jetpack中的常用知识点笔记（1）

一，初识jeptack

jetpack是一个又多个库组成的套件，可以帮助开发者遵循最佳做法，减少样板代码的编写，可以在各种Android版本和设备中一致运行的代码，让开发者精力集中编写重要的代码。
jetpack与AndroidX
AndroidX命名空间中包含Android Jetpack库
AndroidX替代了Android Support Library
AAC（Android Architecture Component）中的组件并入了AndroidX

二，相关组件知识

1，LifeCycle

LifeCycle的诞生是为了解决解耦页面和组件的问题。
使用Lifecycle的好处是：帮助开发者建立感知生命周期的组件。组件在其内部管理自己的生命周期，从而降低耦合度。降低内存泄漏发生的可能性。Activity，Fragment，Service，Application均有lifecycle的支持

• 使用Lifecycle解耦页面和组件
  自定义控件实现LifecycleObserver，

public class MyChronometer extends Chronometer implements LifecycleObserver {

    private long elapsedTime;

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

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    private void startMeter(){
        setBase(SystemClock.elapsedRealtime() - elapsedTime);
        start();
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    private void stopMeter(){
        elapsedTime = SystemClock.elapsedRealtime() - getBase();
        stop();
    }
}

public class Step2Activity extends AppCompatActivity {

    private MyChronometer chronometer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        chronometer = findViewById(R.id.chronometer);
        //  在页面中添加监听
        getLifecycle().addObserver(chronometer);
    }


}

• 使用LifecycleService解耦Service与组件
  举例如下：获取位置监听，首先创建一个MyLocationObserver，实现LifecycleObserver

public class MyLocationObserver implements LifecycleObserver {

    private Context context;
    private LocationManager locationManager;
    private MyLocationListener locationListener;

    public MyLocationObserver(Context context) {
        this.context = context;
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    private void startGetLocation() {
        Log.d("ning","startGetLocation");
        locationManager = (LocationManager) context.getSystemService(Context.LOCATION_SERVICE);
        locationListener = 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, locationListener);
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    private void stopGetLocation() {
        Log.d("ning","stopGetLocation");
        locationManager.removeUpdates(locationListener);
    }


    static class MyLocationListener implements LocationListener{

        @Override
        public void onLocationChanged(Location location) {
            Log.d("ning","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) {

        }
    }

}

然后在定义Service实现LifecycleService

public class MyLocationService extends LifecycleService {

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

最后正常开启服务

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

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

• 使用ProcessLifecycleOwner监听应用程序生命周期
  针对整个应用程序的监听，与Activity数量无关。
  Lifecycle.Event.ON_CREATE只会被调用一次，Lifecycle.Event.ON_DESTROY永远不会被调用
  代码如下，首先创建ApplicationObserver，实现LifecycleObserver

public class ApplicationObserver implements LifecycleObserver {

    private String TAG = "ning";

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    public void onCreate() {
        Log.d(TAG, "Lifecycle.Event.ON_CREATE");
    }


    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    public void onStart() {
        Log.d(TAG, "Lifecycle.Event.ON_START");
    }


    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    public void onResume() {
        Log.d(TAG, "Lifecycle.Event.ON_RESUME");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    public void onPause() {
        Log.d(TAG, "Lifecycle.Event.ON_PAUSE");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    public void onStop() {
        Log.d(TAG, "Lifecycle.Event.ON_STOP");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    public void onDestroy() {
        Log.d(TAG, "Lifecycle.Event.ON_DESTROY");
    }

}

然后在MyAppliaction的调用代码如下

public class MyApplication extends Application{
    @Override
    public void onCreate() {
        super.onCreate();
        ProcessLifecycleOwner.get().getLifecycle()
                .addObserver(new ApplicationObserver());
    }
}

2，ViewModel

ViewModel能够解决常见的三个问题
1)，瞬态数据丢失
2)，异步调用的内存泄漏
3)，类膨胀提高维护难度和测试难度。
ViewModel是介于视图View和数据模型Model之间的桥梁，能够使视图和数据分离也能够保持通信

vm.png

viewmodel的生命周期特性独立于配置变化。
使用注意点，不要向viewmodel中传入Context，会到导致内存泄漏。如果要使用Context，要使用AndroidViewModel的Application。
VM使用代码如下,自定义一个ViewModel继承ViewModel

public class MyViewModel extends AndroidViewModel {

    public int number;

    public MyViewModel(@NonNull Application application) {
        super(application);
    }
}

public class MainActivity extends AppCompatActivity {

    private TextView textView;
    private MyViewModel viewModel;

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

    public void plusNumber(View view) {
        textView.setText(String.valueOf(++viewModel.number));
    }
}

