先看看效果图

1.自定义 View 的基本流程

• 创建 View Class
• 创建 attr 属性文件，确定属性
• View Class 绑定 attr 属性
• onMeasure 测量
• onDraw 绘制
• onTouchEvent ( 用户交互需要处理 )

1.1 创建 View Class

package com.example.view_day05_ratingbar;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;

import androidx.annotation.Nullable;

public class RatingBar extends View {
    private static final String TAG = "wqq";
    private Bitmap mStartNormalBitmap,mStartFocusBitmap;
    private int mGradeNumber = 5;
    private int mCurrentGrade = 0;
    public RatingBar(Context context) {
        this(context, null);
    }

    public RatingBar(Context context, @Nullable @org.jetbrains.annotations.Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public RatingBar(Context context, @Nullable @org.jetbrains.annotations.Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        TypedArray array = context.obtainStyledAttributes(attrs,R.styleable.RatingBar);
        int starNormalID = array.getResourceId(R.styleable.RatingBar_starNormal,0);
        if (starNormalID == 0) {
            throw new RuntimeException("请设置属性 startNormal");
        }

        mStartNormalBitmap = BitmapFactory.decodeResource(getResources(),starNormalID);

        int startFocusId = array.getResourceId(R.styleable.RatingBar_starFocus, 0);
        if (startFocusId == 0) {
            throw new RuntimeException("请设置属性 startFocusId");
        }
        mStartFocusBitmap = BitmapFactory.decodeResource(getResources(), startFocusId);

        mGradeNumber = array.getInt(R.styleable.RatingBar_gradeNumber,0);
        array.recycle();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        //高度 一张图片的高度，自己去实现 padding
        int height = mStartNormalBitmap.getHeight();
        int width = mGradeNumber * mStartFocusBitmap.getWidth();
        setMeasuredDimension(width,height);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        for (int i = 0; i < mGradeNumber; i++) {
            int x = i * mStartFocusBitmap.getWidth();
            if (mCurrentGrade > i) {
                canvas.drawBitmap(mStartFocusBitmap,x,0,null);
            } else {
                canvas.drawBitmap(mStartNormalBitmap,x,0,null);
            }
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        //移动 按下 抬起 处理逻辑都是一样，判断手指的位置，根据当前位置计算出分数，再去刷新界面显示
        switch(event.getAction()) {
            case MotionEvent.ACTION_DOWN: //按下
            case MotionEvent.ACTION_MOVE: // 移动
            //case MotionEvent.ACTION_UP: // 抬起,up事件可以去掉，优化ondraw调用次数
            float moveX = event.getX(); // event.getX()，获取相对于当前控件的位置；event.getRawX()，获取屏幕的x位置
                Log.e(TAG, "movex: "+ moveX + "event:"+ event.getAction());
                int currentGrade = (int)(moveX/mStartFocusBitmap.getWidth() + 1);
            if (currentGrade < 0) {
                currentGrade = 0;
            }
            if (currentGrade > mGradeNumber) {
                currentGrade = mGradeNumber;
            }
            //相等的话，就不用重复绘制了
            if (currentGrade == mCurrentGrade) {
                return true;
            }
            mCurrentGrade = currentGrade;
            //刷新显示
            invalidate();// 由于ondraw流程很长，尽量减少ondraw的调用,减少内存
                break;
        }
        return true;
    }
}

下篇文字会分析 onTouchEvent 源码，为什么要设置返回 true

1.2 创建 attr

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="RatingBar">
        <!--未选中引用-->
        <attr name="starNormal" format="reference" />
        <!--选中引用-->
        <attr name="starFocus" format="reference" />
        <!--最大的分数-->
        <attr name="gradeNumber" format="integer" />
        <!--当前的分数-->
        <attr name="currentGrade" format="integer" />
        <!--星星之间的间距-->
        <attr name="starPadding" format="dimension" />
    </declare-styleable>
</resources>

1.3 创建图片资源

在\app\src\main\res目录下创建文件夹drawable-xhdpi，将星星图片复制进去。

1.4 布局文件

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <com.example.view_day05_ratingbar.RatingBar
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:background="@color/purple_200"
        app:starPadding="50dp"
        app:starFocus="@drawable/select"
        app:starNormal="@drawable/start_normal"
        app:gradeNumber="5"/>

</RelativeLayout>