Android Fragment使用构造函数传参，在签名打包时出错的原因及解决办法

我不是女神ヾ 2022-05-24 02:40 63阅读 0赞

先说结果：

**当我们实例化自定义Fragment时，官方推荐Fragment.setArguments(Bundle bundle)这种方式来传递参数，而不推荐通过构造方法直接来传递参数。**

原因分析：

首先，我们来测试下通过构造方法传递参数的情况：

public class FramentTestActivity extends ActionBarActivity {  
          
        @Override  
        protected void onCreate(Bundle savedInstanceState) {  
            super.onCreate(savedInstanceState);  
            setContentView(R.layout.activity_main);  
      
            if (savedInstanceState == null) {  
                getSupportFragmentManager().beginTransaction()  
                        .add(R.id.container, new TestFragment("param")).commit();  
            }  
              
        }  
      
        public static class TestFragment extends Fragment {  
      
            private String mArg = "non-param";  
              
            public TestFragment() {  
                Log.i("INFO", "TestFragment non-parameter constructor");  
            }  
              
            public TestFragment(String arg){  
                mArg = arg;  
                Log.i("INFO", "TestFragment construct with parameter");  
            }  
      
            @Override  
            public View onCreateView(LayoutInflater inflater, ViewGroup container,  
                    Bundle savedInstanceState) {  
                View rootView = inflater.inflate(R.layout.fragment_main, container,  
                        false);  
                TextView tv = (TextView) rootView.findViewById(R.id.tv);  
                tv.setText(mArg);  
                return rootView;  
            }  
        }  
      
    }

![70][]

可以看到我们传递过来的数据正确的显示了，现在来考虑一个问题，如果设备配置参数发生变化，这里以横竖屏切换来说明问题，显示如下

![70 1][]

发生了什么问题呢？我们传递的参数哪去了？为什么会显示默认值？不急着讨论这个问题，接下来我们来看看Fragment.setArguments(Bundle bundle)这种方式的运行情况

public class FramentTest2Activity extends ActionBarActivity {  
             
            @Override  
            protected void onCreate(Bundle savedInstanceState) {  
                  super.onCreate(savedInstanceState);  
                 setContentView(R.layout. activity_main);  
      
                  if (savedInstanceState == null) {  
                        getSupportFragmentManager().beginTransaction()  
                                     .add(R.id. container, TestFragment.newInstance("param")).commit();  
                 }  
      
           }  
      
            public static class TestFragment extends Fragment {  
      
                  private static final String ARG = "arg";  
                   
                  public TestFragment() {  
                        Log. i("INFO", "TestFragment non-parameter constructor" );  
                 }  
      
                  public static Fragment newInstance(String arg){  
                        TestFragment fragment = new TestFragment();  
                        Bundle bundle = new Bundle();  
                        bundle.putString( ARG, arg);  
                        fragment.setArguments(bundle);  
                         return fragment;  
                 }  
                   
                  @Override  
                  public View onCreateView(LayoutInflater inflater, ViewGroup container,  
                               Bundle savedInstanceState) {  
                        View rootView = inflater.inflate(R.layout. fragment_main, container,  
                                      false);  
                        TextView tv = (TextView) rootView.findViewById(R.id. tv);  
                        tv.setText(getArguments().getString( ARG));  
                         return rootView;  
                 }  
           }  
      
    }

![70 2][]

我们再来看看横竖屏切换后的运行情况

![70 3][]

看到了吧，我们传递的参数在横竖屏切换的情况下完好保存了下来，正确的显示给用户

那么这到底是怎么回事呢，我们知道设备横竖屏切换的话，当前展示给用户的Activity默认情况下会重新创建并展现给用户，那依附于Activity的Fragment会进行如何处理呢，我们可以通过源码来查看

先来看看Activity的onCreate(Bundle saveInstance)方法

protected void onCreate(Bundle savedInstanceState) {  
        if (DEBUG_LIFECYCLE ) Slog.v( TAG, "onCreate " + this + ": " + savedInstanceState);  
        if (mLastNonConfigurationInstances != null) {  
            mAllLoaderManagers = mLastNonConfigurationInstances .loaders ;  
        }  
        if (mActivityInfo .parentActivityName != null) {  
            if (mActionBar == null) {  
                mEnableDefaultActionBarUp = true ;  
            } else {  
                mActionBar .setDefaultDisplayHomeAsUpEnabled( true);  
            }  
        }  
        if (savedInstanceState != null) {  
            Parcelable p = savedInstanceState.getParcelable( FRAGMENTS_TAG );  
            mFragments .restoreAllState(p, mLastNonConfigurationInstances != null  
                    ? mLastNonConfigurationInstances .fragments : null);  
        }  
        mFragments .dispatchCreate();  
        getApplication().dispatchActivityCreated( this , savedInstanceState);  
        mCalled = true ;  
    }

