Android 下雪动画

原文链接 : Snowfall

原文作者 : Styling Android
译文出自 : hanks.xyz
译者 : hanks-zyh
校对者: desmond1121
状态 : 完成

这本是一个愉快的季节，但是，呵呵，胡扯！因为这篇文章的发表时间是2015年的圣诞节，所以我们需要给Style Android用制造出一些节日气氛。感谢读者们，因为有的读者可能没有在庆祝圣诞，有些读者可能还是6月份。
那么问题来了，我们应该做些什么来让这个节日像是真正的节日呢？最简单的方法：带上圣诞帽，拍个照。

tree

看我多么欢乐！
但是我感觉这个图片有些单调，所以来弄点雪花，让雪花飘下来。
我们可以添加一个包含这个图片的自定义View

res/layout/activity_main.xml

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

  <ImageView
    android:id="@+id/image"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_centerInParent="true"
    android:contentDescription="@null"
    android:scaleType="fitCenter"
    android:src="@drawable/tree" />

  <com.stylingandroid.snowfall.SnowView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_alignBottom="@id/image"
    android:layout_alignEnd="@id/image"
    android:layout_alignLeft="@id/image"
    android:layout_alignRight="@id/image"
    android:layout_alignStart="@id/image"
    android:layout_alignTop="@id/image" />
</RelativeLayout>

尽管可以通过继承ImageView来实现自定义View，但我决定将 SnowView 和图片分开，这样每次刷新动画的时候不用重新渲染图片，只刷新 SnowView 就行了

SnowView.java

public class SnowView extends View {
    private static final int NUM_SNOWFLAKES = 150;
    private static final int DELAY = 5;

    private SnowFlake[] snowflakes;

    public SnowView(Context context) {
        super(context);
    }

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

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

    protected void resize(int width, int height) {
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(Color.WHITE);
        paint.setStyle(Paint.Style.FILL);
        snowflakes = new SnowFlake[NUM_SNOWFLAKES];
        for (int i = 0; i < NUM_SNOWFLAKES; i++) {
            snowflakes[i] = SnowFlake.create(width, height, paint);
        }
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        if (w != oldw || h != oldh) {
            resize(w, h);
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        for (SnowFlake snowFlake : snowflakes) {
            snowFlake.draw(canvas);
        }
        getHandler().postDelayed(runnable, DELAY);
    }

    private Runnable runnable = new Runnable() {
        @Override
        public void run() {
            invalidate();
        }
    };
}

代码很简单。在View 的 onSizeChanged 方法中初始化 150 个随机位置的雪花对象。在onDraw方法中画出雪花，然后每隔一段时间就刷新一下位置，需要注意的是onDraw没有立即去执行，而是通过创建一个runnable，这样不会阻塞UI线程
雪花下落是基于Samuel Arbesman的雪花下落的算法。

SnowFlake.java

class SnowFlake {
    private static final float ANGE_RANGE = 0.1f;
    private static final float HALF_ANGLE_RANGE = ANGE_RANGE / 2f;
    private static final float HALF_PI = (float) Math.PI / 2f;
    private static final float ANGLE_SEED = 25f;
    private static final float ANGLE_DIVISOR = 10000f;
    private static final float INCREMENT_LOWER = 2f;
    private static final float INCREMENT_UPPER = 4f;
    private static final float FLAKE_SIZE_LOWER = 7f;
    private static final float FLAKE_SIZE_UPPER = 20f;

    private final Random random;
    private final Point position;
    private float angle;
    private final float increment;
    private final float flakeSize;
    private final Paint paint;

    public static SnowFlake create(int width, int height, Paint paint) {
        Random random = new Random();
        int x = random.getRandom(width);
        int y = random.getRandom(height);
        Point position = new Point(x, y);
        float angle = random.getRandom(ANGLE_SEED) / ANGLE_SEED * ANGE_RANGE + HALF_PI - HALF_ANGLE_RANGE;
        float increment = random.getRandom(INCREMENT_LOWER, INCREMENT_UPPER);
        float flakeSize = random.getRandom(FLAKE_SIZE_LOWER, FLAKE_SIZE_UPPER);
        return new SnowFlake(random, position, angle, increment, flakeSize, paint);
    }

    SnowFlake(Random random, Point position, float angle, float increment, float flakeSize, Paint paint) {
        this.random = random;
        this.position = position;
        this.angle = angle;
        this.increment = increment;
        this.flakeSize = flakeSize;
        this.paint = paint;
    }

    private void move(int width, int height) {
        double x = position.x + (increment * Math.cos(angle));
        double y = position.y + (increment * Math.sin(angle));

        angle += random.getRandom(-ANGLE_SEED, ANGLE_SEED) / ANGLE_DIVISOR;

        position.set((int) x, (int) y);

        if (!isInside(width, height)) {
            reset(width);
        }
    }

    private boolean isInside(int width, int height) {
        int x = position.x;
        int y = position.y;
        return x >= -flakeSize - 1 && x + flakeSize <= width && y >= -flakeSize - 1 && y - flakeSize < height;
    }

    private void reset(int width) {
        position.x = random.getRandom(width);
        position.y = (int) (-flakeSize - 1);
        angle = random.getRandom(ANGLE_SEED) / ANGLE_SEED * ANGE_RANGE + HALF_PI - HALF_ANGLE_RANGE;
    }

    public void draw(Canvas canvas) {
        int width = canvas.getWidth();
        int height = canvas.getHeight();
        move(width, height);
        canvas.drawCircle(position.x, position.y, flakeSize, paint);
    }
}

初始化的时候，雪花的随机位置就已经确定了。这是为了确保雪花不会每次画的时候都在开始的位置。当一个雪花的位置超出Canvas的边界之后，它就会被重新放到顶部的一个随机位置，这样就可以循环利用了，避免了重复创建。
当画雪花下落的每一帧的时候，我们首先给SnowFlake添加一个随机数来改变位置，这样可以模仿有小风吹雪花。
在把雪花画到canvas上之前，我们会进行边界检查(如果需要的话，超出边界的就重新放到顶部)

我一直在不断的调整里面的常量来改变下雪的效果直到我感觉满意为止。

最终效果如下：

当然了，在canvas里面塞这么多东西不是一个好的方法（有其他更好的比如OpenGL），但是，我现在要去吃火鸡了，所以可能要等下一次了。

源文件地址

版权声明：
Part of this code is based upon “Snowfall” by Sam Arbesman, licensed under Creative Commons Attribution-Share Alike 3.0 and GNU GPL license.
Work: https://openprocessing.org/visuals/?visualID= 84771
License:
https://creativecommons.org/licenses/by-sa/3.0/
https://creativecommons.org/licenses/GPL/2.0/

Portions of this page are modifications based on work created and shared by Google and used according to terms described in the Creative Commons 3.0 Attribution License