為什么使用Fragment時必須提供一個無參的構造函數(shù)噩峦？

問題

最近在線上bugly看到一個近兩個版本的出現(xiàn)的一個bug：

#5712 java.lang.NoSuchMethodException
<init> []
com.jess.arms.a.c.onCreate(BaseActivity.java:86)
java.lang.RuntimeException:Unable to start activity 
ComponentInfo{xxxx.VehicleDetailActivity}: 
android.support.v4.app.Fragment$InstantiationException: 
Unable to instantiate fragment xxxx.mvp.ui.fragment.c: 
could not find Fragment constructor

這個bug是方法找不到異常，因為項目中用了mvparms框架瞧筛，剛開始看到異常指向他的BaseActivity類，還以為是框架出了問題导盅。但是后面注意到是項目的VehicleDetailActivity報的could not find Fragment constructor较幌，即fragment構造器找不到的異常。于是馬上將問題定位到了該類新添加的一個DialogFragment上白翻，先看一下這個類的構造方法：

public MorePlanFragment(String modelId, HashMap<String, Integer> map) {
    this.modelId = modelId;
    this.posMap = map;
    this.tabPos = map.get(POS_MAP_TAB);
}

這個MorePlanFragment使用了有參構造函數(shù)乍炉，而問題就出現(xiàn)在這里。

分析

既然報的找不到構造方法的錯誤滤馍，我們先來看一下Fragment的構造函數(shù)：

/**
 * Default constructor.  <strong>Every</strong> fragment must have an
 * empty constructor, so it can be instantiated when restoring its
 * activity's state.  It is strongly recommended that subclasses do not
 * have other constructors with parameters, since these constructors
 * will not be called when the fragment is re-instantiated; instead,
 * arguments can be supplied by the caller with {@link #setArguments}
 * and later retrieved by the Fragment with {@link #getArguments}.
 *
 * <p>Applications should generally not implement a constructor. Prefer
 * {@link #onAttach(Context)} instead. It is the first place application code can run where
 * the fragment is ready to be used - the point where the fragment is actually associated with
 * its context. Some applications may also want to implement {@link #onInflate} to retrieve
 * attributes from a layout resource, although note this happens when the fragment is attached.
 */
public Fragment() {
}

構造函數(shù)上有一段注釋：

默認構造器岛琼。
每一個Fragment必須有一個無參的構造函數(shù)，以便當Activity恢復狀態(tài)時fragment可以實例化巢株。
強烈建議fragment的子類不要有其他的有參構造函數(shù)槐瑞，因為當fragment重新實例化時不會調用這些有參構造函數(shù)；
如果要傳值應該使用setArguments方法阁苞，在需要獲取這些值時調用getArguments方法困檩。

這一段注釋明確的告訴我們使用有參構造函數(shù)會出問題，建議使用無參構造函數(shù)那槽，但是并沒有告訴我們具體是哪里的問題悼沿。我們在Fragment中搜索could not find Fragment constructor這個異常，發(fā)現(xiàn)是在instantiate方法中拋出的骚灸。

public static Fragment instantiate(Context context, String fname, @Nullable Bundle args) {
    try {
        Class<?> clazz = sClassMap.get(fname);
        if (clazz == null) {
            // Class not found in the cache, see if it's real, and try to add it
            clazz = context.getClassLoader().loadClass(fname);
            sClassMap.put(fname, clazz);
        }
        Fragment f = (Fragment) clazz.getConstructor().newInstance();
        if (args != null) {
            args.setClassLoader(f.getClass().getClassLoader());
            f.setArguments(args);
        }
        return f;
    } catch (ClassNotFoundException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (java.lang.InstantiationException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (IllegalAccessException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (NoSuchMethodException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": could not find Fragment constructor", e);
    } catch (InvocationTargetException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": calling Fragment constructor caused an exception", e);
    }
}

