课程链接：http://idcbgp.cn/learn/247
实现自定义Topbar可以大略分为如下三个步骤：

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="Topbar">
        <attr name="title" format="string"/>
        <attr name="titleTextSize" format="dimension"/>
        <attr name="titleTextColor" format="color"/>

        <attr name="leftText" format="string"/>
        <attr name="leftBackground" format="referencecolor"/>
        <attr name="leftTextColor" format="color"/>

        <attr name="rightText" format="string"/>
        <attr name="rightBackground" format="referencecolor"/>
        <attr name="rightTextColor" format="color"/>
    </declare-styleable>
</resources>

public class Topbar extends RelativeLayout{
    private Button leftButton,rightButton;
    private TextView tvTitle;
    // 左边button的属性
    private int leftTextColor;
    private Drawable leftBackground;
    private String leftText;
    // 右边button的属性
    private int rightTextColor;
    private Drawable rightBackground;
    private String rightText;
    // Title属性
    private float titleTextSize;
    private int titleTextColor;
    private String title;
    private LayoutParams leftParams,rightParams,titleParams; // 布局参数对象

    public Topbar(Context context, AttributeSet attrs) {
        super(context, attrs);
        // 获取自定义的属性
        TypedArray ta=context.obtainStyledAttributes(attrs, R.styleable.Topbar);
        // 实例化属性成员
        titleTextSize=ta.getDimension(R.styleable.Topbar_titleTextSize, 0);
        titleTextColor=ta.getColor(R.styleable.Topbar_titleTextColor, 0);
        title=ta.getString(R.styleable.Topbar_title);
        // 省略实例化其他属性成员的代码...

        ta.recycle(); // 回收数据，避免资源浪费

        // 根据context实例化控件
        tvTitle=new TextView(context);
        leftButton=new Button(context);
        rightButton=new Button(context);

        // 为属性成员赋值
        tvTitle.setTextSize(titleTextSize);
        tvTitle.setTextColor(titleTextColor);
        tvTitle.setText(title);
        tvTitle.setGravity(Gravity.CENTER); // 设置TextView文字居中显示
        // 省略其他属性成员的赋值代码...

        setBackgroundColor(0xFFF59563); // 给当前的Topbar设置背景颜色

        // 实例化布局参数对象，另两个布局参数对象的实例化类似，这里省略
        titleParams=new LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,LayoutParams.MATCH_PARENT);
        // 为控件添加布局属性，省略另两个控件的类似操作
        titleParams.addRule(RelativeLayout.CENTER_IN_PARENT,TRUE);
        // 将控件添加到Topbar并传入对应的布局参数对象
        addView(tvTitle, titleParams);
        addView(leftButton, leftParams);
        addView(rightButton, rightParams);
    }
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:custom = "http://schemas.android.com/apk/res-auto" <!-- 设置自定义属性的命名空间 -->
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.imooc.widget.Topbar
        android:layout_width="match_parent"
        android:layout_height="50dp"
        <!-- 使用自定义属性 -->
        custom:leftBackground="@drawable/button_selector"
        custom:leftText="BACK"
        custom:leftTextColor="@color/white"

        custom:rightBackground="@drawable/button_selector"
        custom:rightText="MORE"
        custom:rightTextColor="@color/white"

        custom:title="自定义标题"
        custom:titleTextColor="#123412"
        custom:titleTextSize="15sp">
    </com.imooc.widget.Topbar>

</RelativeLayout>