Custom view to draw bitmap along path

Refer to my old exercise "Animation of moving bitmap along path", the bitmap and path are hard-coded inside custom view. It's a modified version; the thing to be drawn (bitmap/path) are held separated and referenced by custom view in a list, such that it is easy to insert more things.

Please notice:
- I don't thing it's a good approach to do this, I just show a interesting exercise.
- You should not do the calculation (such as move the bitmap) inside onDraw(), it's suggested to do it in another thread.

Create a new class AnimationThing.java. It hold the bitmap and path of individual thing.

package com.blogspot.android_er.androidmovingbitmapalongpath;

import android.graphics.Bitmap;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PathMeasure;

public class AnimationThing {

    Paint paint;
    Path animPath;
    PathMeasure pathMeasure;
    float pathLength;
    Bitmap bm;
    int bm_offsetX, bm_offsetY;
    float step;
    float distance;
    float[] pos;
    float[] tan;
    Matrix matrix;

    public AnimationThing(Paint paint,
                          Path animPath,
                          Bitmap bm,
                          float step) {
        this.paint = paint;

        this.animPath = animPath;
        pathMeasure = new PathMeasure(this.animPath, false);
        pathLength = pathMeasure.getLength();

        this.bm = bm;
        bm_offsetX = bm.getWidth()/2;
        bm_offsetY = bm.getHeight()/2;

        this.step = step;
        distance = 0;
        pos = new float[2];
        tan = new float[2];

        matrix = new Matrix();
    }
}

Create our custom view, AnimationView .java.

package com.blogspot.android_er.androidmovingbitmapalongpath;

import android.content.Context;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.view.View;

import java.util.ArrayList;
import java.util.List;

public class AnimationView extends View {

    List<AnimationThing> animationThingsList;
    public AnimationView(Context context) {
        super(context);
        initAnimationView();
    }

    public AnimationView(Context context, AttributeSet attrs) {
        super(context, attrs);
        initAnimationView();
    }

    public AnimationView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initAnimationView();
    }

    private void initAnimationView(){
        animationThingsList = new ArrayList<>();
    }

    public void insertThing(AnimationThing thing){
        animationThingsList.add(thing);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        for (AnimationThing thing : animationThingsList){

            //!!! Only the path of the last thing will be drawn on screen
            canvas.drawPath(thing.animPath, thing.paint);

            if(thing.distance < thing.pathLength){
                thing.pathMeasure.getPosTan(thing.distance, thing.pos, thing.tan);

                thing.matrix.reset();
                float degrees = (float)(Math.atan2(thing.tan[1], thing.tan[0])*180.0/Math.PI);
                thing.matrix.postRotate(degrees, thing.bm_offsetX, thing.bm_offsetY);
                thing.matrix.postTranslate(thing.pos[0]-thing.bm_offsetX, thing.pos[1]-thing.bm_offsetY);

                canvas.drawBitmap(thing.bm, thing.matrix, null);

                thing.distance += thing.step;
            }else{
                thing.distance = 0;
            }
        }

        invalidate();
    }
}

Layout, activity_main.xml.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp"
    android:orientation="vertical"
    tools:context="com.blogspot.android_er.androidmovingbitmapalongpath.MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:autoLink="web"
        android:text="http://android-er.blogspot.com/"
        android:textStyle="bold" />

    <com.blogspot.android_er.androidmovingbitmapalongpath.AnimationView
        android:id="@+id/MyAnimationView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#F0F0F0"/>
</LinearLayout>

MainActivity.java

package com.blogspot.android_er.androidmovingbitmapalongpath;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    AnimationView myAnimationView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myAnimationView = (AnimationView)findViewById(R.id.MyAnimationView);

        prepareThings();
    }

    private void prepareThings(){
        Paint paint;
        Path animPath;
        float step;
        Bitmap bm;

        paint = new Paint();
        paint.setColor(Color.BLUE);
        paint.setStrokeWidth(1);
        paint.setStyle(Paint.Style.STROKE);

        bm = BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher);

        animPath = new Path();
        animPath.moveTo(100, 100);
        animPath.lineTo(200, 100);
        animPath.lineTo(300, 50);
        animPath.lineTo(400, 150);
        animPath.lineTo(100, 300);
        animPath.lineTo(600, 300);
        animPath.lineTo(100, 100);
        animPath.close();

        step = 1;

        AnimationThing thing = new AnimationThing(paint, animPath, bm, step);
        myAnimationView.insertThing(thing);

        //The second thing
        bm = BitmapFactory.decodeResource(getResources(), android.R.drawable.ic_menu_add);

        animPath.reset();
        animPath.addCircle(400, 400, 300, Path.Direction.CW);
        step = 3;
        thing = new AnimationThing(paint, animPath, bm, step);
        myAnimationView.insertThing(thing);
    }
}