看上面的代碼我們可以知道糟趾，F(xiàn)ragment的實例化是通過調用類對象的getConstructor()方法獲取構造器對象并調用其newInstance()方法創(chuàng)建對象的。此時還會將args參數(shù)設置給Fragment∩跎現(xiàn)在找到了具體報錯的地方义郑，但是這個方法是在哪里調用觸發(fā)的呢？在Fragment沒有找到調用的地方鳖藕，由于Fragment是由FragmentManager管理的魔慷，在該類發(fā)現(xiàn)是在restoreAllState方法中調用的伺通。

void restoreAllState(Parcelable state, FragmentManagerNonConfig nonConfig) {
        // Build the full list of active fragments, instantiating them from
        // their saved state.
        mActive = new SparseArray<>(fms.mActive.length);
        for (int i=0; i<fms.mActive.length; i++) {
            FragmentState fs = fms.mActive[i];
            if (fs != null) {
                FragmentManagerNonConfig childNonConfig = null;
                if (childNonConfigs != null && i < childNonConfigs.size()) {
                    childNonConfig = childNonConfigs.get(i);
                }
                ViewModelStore viewModelStore = null;
                if (viewModelStores != null && i < viewModelStores.size()) {
                    viewModelStore = viewModelStores.get(i);
                }
                Fragment f = fs.instantiate(mHost, mContainer, mParent, childNonConfig,
                        viewModelStore);
                if (DEBUG) Log.v(TAG, "restoreAllState: active #" + i + ": " + f);
                mActive.put(f.mIndex, f);
                // Now that the fragment is instantiated (or came from being
                // retained above), clear mInstance in case we end up re-restoring
                // from this FragmentState again.
                fs.mInstance = null;
            }
        }
    ...   
    }

這方法名意為恢復所有的狀態(tài)记舆，而其中注釋為創(chuàng)建激活Fragment的列表，并將他們從保存的狀態(tài)中實例化魁亦。這個方法應該是Fragment重新實例化時調用的方法喉誊。該方法在Fragment的restoreChildFragmentState被調用邀摆。

void restoreChildFragmentState(@Nullable Bundle savedInstanceState) {
    if (savedInstanceState != null) {
        Parcelable p = savedInstanceState.getParcelable(
                FragmentActivity.FRAGMENTS_TAG);
        if (p != null) {
            if (mChildFragmentManager == null) {
                instantiateChildFragmentManager();
            }
            mChildFragmentManager.restoreAllState(p, mChildNonConfig);
            mChildNonConfig = null;
            mChildFragmentManager.dispatchCreate();
        }
    }
}

restoreChildFragmentState方法又在Fragment的onCreate方法中調用，這里將保存的savedInstanceState狀態(tài)又傳遞給了restoreChildFragmentState以完成Fragment的重新實例化伍茄。

@CallSuper
public void onCreate(@Nullable Bundle savedInstanceState) {
    mCalled = true;
    restoreChildFragmentState(savedInstanceState);
    if (mChildFragmentManager != null
            && !mChildFragmentManager.isStateAtLeast(Fragment.CREATED)) {
        mChildFragmentManager.dispatchCreate();
    }
}

結論

經過以上的分析栋盹，我們就知道了為什么這個錯誤出在了Fragment的有參構造函數(shù)上。因為當Fragment因為某種原因重新創(chuàng)建時敷矫，會調用到onCreate方法傳入之前保存的狀態(tài)例获，在instantiate方法中通過反射無參構造函數(shù)創(chuàng)建一個Fragment汉额，并且為Arguments初始化為原來保存的值，而此時如果沒有無參構造函數(shù)就會拋出異常榨汤，造成程序崩潰蠕搜。
所以Fragment的構造函數(shù)以及參數(shù)傳遞正確使用方式為如下：

public static MorePlanFragment newInstance(String modelId,HashMap<String, Integer> map) {
    Bundle args = new Bundle();
    args.putString("modelId",modelId);
    args.putSerializable("map",map);
    MorePlanFragment fragment = new MorePlanFragment();
    fragment.setArguments(args);
    return fragment;
}

為什么使用Fragment時必須提供一個無參的構造函數(shù)族淮？

為什么使用Fragment時必須提供一個無參的構造函數(shù)？

問題

分析

結論