前言
前面介绍ViewModel时，说到ViewModel常与livedata联合使用；但是ViewModel不单单是为了livedata而创建的，它可以帮助我们保存UI相关的数据，维护与UI一直的生命周期并排除config change造成的影响；这里再介绍一种ViewModel的使用场景；如sdk中要实现无UI接口，你不知道具体的接入方的逻辑是怎么样的，比如要通过一个flag来控制不同的逻辑，但是因为接入方的界面不是你实现的，你怎么添加这个flag呢； 就是通过ViewModel，实际上将你的flag标志和接入方的实例对象绑定起来；同时前面说过HolderFragment的思路是值得借鉴的，我们在本文中也会看看如何使用
ViewModel使用
创建ViewModel
public class MyViewModel extends ViewModel {
private boolean mflag;
public boolean isflag() {
return mflag;
}
public void setFlag(boolean mFlag) {
this.mflag = mFlag;
}
}
将ViewModel实例与接入方界面绑定
MyViewModel myViewModel = ViewModelProviders.of(activity).get(MyViewModel.class);
MyViewModel myViewModel = ViewModelProviders.of(fragment).get(MyViewModel.class);
这样就可以通过创建viewmodel示例对象来操作与activity/fragment相关的flag；
局限性
但是这种方法是有局限性的；我们看下ViewModelProviders， ViewModelStores的接口
/**
* Creates a {@link ViewModelProvider}, which retains ViewModels while a scope of given Activity
* is alive. More detailed explanation is in {@link ViewModel}.
*
* It uses {@link ViewModelProvider.AndroidViewModelFactory} to instantiate new ViewModels.
*
* @param activity an activity, in whose scope ViewModels should be retained
* @return a ViewModelProvider instance
*/
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity) {
return of(activity, null);
}
/**
* Creates a {@link ViewModelProvider}, which retains ViewModels while a scope of given
* {@code fragment} is alive. More detailed explanation is in {@link ViewModel}.
*
* It uses the given {@link Factory} to instantiate new ViewModels.
*
* @param fragment a fragment, in whose scope ViewModels should be retained
* @param factory a {@code Factory} to instantiate new ViewModels
* @return a ViewModelProvider instance
*/
@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(ViewModelStores.of(fragment), factory);
}
/**
* Creates a {@link ViewModelProvider}, which retains ViewModels while a scope of given Activity
* is alive. More detailed explanation is in {@link ViewModel}.
*
* It uses the given {@link Factory} to instantiate new ViewModels.
*
* @param activity an activity, in whose scope ViewModels should be retained
* @param factory a {@code Factory} to instantiate new ViewModels
* @return a ViewModelProvider instance
*/
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity,
@Nullable Factory factory) {
Application application = checkApplication(activity);
if (factory == null) {
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
return new ViewModelProvider(ViewModelStores.of(activity), factory);
}
/**
* Returns the {@link ViewModelStore} of the given activity.
*
* @param activity an activity whose {@code ViewModelStore} is requested
* @return a {@code ViewModelStore}
*/
@NonNull
@MainThread
public static ViewModelStore of(@NonNull FragmentActivity activity) {
if (activity instanceof ViewModelStoreOwner) {
return ((ViewModelStoreOwner) activity).getViewModelStore();
}
return holderFragmentFor(activity).getViewModelStore();
}
/**
* Returns the {@link ViewModelStore} of the given fragment.
