如果您管理将每个事务添加到后退堆栈的流程，那么您可以这样做，以便在用户按后退按钮时显示上一个片段(您也可以映射home按钮)。

@Override
public void onBackPressed() {
    if (getFragmentManager().getBackStackEntryCount() > 0)
        getFragmentManager().popBackStack();
    else
        super.onBackPressed();
}

在您的活动中添加此代码

@Override

public void onBackPressed() {
    if (getFragmentManager().getBackStackEntryCount() == 0) {
        super.onBackPressed();
    } else {
        getFragmentManager().popBackStack();
    }
}

并在commit()之前在Fragment中添加这一行

ft.addToBackStack(任何名称);

我们创建了一个小的库来处理多个片段和/或活动中的背压。使用就像在gradle文件中添加依赖一样简单:

compile 'net.skoumal.fragmentback:fragment-back:0.1.0'

让你的片段实现BackFragment接口:

public abstract class MyFragment extends Fragment implements BackFragment {

    public boolean onBackPressed() {

        // -- your code --

        // return true if you want to consume back-pressed event
        return false;
    }

    public int getBackPriority() {
        return NORMAL_BACK_PRIORITY;
    }
}

通知你的片段关于背压:

public class MainActivity extends AppCompatActivity {

    @Override
    public void onBackPressed() {
        // first ask your fragments to handle back-pressed event
        if(!BackFragmentHelper.fireOnBackPressedEvent(this)) {
            // lets do the default back action if fragments don't consume it
            super.onBackPressed();
        }
    }
}

欲了解更多详细信息和其他用例，请访问GitHub页面:

https://github.com/skoumalcz/fragment-back

你可以使用from getActionBar().setDisplayHomeAsUpEnabled():

@Override
public void onBackStackChanged() {
    int backStackEntryCount = getFragmentManager().getBackStackEntryCount();

    if(backStackEntryCount > 0){
        getActionBar().setDisplayHomeAsUpEnabled(true);
    }else{
        getActionBar().setDisplayHomeAsUpEnabled(false);
    }
}

对于那些谁使用静态片段

在这种情况下，如果你有一个静态片段，那么它会更可取。 为片段创建一个实例对象

private static MyFragment instance=null;

在MyFragment的onCreate()中初始化该实例

  instance=this;

也可以创建一个函数来获取Instance

 public static MyFragment getInstance(){
   return instance;
}

也可以创建函数

public boolean allowBackPressed(){
    if(allowBack==true){
        return true;
    }
    return false;
}


 //allowBack is a boolean variable that will be set to true at the action 
 //where you want that your backButton should not close activity. In my case I open 
 //Navigation Drawer then I set it to true. so when I press backbutton my 
 //drawer should be get closed

public void performSomeAction(){
    //.. Your code
    ///Here I have closed my drawer
}

在你能做的活动中

@Override
public void onBackPressed() {

    if (MyFragment.getInstance().allowBackPressed()) { 
        MyFragment.getInstance().performSomeAction();
    }
    else{
        super.onBackPressed();
    }
}

检查后台工作完美

@Override
public boolean onKeyDown(int keyCode, KeyEvent event)
{
    if (keyCode == KeyEvent.KEYCODE_BACK)
    {
        if (getFragmentManager().getBackStackEntryCount() == 1)
        {
            // DO something here since there is only one fragment left
            // Popping a dialog asking to quit the application
            return false;
        }
    }
    return super.onKeyDown(keyCode, event);
}