最近在開發(fā)中遇到一個crash，仔細(xì)研究了一下捞蚂，記錄一下：
先說結(jié)論：使用Fragment時妇押，要聲明一個無參的構(gòu)造函數(shù)，否則在狀態(tài)恢復(fù)時會出現(xiàn)crash
因為當(dāng)Fragment因為某種原因重新創(chuàng)建時姓迅，會調(diào)用到onCreate方法傳入之前保存的狀態(tài)敲霍，在instantiate方法中通過反射無參構(gòu)造函數(shù)創(chuàng)建一個Fragment，并且為Arguments初始化為原來保存的值丁存，而此時如果沒有無參構(gòu)造函數(shù)就會拋出異常色冀，造成程序崩潰。

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.demo/com.demo.activity.DemoActivity}: androidx.fragment.app.Fragment$InstantiationException: Unable to instantiate fragment com.demo.fragment.DemoFragment: could not find Fragment constructor
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2944)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3079)
    at android.app.ActivityThread.handleRelaunchActivityInner(ActivityThread.java:4815)
    at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4724)
    at android.app.servertransaction.ActivityRelaunchItem.execute(ActivityRelaunchItem.java:69)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1836)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6702)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:911)
Caused by: androidx.fragment.app.Fragment$InstantiationException: Unable to instantiate fragment com.demo.fragment.DemoFragment: could not find Fragment constructor
    at androidx.fragment.app.Fragment.instantiate(Fragment.java:563)
    at androidx.fragment.app.FragmentContainer.instantiate(FragmentContainer.java:57)
    at androidx.fragment.app.FragmentManager$3.instantiate(FragmentManager.java:390)
    at androidx.fragment.app.FragmentStateManager.<init>(FragmentStateManager.java:74)
    at androidx.fragment.app.FragmentManager.restoreSaveState(FragmentManager.java:2454)
    at androidx.fragment.app.FragmentController.restoreSaveState(FragmentController.java:196)
    at androidx.fragment.app.FragmentActivity.onCreate(FragmentActivity.java:287)
    at androidx.appcompat.app.AppCompatActivity.onCreate(AppCompatActivity.java:115)
    at com.demo.activity.BaseActivity.onCreate(BaseActivity.java:110)
    at com.demo.activity.DemoActivity.onCreate(DemoActivity.java:81)
    at android.app.Activity.performCreate(Activity.java:7136)
    at android.app.Activity.performCreate(Activity.java:7127)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2924)
    ... 13 more
Caused by: java.lang.NoSuchMethodException: <init> []
    at java.lang.Class.getConstructor0(Class.java:2327)
    at java.lang.Class.getConstructor(Class.java:1725)
    at androidx.fragment.app.Fragment.instantiate(Fragment.java:548)
    ... 26 more

根據(jù)堆棧提示柱嫌，找到了出問題的地方Fragmet類：

fragment.png

onCreate.png

FragmentManager.restoreSaveState

FragmentActivity.png

Parcelable saveAllState() {
    // Make sure all pending operations have now been executed to get
    // our state update-to-date.
    forcePostponedTransactions();
    endAnimatingAwayFragments();
    execPendingActions();
    // 省略部分代碼......
 
    // First collect all active fragments.
    int size = mActive.size();
    // 省略部分代碼......
 
    // Build list of currently added fragments.
    size = mAdded.size();
    // 省略部分代碼......
 
    // Now save back stack.
    // 省略部分代碼......
 
    FragmentManagerState fms = new FragmentManagerState();
    fms.mActive = active;
    fms.mAdded = added;
    fms.mBackStack = backStack;
    if (mPrimaryNav != null) {
        fms.mPrimaryNavActiveWho = mPrimaryNav.mWho;
    }
    fms.mNextFragmentIndex = mNextFragmentIndex;
    return fms;
}

可以看到杨赤，saveAllState() 返回的對象其實就是FragmentManagerState敞斋，這里返回Parcelable而不是FragmentManagerState對象主要是方便數(shù)據(jù)的持久化處理。
因此在恢復(fù)狀態(tài)時 FragmentManager.restoreSaveState方法可以直接將Parcelable對象強轉(zhuǎn)為FragmentManagerState對象疾牲。
FragmentManager中構(gòu)建了默認(rèn)的FragmentFractory植捎，F(xiàn)actory中重寫了instantiate方法，調(diào)用了FragmentHostCallback的instantiate方法阳柔。該方法最終調(diào)用了Fragment類的中靜態(tài)方法焰枢。
即文章開始提到的 Fragment.instantiate 方法，在instantiate中嘗試用無參構(gòu)造函數(shù)創(chuàng)建fragment實例時由于找不到無參的構(gòu)造函數(shù)而報錯
此外需要注意舌剂，創(chuàng)建fragment涉及到相關(guān)參數(shù)保存的操作济锄，官方建調(diào)用fragment.setArguments(args)方法，系統(tǒng)會再恢復(fù)狀態(tài)時同步恢復(fù)這些參數(shù)霍转，從而避免業(yè)務(wù)數(shù)據(jù)的丟失荐绝。

/**
 * Create a new instance of a Fragment with the given class name.  This is
 * the same as calling its empty constructor, setting the {@link ClassLoader} on the
 * supplied arguments, then calling {@link #setArguments(Bundle)}.
 *
 * ....省略部分
 *
 */
/**
 * Supply the construction arguments for this fragment.
 * The arguments supplied here will be retained across fragment destroy and
 * creation.
 * <p>This method cannot be called if the fragment is added to a FragmentManager and
 * if {@link #isStateSaved()} would return true.</p>
 */
public void setArguments(@Nullable Bundle args) {
    if (mFragmentManager != null && isStateSaved()) {
        throw new IllegalStateException("Fragment already added and state has been saved");
    }
    mArguments = args;
}