*
* @param fragment a fragment whose {@code ViewModelStore} is requested
* @return a {@code ViewModelStore}
*/
@NonNull
@MainThread
public static ViewModelStore of(@NonNull Fragment fragment) {
if (fragment instanceof ViewModelStoreOwner) {
return ((ViewModelStoreOwner) fragment).getViewModelStore();
}
return holderFragmentFor(fragment).getViewModelStore();
}
我们看到传入的参数只支持support的activity/fragment； 前面我们也分析过HolderFragment，说明了其是为了向前适配的， 我们看下能否使用HolderFragment呢？
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class HolderFragment extends Fragment implements ViewModelStoreOwner
很可惜是不行的，这个类里面的接口都是隐藏的；
这个就比较奇怪了，明明底层已经做了适配，但不把上层接口暴露出来；这个底层有什么用呢？
原因： 适配是适配了androidx的support activity/fragment；而不是normal Activity/Fragment(android.app.activity)
参考：
Before androidx FragmentActivity.java
androidx FragmentActivity.java
借鉴HolderFragment
虽然不明白google为什么没有为normal activity提供接口；但是既然没有开放接口，我们也只能另想办法了； 我们可以写一个类HolderFragment，然后将其依赖到接入方的normal activity/fragment；然后再通过其得到对应的ViewModel中保存的flag; 如下：
public class HolderFragmentForNormalUI extends Fragment implements ViewModelStoreOwner {
private static final String LOG_TAG = "ViewModelStores";
private static final HolderFragmentForNormalUI.HolderFragmentManager sHolderFragmentManager = new HolderFragmentForNormalUI.HolderFragmentManager();
/**
* @hide
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static final String HOLDER_TAG =
"com.bytedance.ttgame.sdk.module.account.utils.HolderFragmentForNormalUI";
private ViewModelStore mViewModelStore = new ViewModelStore();
public HolderFragmentForNormalUI() {
setRetainInstance(true);
}
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
sHolderFragmentManager.holderFragmentCreated(this);
}
@Override
public void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
}
@Override
public void onDestroy() {
super.onDestroy();
mViewModelStore.clear();
}
@NonNull
@Override
public ViewModelStore getViewModelStore() {
return mViewModelStore;
}
public static HolderFragmentForNormalUI holderFragmentFor(Activity activity) {
return sHolderFragmentManager.holderFragmentFor(activity);
}
public static HolderFragmentForNormalUI holderFragmentFor(Fragment fragment) {
return sHolderFragmentManager.holderFragmentFor(fragment);
}
@SuppressWarnings("WeakerAccess")
static class HolderFragmentManager {
private Map mNotCommittedActivityHolders = new HashMap<>();
private Map mNotCommittedFragmentHolders = new HashMap<>();
private Application.ActivityLifecycleCallbacks mActivityCallbacks =
new EmptyActivityLifecycleCallbacksForNormalUI() {
@Override
public void onActivityDestroyed(Activity activity) {
HolderFragmentForNormalUI fragment = mNotCommittedActivityHolders.remove(activity);
if (fragment != null) {
Log.e(LOG_TAG, "Failed to save a ViewModel for " + activity);
}
}
};
private boolean mActivityCallbacksIsAdded = false;
void holderFragmentCreated(Fragment holderFragment) {
mNotCommittedActivityHolders.remove(holderFragment.getActivity());
}
private static HolderFragmentForNormalUI findHolderFragment(FragmentManager manager) {
if (manager.isDestroyed()) {
throw new IllegalStateException("Can't access ViewModels from onDestroy");
}
Fragment fragmentByTag = manager.findFragmentByTag(HOLDER_TAG);
if (fragmentByTag != null && !(fragmentByTag instanceof HolderFragmentForNormalUI)) {
throw new IllegalStateException("Unexpected "
+ "fragment instance was returned by HOLDER_TAG");
}
return (HolderFragmentForNormalUI) fragmentByTag;
}
private static HolderFragmentForNormalUI createHolderFragment(FragmentManager fragmentManager) {
HolderFragmentForNormalUI holder = new HolderFragmentForNormalUI();
fragmentManager.beginTransaction().add(holder, HOLDER_TAG).commitAllowingStateLoss();
return holder;
}
HolderFragmentForNormalUI holderFragmentFor(Activity activity) {
FragmentManager fm = activity.getFragmentManager();
HolderFragmentForNormalUI holder = findHolderFragment(fm);
if (holder != null) {
return holder;
}
holder = mNotCommittedActivityHolders.get(activity);
if (holder != null) {
return holder;
}
if (!mActivityCallbacksIsAdded) {
mActivityCallbacksIsAdded = true;
activity.getApplication().registerActivityLifecycleCallbacks(mActivityCallbacks);
}
holder = createHolderFragment(fm);
mNotCommittedActivityHolders.put(activity, holder);
return holder;
}
HolderFragmentForNormalUI holderFragmentFor(Fragment parentFragment) {
FragmentManager fm = parentFragment.getChildFragmentManager();
HolderFragmentForNormalUI holder = findHolderFragment(fm);
if (holder != null) {
return holder;
}
holder = createHolderFragment(fm);
return holder;
}
}
使用方式(activity为android.app.Activity)
MyViewModel myViewModel = new ViewModelProvider(HolderFragmentForNormalUI.holderFragmentFor(activity).getViewModelStore(),
ViewModelProvider.AndroidViewModelFactory.getInstance(activity.getApplication())).get(SendCodeViewModel.class);
总结
ViewModel与activity/fragment依赖的关系可以用来给无法操作的界面添加相应属性，标志位等；适用于我们抽取无UI接口时，有些地方需要与界面有耦合关系的情况（当然理想情况下这是不合理的，可惜现实往往并不总能尽如人意）； jectpack组件的使用方式还有很多，这都有待于我们探索；