Usage of ViewModel and livedata in jetpack

ViewModel Introduce

ViewModel Separate the data needed by the page from the page , The page only needs to deal with user interaction and presentation data . Is between View（UI） and Model（ data ） The bridge between , So that the view and data can be separated , And keep the communication going .
ViewModel Independent of configuration changes , for example activity As you rotate, the page reconstructs , The life cycle will end and start again , But it won't affect ViewModel Life cycle of , Or that one? ViewModel.
ViewModel The instantiation process of is through ViewModelProvider To complete ,ViewModelProvider Will judge ViewModel Whether there is , If it exists weakly, it returns directly , Otherwise, create a new ViewModel.

ViewModel Instantiation

val timerViewModel = ViewModelProvider(this).get(TimerViewModel::class.java)

ViewModelProvider Method requires a ViewModelStoreOwner Parameters ,

   public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
        this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
                ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
                : NewInstanceFactory.getInstance());
    }

We use the context of the current page this As ViewModelProvider Parameters of , This is because in the androidx Take our activity In the end, he inherited it ComponentActivity, And this activity It has been implemented for us by default ViewModelStoreOwner Interface
ViewModelStoreOwner Interface , This interface defines getViewModelStore Method and return ViewModelStore object

@SuppressWarnings("WeakerAccess")
public interface ViewModelStoreOwner {
    /**
     * Returns owned {@link ViewModelStore}
     *
     * @return a {@code ViewModelStore}
     */
    @NonNull
    ViewModelStore getViewModelStore();
}

ViewModelStore Maintained in object ViewModel, It can be seen that ViewModel In fact, it is based on HashMap<String, ViewModel> In the form of , So it's independent of activity There is a life cycle of , So it's using viewmodel Don't pass in any type of Context Or with Context Object reference of , As a result, the page cannot be destroyed , This causes a memory leak (activity At the time of destruction , Its own reference has not been broken and cannot be destroyed, which will cause a memory leak - Applied for a piece of memory , But it wasn't released in time , And this memory will be occupied all the time and can't be allocated any more )

public class ViewModelStore {

    private final HashMap<String, ViewModel> mMap = new HashMap<>();

    final void put(String key, ViewModel viewModel) {
        ViewModel oldViewModel = mMap.put(key, viewModel);
        if (oldViewModel != null) {
            oldViewModel.onCleared();
        }
    }

    final ViewModel get(String key) {
        return mMap.get(key);
    }

    Set<String> keys() {
        return new HashSet<>(mMap.keySet());
    }

    /**
     *  Clears internal storage and notifies ViewModels that they are no longer used.
     */
    public final void clear() {
        for (ViewModel vm : mMap.values()) {
            vm.clear();
        }
        mMap.clear();
    }
}

If in ViewModel You have to use context, have access to AndroidViewModel class , It is ViewModel Subclasses of , receive Application As Context, It also means that its life cycle is the same as application Same .

ViewModel Source code creation

public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        ViewModel viewModel = mViewModelStore.get(key);

        if (modelClass.isInstance(viewModel)) {
            if (mFactory instanceof OnRequeryFactory) {
                ((OnRequeryFactory) mFactory).onRequery(viewModel);
            }
            return (T) viewModel;
        } else {
            //noinspection StatementWithEmptyBody
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }
        if (mFactory instanceof KeyedFactory) {
            viewModel = ((KeyedFactory) mFactory).create(key, modelClass);
        } else {
            viewModel = mFactory.create(modelClass);
        }
        mViewModelStore.put(key, viewModel);
        return (T) viewModel;
    }

Use

rely on

   implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.3.0'

viewmodel Is an abstract class , There's a... In it onCleared() Method , When viewmodel No longer needed , That is, the related activity Be destroyed when the , This method is called by the system . You can perform some resource release operations , But because of the rotation of the screen activity The reconstruction of does not trigger this method , Does not affect the viewmodel Life cycle of

Let's write a viewmodel Implementation class of TimerViewModel, Maintain a timer inside , And then in activity Instantiate it , Take a look at when the program is in the foreground and background switching and horizontal and vertical screen switching , Will the timer reset ,

class TimerViewModel:ViewModel() {
    private var timer:Timer?=null
    private var currentSecond:Int = 0

    fun startTiming(){
        if (timer==null){
            currentSecond = 0
            timer = Timer()
            val timerTask:TimerTask = MyTask()
            timer?.schedule(timerTask,1000,1000)
        }
    }
    private inner class MyTask:TimerTask(){
        override fun run() {
            currentSecond ++
            if (onTimeChangeListener!=null){
                onTimeChangeListener.onTimeChange(currentSecond)
            }
        }
    }
    interface OnTimeChangeListener{
        fun onTimeChange(second:Int)
    }
    private lateinit var onTimeChangeListener:OnTimeChangeListener
    fun setOnTimeChangeListener(onTimeChangeListener:OnTimeChangeListener){
        this.onTimeChangeListener = onTimeChangeListener
    }

    /**
     *  When viewmodel No longer needed , That is, the related activity Be destroyed when the , This method is called by the system 
     *  You can perform some resource release operations , But because of the rotation of the screen activity The reconstruction of does not trigger this method , Does not affect the viewmodel Life cycle of 
     */
    override fun onCleared() {
        super.onCleared()
    }
}

activity and viewmodel relation

 private fun init() {
        val timerViewModel = ViewModelProvider(this).get(TimerViewModel::class.java)
        timerViewModel.setOnTimeChangeListener(object : TimerViewModel.OnTimeChangeListener {
            override fun onTimeChange(second: Int) {
                Log.e(TAG, "onTimeChange:$second ")
            }
        })
        timerViewModel.startTiming()
    }

It turns out that , stay activity When you are in the background and switching between horizontal and vertical screens ,viewmodel There's no change in the life cycle of

ViewModel And onSaveInstanceState() The difference between

onSaveInstanceState() Method can also solve the problem of data loss caused by screen rotation , however onSaveInstanceState() Method can only hold a small amount of , Data that supports serialization , and ViewModel There is no such restriction , It supports all the data in the page
When the page is completely destroyed ,ViewModel There is no data in , and onSaveInstanceState() There is no limit to the method .

LiveData Introduce

LiveData It's an observable data class container , Data can be packaged , Be an observed person , When the data changes, it can be captured by the observer .
LiveData Is an abstract class , We usually use subclasses of it MutableLiveData
LiveData Be able to sense the life cycle of a page , Check whether the current page is activated , Only when it is activated can it be notified , Otherwise it will be destroyed , binding activity The process is as follows

MutableLiveData usage

private var currentSecond:MutableLiveData<Int> = MutableLiveData()

private fun initComponent() {
        val timerViewModel = ViewModelProvider(this).get(TimerViewModel::class.java)
        var liveData = timerViewModel.getCurrentSecond()
        liveData.observe(this, Observer {
            Log.e(TAG, "initComponent::$it ")
        })
    }

Sample code -view_model Branch