetLocation");
        locationManager.removeUpdates(myLocationListener);
    }

    static class MyLocationListener implements LocationListener {

        @Override
        public void onLocationChanged(Location location) {
            Log.d("liang", "location changed" + location.toString());
        }

        @Override
        public void onStatusChanged(String provider, int status, Bundle extras) {

        }

        @Override
        public void onProviderEnabled(String provider) {

        }

        @Override
        public void onProviderDisabled(String provider) {

        }
    }
}

可以在 Terminal 终端中通过一下命令改变位置信息

adb -s emulator-5554 emu geo fix 121.4961236714487 31.244010934431345

1.帮助开发者建立可感知生命周期的组件
2.组件在其内部管理自己的生命周期，从而降低模块耦合度
3.降低内存泄漏发生的可能性
4.Activity、Fragment、Service、Application均有LifeCycle支持

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