由于我们的Fragment是由FragmentManager来管理，所以可以跟进FragmentManager.restoreAllState()方法，通过对当前活动的Fragmnet找到下面的代码块

for (int i=0; i<fms.mActive.length; i++) {  
               FragmentState fs = fms.mActive[i];  
               if (fs != null) {  
                  Fragment f = fs.instantiate(mActivity, mParent);  
                   if (DEBUG) Log.v(TAG, "restoreAllState: active #" + i + ": " + f);  
                   mActive.add(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;  
               } else {  
                   mActive.add(null);  
                    if (mAvailIndices == null) {  
                        mAvailIndices = new ArrayList<Integer>();  
                   }  
                   if (DEBUG) Log.v(TAG, "restoreAllState: avail #" + i);  
                   mAvailIndices.add(i);  
               }  
    }

接下来我们可以看看FragmentState.instantitate()方法的实现

可以看到最终转入到Fragment. instantitate()方法

public Fragment instantiate(Activity activity, Fragment parent) {  
            if (mInstance != null) {  
                return mInstance ;  
            }  
             
            if (mArguments != null) {  
                mArguments .setClassLoader(activity.getClassLoader());  
            }  
             
            mInstance = Fragment.instantiate(activity, mClassName , mArguments );  
             
            if (mSavedFragmentState != null) {  
                mSavedFragmentState .setClassLoader(activity.getClassLoader());  
                mInstance .mSavedFragmentState = mSavedFragmentState ;  
            }  
            mInstance .setIndex(mIndex , parent);  
            mInstance .mFromLayout = mFromLayout ;  
            mInstance .mRestored = true;  
            mInstance .mFragmentId = mFragmentId ;  
            mInstance .mContainerId = mContainerId ;  
            mInstance .mTag = mTag ;  
            mInstance .mRetainInstance = mRetainInstance ;  
            mInstance .mDetached = mDetached ;  
            mInstance .mFragmentManager = activity.mFragments;  
            if (FragmentManagerImpl.DEBUG) Log.v(FragmentManagerImpl.TAG,  
                    "Instantiated fragment " + mInstance );  
      
            return mInstance ;  
        }

可以看到最终转入到Fragment.instantitate()方法

public static Fragment instantiate(Context context, String fname, 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.newInstance();  
           if (args != null) {  
               args.setClassLoader(f.getClass().getClassLoader());  
               f. mArguments = 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);  
       }

通过此方法可以看到，最终会通过反射无参构造实例化一个新的Fragment，并且给mArgments初始化为原先的值，而原来的Fragment实例的数据都丢失了，并重新进行了初始化

通过上面的分析，我们可以知道Activity重新创建时，会重新构建它所管理的Fragment，原先的Fragment的字段值将会全部丢失，但是通过Fragment.setArguments(Bundle bundle)方法设置的bundle会保留下来。所以尽量使用Fragment.setArguments(Bundle bundle)方式来传递参数

/**
         * 初始化fragmentlist
         */
        private void initFragmentList() {
            mListFragment = new ArrayList<>();
            for (int i = 0; i < 7; i++) {
                RemoteStepFragment fragment = new RemoteStepFragment();
                Bundle bundle = new Bundle();
                bundle.putInt("TAG", i);
                fragment.setArguments(bundle);
                mListFragment.add(fragment);
            }
        }

fragment中

// 获取传递的参数
    		mTag = getArguments().getInt("TAG");

完！！！

[70]: /images/20220524/77f697444bcc460eaba0999980bc5f07.png
[70 1]: /images/20220524/1499387539774c95b4762c06b0f58cbe.png
[70 2]: /images/20220524/743c2da19da44f11932e227f0197f719.png
[70 3]: /images/20220524/a7c4b5465216480291fb90f27406ba28.png