考虑TabbedActivity的这部分代码：
public class MainActivity extends AppCompatActivity {
/**
* The {@link android.support.v4.view.PagerAdapter} that will provide
* fragments for each of the sections. We use a
* {@link FragmentPagerAdapter} derivative, which will keep every
* loaded fragment in memory. If this becomes too memory intensive, it
* may be best to switch to a
* {@link android.support.v4.app.FragmentStatePagerAdapter}.
*/
private SectionsPagerAdapter mSectionsPagerAdapter;
/**
* The {@link ViewPager} that will host the section contents.
*/
private ViewPager mViewPager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
// Create the adapter that will return a fragment for each of the three
// primary sections of the activity.
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.container);
mViewPager.setAdapter(mSectionsPagerAdapter);
TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
tabLayout.setupWithViewPager(mViewPager);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
/**
* The fragment argument representing the section number for this
* fragment.
*/
private static final String ARG_SECTION_NUMBER = "section_number";
public PlaceholderFragment() {
}
/**
* Returns a new instance of this fragment for the given section
* number.
*/
public static PlaceholderFragment newInstance(int sectionNumber) {
PlaceholderFragment fragment = new PlaceholderFragment();
Bundle args = new Bundle();
args.putInt(ARG_SECTION_NUMBER, sectionNumber);
fragment.setArguments(args);
return fragment;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState)
{
switch (getArguments().getInt(ARG_SECTION_NUMBER))
{
case 1:
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
return rootView;
case 2:
View rootView2 = inflater.inflate(R.layout.fragment_main, container, false);
Toast.makeText(getContext(),"FRAGMENT 2 BUILT WHEN WE VIEW FRAGMENT 1! ",Toast.LENGTH_LONG).show();
return rootView2;
case 3:
View rootView3 = inflater.inflate(R.layout.fragment_main, container, false);
return rootView3;
default:
View rootView0 = inflater.inflate(R.layout.fragment_main, container, false);
return rootView0;
}
}
}
/**
* A {@link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the sections/tabs/pages.
*/
public class SectionsPagerAdapter extends FragmentPagerAdapter {
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
// getItem is called to instantiate the fragment for the given page.
// Return a PlaceholderFragment (defined as a static inner class below).
return PlaceholderFragment.newInstance(position + 1);
}
@Override
public int getCount() {
// Show 3 total pages.
return 3;
}
@Override
public CharSequence getPageTitle(int position) {
switch (position) {
case 0:
return "SECTION 1";
case 1:
return "SECTION 2";
case 2:
return "SECTION 3";
}
return null;
}
}
}
当我们在案例1中时，案例2被调用，如果我在第二个片段中放置了我想要看的Toast，我在第一个看起来因为第二个是之前构建的。我想只在片段在我的TabbedActivity中可视化时才调用片段的创建。我怎么能这样做？
答案 0 :(得分：1)
TabbedActivity在核心中使用ViewPager来提供滑动行为。据我了解，您希望片段只有在可见时才会被实例化。但是，默认情况下，ViewPager始终使用1个预取项加载片段（或项）。您可以更改该金额，但最低为1。以下是ViewPager类的代码：
public void setOffscreenPageLimit(int limit) {
if (limit < DEFAULT_OFFSCREEN_PAGES) {
Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to "
+ DEFAULT_OFFSCREEN_PAGES);
limit = DEFAULT_OFFSCREEN_PAGES;
}
if (limit != mOffscreenPageLimit) {
mOffscreenPageLimit = limit;
populate();
}
}
因此，使用ViewPager，您无法实现目标。您可以使用TabLayout实现类似的行为
使用TabLayout的XML布局：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<android.support.v7.widget.Toolbar
android:layout_width="match_parent"
android:layout_height="?android:attr/actionBarSize"
android:background="@color/colorPrimary"
app:contentInsetStart="0dp">
<android.support.design.widget.TabLayout
android:id="@+id/tabLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:tabIndicatorColor="#000000"
app:tabIndicatorHeight="3dp">
<android.support.design.widget.TabItem
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Tab 1" />
<android.support.design.widget.TabItem
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Tab 2" />
<android.support.design.widget.TabItem
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Tab 3" />
<android.support.design.widget.TabItem
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Tab 4" />
</android.support.design.widget.TabLayout>
</android.support.v7.widget.Toolbar>
<FrameLayout
android:id="@+id/fragmentContainer"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1" />
</LinearLayout>
您还应该为此活动设置NoActionBar主题
活动代码：
public class TabbedActivity extends AppCompatActivity {
private SparseArrayCompat<Fragment> fragments = new SparseArrayCompat<>();
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.tabbed_activity);
setTitle("");
fragments.put(0, new Fragment()); //Here you need to instantiate your fragments
fragments.put(1, new Fragment()); // instead of new Fragment()
fragments.put(2, new Fragment()); // like new Tab1Fragment()
fragments.put(3, new Fragment());
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
TabLayout tabLayout = (TabLayout) findViewById(R.id.tabLayout);
tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
@Override
public void onTabSelected(TabLayout.Tab tab) {
changeFragment(tab.getPosition());
}
@Override
public void onTabUnselected(TabLayout.Tab tab) {
}
@Override
public void onTabReselected(TabLayout.Tab tab) {
}
});
changeFragment(0);
}
private void changeFragment(int position) {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction transaction = fragmentManager.beginTransaction();
Fragment fragment = fragments.get(position);
String transactionTag = fragment.getClass().getSimpleName();
if (fragmentManager.getBackStackEntryCount() > 0 &&
fragmentManager.getBackStackEntryAt(fragmentManager.getBackStackEntryCount() - 1).getName()
.equalsIgnoreCase(transactionTag)) {
return;
}
transaction.replace(R.id.fragmentContainer, fragment, transactionTag);
transaction.commitAllowingStateLoss();
}
}
TabLayout非常灵活，请在此处详细了解（https://guides.codepath.com/android/google-play-style-tabs-using-tablayout）