S2. Android 常用控件

    <Button
        android:id="@+id/testBtn"
        android:layout_width="150dp"
        android:layout_height="43dp"
        android:onClick="onClick"
        android:text="测试按钮"
        tools:layout_editor_absoluteX="130dp"
        tools:layout_editor_absoluteY="46dp" />

    protected void onClick(View v){
        switch (v.getId()){
            case R.id.testBtn:
                Toast.makeText(this, "测试按钮被点击", Toast.LENGTH_SHORT).show();
        }
    }

    //用户点击返回键时触发
    private static final int TIME_INTERVAL = 2000;
    private long mBackPressed = 0;
    public void onBackPressed(){
        long cMills = System.currentTimeMillis();
        if(cMills - mBackPressed > TIME_INTERVAL){//如果两次点击时间间隔超过 TIME_INTERVAL，则执行“提示退出”操作
            mBackPressed = cMills;
            Toast.makeText(this, "再按一次退出", Toast.LENGTH_SHORT).show();
        }else{//如果两次点击在 TIME_INTERVAL 时间间隔内，则执行“退出”操作
            super.onBackPressed();
        }
    }

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:android="http://schemas.android.com/apk/res/android">

    <item
        android:id="@+id/navigation_home"
        android:icon="@drawable/ic_home_black_24dp"
        android:title="@string/title_home" />

    <item
        android:id="@+id/navigation_dashboard"
        android:icon="@drawable/ic_dashboard_black_24dp"
        android:title="@string/title_dashboard" />

    <item
        android:id="@+id/navigation_notifications"
        android:icon="@drawable/ic_notifications_black_24dp"
        android:title="@string/title_notifications" />

</menu>

<resources>
    <string name="app_name">HelloWorld</string>
    <string name="title_home">主页</string>
    <string name="title_dashboard">导航</string>
    <string name="title_notifications">通知</string>
</resources>