Android Fragment 监听返回键

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直接给代码*-*

Fragment：

  public boolean onKeyDown(int keyCode, KeyEvent event)
    {
        if ((keyCode == KeyEvent.KEYCODE_BACK))
        {
            //执行操作
            ((MainActivity) Objects.requireNonNull(getActivity())).viewPager.setCurrentItem(3); 
        
        }
        return true;
    }

MainActivity：

 
 @BindView(R.id.viewPager)
 public ViewPager viewPager;

 Fragment currentFragment;

   viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener()
        {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels)
            {
            
            }
            
            @Override
            public void onPageSelected(int position)
            {
                FragmentManager fragmentManager = MainActivity.this.getSupportFragmentManager();
                List<Fragment> fragments = fragmentManager.getFragments();
                currentFragment=fragments.get(position);
                ……
            }
            
            @Override
            public void onPageScrollStateChanged(int state)
            {
            }
        });

setupViewPager(viewPager);

 private void setupViewPager(ViewPager viewPager)
    {
        
        adapter = new ViewFragmentPagerAdapter(getSupportFragmentManager());
        
        adapter.addFragment(MainFragment.newInstance(getResources().getString(R.string.app_title)));
        adapter.addFragment(new Fragment());
        ……
        viewPager.setAdapter(adapter);
        viewPager.setOffscreenPageLimit(5);
    }

   // 用来计算返回键的点击间隔时间
    private long exitTime = 0;
    
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event)
    {
        if(currentFragment!=null&&currentFragment instanceof AboutApp){
            ((AboutApp)currentFragment).onKeyDown(keyCode, event);
            return true;
        } else  if (isTaskRoot())
        {
            if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN)
            {
                if ((System.currentTimeMillis() - exitTime) > 2000)
                {
                    Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
                    exitTime = System.currentTimeMillis();
                }
                else
                {
                    finish();
                }
                return true;
            }
        }
        
        return super.onKeyDown(keyCode, event);
    }