package example.antonio.activitydemo;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.support.v4.app.*;

public class MainActivity extends FragmentActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final FragmentManager fragmentManager = getSupportFragmentManager();

        Button button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                FirstFragment fragment = new FirstFragment();
                FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

                fragmentTransaction.add(R.id.container,fragment);
                fragmentTransaction.commit();
            }
        });

        Button button2 = (Button) findViewById(R.id.button2);
        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                SecondFragment fragment = new SecondFragment();
                FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

                fragmentTransaction.replace(R.id.container,fragment);
                fragmentTransaction.commit();
            }
        });
    }
}

FirstFragment

package example.antonio.activitydemo;

import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.ViewGroup;
import android.view.View;

public class FirstFragment extends Fragment {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInsanceState) {
        return inflater.inflate(R.layout.first_fragment,container,false);
    }
}

SecondFragment

package example.antonio.activitydemo;

import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.ViewGroup;
import android.view.View;

public class SecondFragment extends Fragment {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater,Bundle savedInsanceState) {
        return inflater.inflate(R.layout.second_fragment,false);
    }
}

他们的XML文件：

main_activity.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/container"
    android:orientation="vertical">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start1"
        android:id="@+id/button"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start2"
        android:id="@+id/button2"/>

</LinearLayout>

first_fragment.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="#000000">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="First!"
        android:textColor="#ffffff"/>


</FrameLayout>

second_fragment.xml就像前一个一样,只有android：text =“Second！”

这是奇怪的事情,当我点击4次Start1按钮然后我得到4倍的单词First！,直到这里一切正常.如果我单击Start2按钮,我会期望删除前面的4个片段并添加一个新的片段,其外观为Second！.这是我从文档中理解的：

Replace an existing fragment that was added to a container. This is essentially the same as calling remove(Fragment) for all currently added fragments that were added with the same containerViewId and then add(int,Fragment,String) with the same arguments given here.

但这是我得到的：

我试着阅读源代码,也许我发现了什么是“问题”,我想知道你的想法,如果是我,我在某个地方犯了错误.

以下是BackStackRecord.java的片段,run()方法：

case OP_REPLACE: {
                    Fragment f = op.fragment;
                    int containerId = f.mContainerId;
                    if (mManager.mAdded != null) {
                        for (int i = 0; i < mManager.mAdded.size(); i++) {
                            Fragment old = mManager.mAdded.get(i);
                            if (FragmentManagerImpl.DEBUG) {
                                Log.v(TAG,"OP_REPLACE: adding=" + f + " old=" + old);
                            }
                            if (old.mContainerId == containerId) {
                                if (old == f) {
                                    op.fragment = f = null;
                                } else {
                                    if (op.removed == null) {
                                        op.removed = new ArrayList<Fragment>();
                                    }
                                    op.removed.add(old);
                                    old.mNextAnim = op.exitAnim;
                                    if (mAddToBackStack) {
                                        old.mBackStackNesting += 1;
                                        if (FragmentManagerImpl.DEBUG) {
                                            Log.v(TAG,"Bump nesting of "
                                                    + old + " to " + old.mBackStackNesting);
                                        }
                                    }
                                    mManager.removeFragment(old,mTransition,mTransitionStyle);
                                }
                            }
                        }
                    }

这是FragmentManager.java的片段：

public void removeFragment(Fragment fragment,int transition,int transitionStyle) {
        if (DEBUG) Log.v(TAG,"remove: " + fragment + " nesting=" + fragment.mBackStackNesting);
        final boolean inactive = !fragment.isInBackStack();
        if (!fragment.mDetached || inactive) {
            if (false) {
                // Would be nice to catch a bad remove here,but we need
                // time to test this to make sure we aren't crashes cases
                // where it is not a problem.
                if (!mAdded.contains(fragment)) {
                    throw new IllegalStateException("Fragment not added: " + fragment);
                }
            }
            **if (mAdded != null) {
                mAdded.remove(fragment);
            }**
            if (fragment.mHasMenu && fragment.mMenuVisible) {
                mNeedMenuInvalidate = true;
            }
            fragment.mAdded = false;
            fragment.mRemoving = true;
            moveToState(fragment,inactive ? Fragment.INITIALIZING : Fragment.CREATED,transition,transitionStyle,false);
        }
    }

正如您所看到的,removeFragment方法从mAdded中删除了一个frament,但是在run()方法之后,i索引没有被修改,它从它离开的地方重新启动,这样就会丢失一些元素……