3，LiveData

LiveData和ViewModel的关系是在ViewModel中的数据发生变化时通知页面。
LiveData的优势有以下几点
1）确保界面数据合理状态
2）不会发生内存泄漏
3）不会因Activity停止而导致崩溃
4）不再需要手动处理生命周期
5）数据始终保持最新状态
6）适当的配置更改
7）共享资源
····
代码如下，创建一个ViewModel

public class MyViewModel extends ViewModel {
    private MutableLiveData currentSecond;

    public MutableLiveData getCurrentSecond() {
        if(currentSecond == null){
            currentSecond = new MutableLiveData<>();
            currentSecond.setValue(0);
        }
        return currentSecond;
    }
}

下面是Activity中的代码

public class MainActivity extends AppCompatActivity {

    private MyViewModel viewModel;

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

        viewModel.getCurrentSecond().observe(this, new Observer() {
            @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 postValue
                //UI线程 setValue
                viewModel.getCurrentSecond().postValue(viewModel.getCurrentSecond().getValue()+1);
            }
        },1000,1000);
    }
}

4，DataBinding

• 让布局文件承担了部分原本属于页面的工作，使页面与布局耦合度进一步降低
  自定义BindingAdapter
  加载网络图片。
  方法重载，加载本地图片。
  多参数重载。
  布局文件中的代码如下：

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ActivityMainBinding activityMainBinding = DataBindingUtil.setContentView(this,R.layout.activity_main);
        Idol idol = new Idol("斯嘉丽.约翰逊",4);
        activityMainBinding.setIdol(idol);

        activityMainBinding.setEventHandle(new EventHandleListener(this));
    }

public class EventHandleListener {
    private Context context;

    public EventHandleListener(Context context) {
        this.context = context;
    }

    public void buttonOnClick(View view){
        Toast.makeText(context,"喜欢",Toast.LENGTH_SHORT).show();
    }
}

当布局文件用到include引用布局的时候怎样传递数据类。
首先是activity_main.xml

sub.xml文件代码入下

自定义BindAdapter代码如下

public class ImageViewBindingAdapter {

    //加载网络图片
    @BindingAdapter("image")
    public static void setImage(ImageView imageView, String url){
        if(!TextUtils.isEmpty(url)){
            Picasso.get()
                    .load(url)
                    .placeholder(R.drawable.ic_launcher_background)
                    .into(imageView);
        }else{
            imageView.setBackgroundColor(Color.GRAY);
        }
    }

    //加载本地图片
    @BindingAdapter("image")
    public static void setImage(ImageView imageView, int resId){
        imageView.setImageResource(resId);
    }

    //参数可选，网络图片为空时，加载本地图片
    @BindingAdapter(value = {"image", "defaultImageResource"}, requireAll = false)
    public static void setImage(ImageView imageView, String url, int resId){
        if(!TextUtils.isEmpty(url)){
            Picasso.get()
                    .load(url)
                    .placeholder(R.drawable.ic_launcher_background)
                    .into(imageView);
        }else{
            imageView.setImageResource(resId);
        }
    }

}

• 双向绑定
  BaseObservable与ObservableFiled
  BaseObservable示例代码如下

public class UserViewModel extends BaseObservable {

    private User user;

    public UserViewModel(){
        this.user = new User("Jack");
    }

    @Bindable
    public String getUserName(){
        return user.userName;
    }

    public void setUserName(String userName){
        if(userName != null && !userName.equals(user.userName)){
            user.userName = userName;
            Log.d("ning","set username:"+userName);
            notifyPropertyChanged(BR.userName);
        }
    }
}

ObservableFiled的示例代码

public class UserViewModel {

    private ObservableField userObservableField;

    public UserViewModel(){
        User user = new User("Jack");
        userObservableField = new ObservableField<>();
        userObservableField.set(user);
    }

    public String getUserName(){
        return userObservableField.get().userName;
    }

    public void setUserName(String userName){
        Log.d("ning","userObservableField:"+userName);
        userObservableField.get().userName = userName;
    }
}

• RecyclerView的绑定
  Adapter适配器中的代码如下：

public class RecyclerViewAdapter extends RecyclerView.Adapter {

    List idols;

    public RecyclerViewAdapter(List idols) {
        this.idols = idols;
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        ItemBinding itemBinding = DataBindingUtil.inflate(LayoutInflater.from(parent.getContext()),
                R.layout.item,
                parent,
                false);
        return new MyViewHolder(itemBinding);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        Idol idol = idols.get(position);
        holder.itemBinding.setIdol(idol);
    }

    @Override
    public int getItemCount() {
        return idols.size();
    }

    static class MyViewHolder extends RecyclerView.ViewHolder{

        private ItemBinding itemBinding;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
        }

        public MyViewHolder(ItemBinding itemBinding) {
            super(itemBinding.getRoot());
            this.itemBinding = itemBinding;
        }
    }
}

• DataBinding的优势
  不需要findViewById，项目更加简洁，可读性更高
  布局文件可以包含简单的业务逻辑