Snowfall--降雪效果

翻译于Snowfall
效果图就不贴了，就是一个下雪效果。可以想到。。。
我们这样做，将这个自动一布局在Image上面。这样的话也可以依附其他布局。

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
让我们来看这个自定义的View：

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。当布局改变的时候，初始化150个雪花对象，随机放置。在onDraw()方法中绘制所有的雪花，在子线程中每隔一段时间刷新invalidate()重新绘制。

雪花下落的算法在此
每个雪花对象的具体实现：

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);
    }
}

首先确定每一朵雪花在画布上的任意位置。这是确保当雪花是第一次绘制时，降雪已经开始在下，而不是全在开始位置。当雪花超出画布上的边界，它被重新定位到一个随机水平位置在顶部，这样我们的雪花就循环起来了，以避免不必要的对象的创建。
我们也可以随意改变位置出现不同的效果，不妨试试吧。
最终效果如下：
youtube
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: http://openprocessing.org/visuals/?visualID= 84771
License:
http://creativecommons.org/licenses/by-sa/3.0/
http://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

Snowfall--降雪效果

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