ViewModel, a part of the MVVM framework, its main function is to process business logic, data distribution, etc. It is a bridge connecting Mode and View. Compared with MVP, it is similar to P.
The ViewModel class has the following advantages:
- The ViewModel class is life-aware and has the same life cycle as the Activity bound to it. It can solve unnecessary troubles caused by the sudden destruction of the Activity when requesting the network.
- The data will not be lost if the device information changes (cut between horizontal and vertical screens). In fact, it only has one life cycle, which is triggered when the page is detected to be destroyed.
- Another feature of ViewModel is that you can use ViewModel to share data between Fragments of the same Activity.
- The ViewModel bound to each Activity is independent (between Activities)
1. Start using
Add dependencies
以前是需要添加依赖的，现在
方式一： implementation 'androidx.appcompat:appcompat:1.2.0',//这里面也包含ViewModel,LiveData等，
方式二： 也可以单独添加
implementation "androidx.fragment:fragment:1.1.0"
implementation "androidx.lifecycle:lifecycle-viewmodel:2.1.0"
implementation "androidx.lifecycle:lifecycle-extensions:2.1.0"
2、Activity
2.1, Create a new ViewModel
public class MyViewModel extends ViewModel {
@Override
protected void onCleared() {
super.onCleared();
}
}
The onCleared method is unique to ViewModel. It will be called when the Activity actually exits, not after it is destroyed, because onDestroy will also be called when the screen is rotated. So we can cancel network requests etc here. When you do not do any operations during normal development, if there is a network request and the Activity is destroyed, it is very likely that the request will successfully return the result to the activity, causing unnecessary troubles such as leakage.
Solution:
You can use ViewModel in combination with onCleared(), call.cancel(), etc.
When the network requests, if the activity is suddenly destroyed, the viewmodel bound to it will also be destroyed. At the same time, we cancel the network connection interface (call.cancel()) in the onCleared() method. Even if there is data returned, the activity will not accept to the notification, because the data is monitored from livedata, and only when livedata triggers an update, the Activity will receive the notification.
(Of course there are many other ways)
2.2. Binding Activity
Here are the following ways:
- ViewModelProviders.of(this).get(class)（过时）
- new ViewModelProvider(this,factory).get(class)（常用）
- new ViewModelProvider.NewInstanceFactory().create(class);
The first way: ViewModelProviders.of(this).get(class)
ViewModelProviders.of(this).get(MyViewModel.class);
implementation "androidx.fragment:fragment:1.1.0"
implementation "androidx.lifecycle:lifecycle-viewmodel:2.1.0"
implementation "androidx.lifecycle:lifecycle-extensions:2.1.0"
This method is among these dependencies. Of course, if yours is different from mine, it doesn't matter. This is not the point. This method is relatively old, and it can also be found in the source code that the of method of ViewModelProviders actually calls ViewModelProvider and automatically creates a factory.
----------------ViewModelProviders.class----------------
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull Fragment fragment, @Nullable Factory factory) {
Application application = checkApplication(checkActivity(fragment));
if (factory == null) {
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
return new ViewModelProvider(fragment.getViewModelStore(), factory);
}
The second way: new ViewModelProvider(this,factory).get(class)
ViewModelProvider(this, new ViewModelProvider.NewInstanceFactory()).get(MyViewModel.class);
Pass in a system factory. Create creates a new instance, while get searches the HashMap first. If it cannot find it, it creates a new instance. It is also the reason why the reconstructed Viewmodel is the same object. He will put the object in HashMap<String, ViewModel> mMap = new HashMap<>(); in the ViewModelStore class. This method is relatively new and is generally created this way.
public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
//第一步：先查找
ViewModel viewModel = mViewModelStore.get(key);
if (modelClass.isInstance(viewModel)) {
//noinspection unchecked
return (T) viewModel;
} else {
//noinspection StatementWithEmptyBody
if (viewModel != null) {
// TODO: log a warning.
}
}
//第二步：没有则创建，并保存在mViewModelStore中
if (mFactory instanceof KeyedFactory) {
viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
} else {
viewModel = (mFactory).create(modelClass);
}
mViewModelStore.put(key, viewModel);
//noinspection unchecked
return (T) viewModel;
}
The third way: new ViewModelProvider.NewInstanceFactory().create(class);
MyViewModel myViewModel1 = new ViewModelProvider.NewInstanceFactory().create(MyViewModel.class);
Write a LiveData below. If you don’t understand LiveData, you can read the previous blog. The table of contents is at the top.
public class MyViewModel extends ViewModel {
private MutableLiveData<String> mStr= new MutableLiveData<>();
public MutableLiveData<String> getmStr() {
return mStr;
}
public void setmStr(String s) {
if(mStr==null){
mStr = new MutableLiveData<>();
}
mStr.setValue(s);
}
@Override
protected void onCleared() {
super.onCleared();
}
}
all codes
public class JPackActivity extends AppCompatActivity {
private ActyJpackLayoutBinding mBinding;
private MyViewModel myViewModel;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//通过databinding 把activity和view绑定
mBinding = DataBindingUtil.setContentView(this,R.layout.acty_jpack_layout);
//通过ViewModeProvider 把activity和ViewModel绑定起来。
myViewModel = new ViewModelProvider(this, new ViewModelProvider.NewInstanceFactory()).get(MyViewModel.class);
myViewModel.getmStr().observe(this, new Observer<String>() {
@Override
public void onChanged(String s) {
mBinding.textShow.setText(s);
}
});
}
@Override
protected void onDestroy() {
super.onDestroy();
//移除观察者
}
3、Fragment
Use the same as in Activity. However, it should be noted that the difference between passing this and getActivity
- this: This ViewModel is independent and only serves this Fragment alone. Other Fragments cannot obtain the ViewModel with the same memory address.
- getActivity: The ViewModel scope obtained by using getActivity() is in the Activity and all the fragments it creates, which means that you also get the ViewModel with the same memory address in other Fragments
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
MyViewModel myViewModel = new ViewModelProvider(getActivity(), new ViewModelProvider.NewInstanceFactory()).get(MyViewModel.class);
return inflater.inflate(R.layout.fragment_blank, container, false);
}
Of course, there is another point to note: get("key1",MyViewModel.class);, the get method can pass the key value, and the ViewModel data created by different keys is independent.
4. Summary
It is very simple to use, and you need to practice more so that you will not be stumped by those xx interviewers asking how to implement the steps.
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