前言
前面介紹ViewModel時，說到ViewModel常與livedata聯(lián)合使用迫摔；但是ViewModel不單單是為了livedata而創(chuàng)建的，它可以幫助我們保存UI相關(guān)的數(shù)據(jù)泥从，維護(hù)與UI一直的生命周期并排除config change造成的影響句占；這里再介紹一種ViewModel的使用場景；如sdk中要實(shí)現(xiàn)無UI接口躯嫉，你不知道具體的接入方的邏輯是怎么樣的纱烘，比如要通過一個flag來控制不同的邏輯，但是因?yàn)榻尤敕降慕缑娌皇悄銓?shí)現(xiàn)的祈餐，你怎么添加這個flag呢擂啥； 就是通過ViewModel，實(shí)際上將你的flag標(biāo)志和接入方的實(shí)例對象綁定起來帆阳；同時前面說過HolderFragment的思路是值得借鑒的哺壶，我們在本文中也會看看如何使用
ViewModel使用
創(chuàng)建ViewModel
public class MyViewModel extends ViewModel {
private boolean mflag;
public boolean isflag() {
return mflag;
}
public void setFlag(boolean mFlag) {
this.mflag = mFlag;
}
}
將ViewModel實(shí)例與接入方界面綁定
MyViewModel myViewModel = ViewModelProviders.of(activity).get(MyViewModel.class);
MyViewModel myViewModel = ViewModelProviders.of(fragment).get(MyViewModel.class);
這樣就可以通過創(chuàng)建viewmodel示例對象來操作與activity/fragment相關(guān)的flag；
局限性
但是這種方法是有局限性的；我們看下ViewModelProviders山宾， 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}.
* <p>
* 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}.
* <p>
* 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}.
* <p>
* 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();
}
我們看到傳入的參數(shù)只支持support的activity/fragment至扰； 前面我們也分析過HolderFragment，說明了其是為了向前適配的塌碌， 我們看下能否使用HolderFragment呢渊胸？
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class HolderFragment extends Fragment implements ViewModelStoreOwner
很可惜是不行的旬盯，這個類里面的接口都是隱藏的台妆；
這個就比較奇怪了，明明底層已經(jīng)做了適配胖翰，但不把上層接口暴露出來接剩；這個底層有什么用呢？
原因： 適配是適配了androidx的support activity/fragment萨咳；而不是normal Activity/Fragment(android.app.activity)
參考：
Before androidx FragmentActivity.java
androidx FragmentActivity.java
借鑒HolderFragment
雖然不明白google為什么沒有為normal activity提供接口懊缺；但是既然沒有開放接口，我們也只能另想辦法了培他； 我們可以寫一個類HolderFragment鹃两，然后將其依賴到接入方的normal activity/fragment；然后再通過其得到對應(yīng)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<Activity, HolderFragmentForNormalUI> mNotCommittedActivityHolders = new HashMap<>();
private Map<Fragment, HolderFragmentForNormalUI> 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為android.app.Activity)
MyViewModel myViewModel = new ViewModelProvider(HolderFragmentForNormalUI.holderFragmentFor(activity).getViewModelStore(),
ViewModelProvider.AndroidViewModelFactory.getInstance(activity.getApplication())).get(SendCodeViewModel.class);
總結(jié)
ViewModel與activity/fragment依賴的關(guān)系可以用來給無法操作的界面添加相應(yīng)屬性舀凛，標(biāo)志位等俊扳；適用于我們抽取無UI接口時，有些地方需要與界面有耦合關(guān)系的情況（當(dāng)然理想情況下這是不合理的猛遍，可惜現(xiàn)實(shí)往往并不總能盡如人意）馋记； jectpack組件的使用方式還有很多，這都有待于我們探索懊烤；
