我们从下面几个方面分析Android jetpack viewmodel的原理

• viewmodel涉及的类以及数据结构
• viewmodel的启动
• viewmodel的恢复

viewmodel的启动

FragmentActivity
mFragment.attachHost(null)

FragmentController
public void attachHost(Fragment parent)
mHost.mFragmentManager.attachController(mHost, mHost, parent)

FragmentManager
void attachController(FragmentHostCallback<?> host, FragmentContainer container, Fragment parent){
//get the fragmentManagerViewModel
if(parent != null){
  mNonConfig = parent.mFragmentManager.getChildNonConfig(parent);
}else if(host instanceof ViewModelStoreOwner){
  ViewModelStore viewModelStore = ((ViewModelStoreOwner)host).getViewModelStore();
  mNonConfig = FragmentManagerViewModel.getInstance(viewModelStore)
}else{
  mNonConfig = new FragmentManagerViewModel(false)
}
}

viewmodel的恢复

Activity
 ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
            int configChanges, boolean getNonConfigInstance, String reason) {
       ...
            if (getNonConfigInstance) {
                try {
                    r.lastNonConfigurationInstances
                            = r.activity.retainNonConfigurationInstances();
                } catch (Exception e) {
                    if (!mInstrumentation.onException(r.activity, e)) {
                        throw new RuntimeException(
                                "Unable to retain activity "
                                + r.intent.getComponent().toShortString()
                                + ": " + e.toString(), e);
                    }
                }
            }
           ...
    }
//把mViewModelStore保存在new 出来的NonConfigurationInstances实例中
ComponentActivity
public final Object onRetainNonConfigurationInstance() {
        // Maintain backward compatibility.
        Object custom = onRetainCustomNonConfigurationInstance();

        ViewModelStore viewModelStore = mViewModelStore;
        if (viewModelStore == null) {
            // No one called getViewModelStore(), so see if there was an existing
            // ViewModelStore from our last NonConfigurationInstance
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                viewModelStore = nc.viewModelStore;
            }
        }

        if (viewModelStore == null && custom == null) {
            return null;
        }

        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.custom = custom;
        nci.viewModelStore = viewModelStore;
        return nci;
    }