Android 源码实现雪花飘落效果

package com.stylingandroid.snowfall; import android.graphics.Canvas; import android.graphics.Paint; import android.graphics.Point; 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); } }