编写Android应用程序代码：请设计一个“掌上驴友”的软件，其中登录界面上密码输入部分要求密码显示。登陆后的中心框架界面用选项卡的形式来设计，“掌上驴友”的功能分为四个模块。

好的，以下是一个简单的Android应用程序代码示例，实现了“掌上驴友”软件的基本功能。请注意，此代码只是示例，您需要根据自己的需求进行修改和扩展。

import android.os.Bundle;
import android.support.design.widget.TabLayout;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    private TabLayout tabLayout;
    private ViewPager viewPager;
    private ViewPagerAdapter viewPagerAdapter;

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

        tabLayout = (TabLayout) findViewById(R.id.tab_layout);
        viewPager = (ViewPager) findViewById(R.id.view_pager);
        viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());

        // 添加四个模块页面
        viewPagerAdapter.addFragment(new Module1Fragment(), "模块1");
        viewPagerAdapter.addFragment(new Module2Fragment(), "模块2");
        viewPagerAdapter.addFragment(new Module3Fragment(), "模块3");
        viewPagerAdapter.addFragment(new Module4Fragment(), "模块4");

        viewPager.setAdapter(viewPagerAdapter);
        tabLayout.setupWithViewPager(viewPager);

        // 在登录界面上密码输入部分要求密码显示
        PasswordEditText passwordEditText = (PasswordEditText) findViewById(R.id.password_edit_text);
        passwordEditText.setTransformationMethod(new PasswordTransformationMethod());
    }
}

其中，`Module1Fragment`、`Module2Fragment`、`Module3Fragment`和`Module4Fragment`是四个模块的页面。您需要根据自己的需求编写这些页面的代码。

另外，`PasswordEditText`是一个自定义的带密码显示功能的EditText控件，您需要自己实现这个控件。以下是一个简单的示例代码：

import android.content.Context;
import android.graphics.drawable.Drawable;
import android.support.v4.content.ContextCompat;
import android.text.InputType;
import android.text.method.PasswordTransformationMethod;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.widget.EditText;

public class PasswordEditText extends EditText {

    private Drawable showPasswordDrawable;
    private Drawable hidePasswordDrawable;
    private boolean isPasswordVisible = false;

    public PasswordEditText(Context context) {
        super(context);
        init();
    }

    public PasswordEditText(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public PasswordEditText(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    private void init() {
        // 获取显示密码和隐藏密码的drawable
        showPasswordDrawable = ContextCompat.getDrawable(getContext(), R.drawable.ic_show_password);
        hidePasswordDrawable = ContextCompat.getDrawable(getContext(), R.drawable.ic_hide_password);

        // 设置显示密码的drawable的点击事件
        showPasswordDrawable.setCallback(new Callback() {
            @Override
            public void invalidateDrawable(Drawable who) {
                invalidate();
            }

            @Override
            public void scheduleDrawable(Drawable who, Runnable what, long when) {
                scheduleDrawable(who, what, when);
            }

            @Override
            public void unscheduleDrawable(Drawable who, Runnable what) {
                unscheduleDrawable(who, what);
            }
        });
        showPasswordDrawable.setBounds(0, 0, showPasswordDrawable.getIntrinsicWidth(), showPasswordDrawable.getIntrinsicHeight());

        // 设置隐藏密码的drawable的点击事件
        hidePasswordDrawable.setCallback(new Callback() {
            @Override
            public void invalidateDrawable(Drawable who) {
                invalidate();
            }

            @Override
            public void scheduleDrawable(Drawable who, Runnable what, long when) {
                scheduleDrawable(who, what, when);
            }

            @Override
            public void unscheduleDrawable(Drawable who, Runnable what) {
                unscheduleDrawable(who, what);
            }
        });
        hidePasswordDrawable.setBounds(0, 0, hidePasswordDrawable.getIntrinsicWidth(), hidePasswordDrawable.getIntrinsicHeight());

        // 设置EditText的输入类型为密码
        setInputType(InputType.TYPE_CLASS_TEXT | InputType.TYPE_TEXT_VARIATION_PASSWORD);
        setTransformationMethod(new PasswordTransformationMethod());

        // 设置EditText的右侧drawable为显示密码的drawable
        setCompoundDrawables(null, null, showPasswordDrawable, null);

        // 设置EditText的drawable的点击事件
        setOnTouchListener(new OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                if (event.getAction() == MotionEvent.ACTION_UP) {
                    // 判断是否点击了drawable
                    if (event.getX() >= getWidth() - getCompoundPaddingRight()) {
                        // 切换密码可见性
                        isPasswordVisible = !isPasswordVisible;
                        if (isPasswordVisible) {
                            setCompoundDrawables(null, null, hidePasswordDrawable, null);
                            setInputType(InputType.TYPE_CLASS_TEXT);
                        } else {
                            setCompoundDrawables(null, null, showPasswordDrawable, null);
                            setInputType(InputType.TYPE_CLASS_TEXT | InputType.TYPE_TEXT_VARIATION_PASSWORD);
                        }
                        setSelection(length());
                    }
                }
                return false;
            }
        });
    }
}