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。
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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"
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/
© 2015, Mark Allison. All rights reserved. This article originally appeared on Styling Android.
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
