简单实现满屏表情下落的动画效果，你也可以

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首先我相信大家一定玩过微信吧。之前在玩微信的时候，给好友发一句“圣诞快乐”就会有满屏的圣诞树往下掉，当时觉得这个动画好酷。正好在公司的项目中需要用到这样的动画效果。于是写了一个小Demo,就有了这篇文章。

下图是做出的相关效果：

这里写图片描述

看完上面的效果图，大家一定都迫不及待地想要试一试了，那就让我们来动手吧。

首先我们定义一个实体类DropLook:

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/**
 * 下落的表情
 */
public class DropLook {

    // x轴坐标
    private float x;
    // y轴坐标
    private float y;
    // 初始旋转角度
    private float rotation;
    // 下落速度
    private float speed;
    // 旋转速度
    private float rotationSpeed;
    // 宽度
    private int width;
    // 高度
    private int height;
    // 图片
    private Bitmap bitmap;

    public float getX() {
        return x;
    }

    public void setX(float x) {
        this.x = x;
    }

    public float getY() {
        return y;
    }

    public void setY(float y) {
        this.y = y;
    }

    public float getRotationSpeed() {
        return rotationSpeed;
    }

    public void setRotationSpeed(float rotationSpeed) {
        this.rotationSpeed = rotationSpeed;
    }

    public float getRotation() {
        return rotation;
    }

    public void setRotation(float rotation) {
        this.rotation = rotation;
    }

    public float getSpeed() {
        return speed;
    }

    public void setSpeed(float speed) {
        this.speed = speed;
    }

    public int getWidth() {
        return width;
    }

    public void setWidth(int width) {
        this.width = width;
    }

    public Bitmap getBitmap() {
        return bitmap;
    }

    public void setBitmap(Bitmap bitmap) {
        this.bitmap = bitmap;
    }

    public int getHeight() {
        return height;
    }

    public void setHeight(int height) {
        this.height = height;
    }

}

我们定义的实体类很简单，只是设置了如宽高、x，y坐标、下落速度等。接下来我们再创建一个DropLookFactory类，用来创建DropLook对象。

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public class DropLookFactory {

    private DropLookFactory() {

    }

    public static DropLook createDropLook(int width, int height,Bitmap originalBitmap) {
        DropLook look = new DropLook();
        if (originalBitmap == null) {
            throw new NullPointerException("originalBitmap cannot be null");
        }
        // 设置与图片等宽
        look.setWidth(originalBitmap.getWidth());
        // 设置与图片等高
        look.setHeight(originalBitmap.getHeight());
        // 设置起始位置的X坐标
        look.setX((float) Math.random() * (width - look.getWidth()));
        // 设置起始位置的Y坐标
        look.setY((float) Math.random() * (height - look.getHeight()));
        // 设置速度
        look.setSpeed(20 + (float) Math.random() * 40);
        // 设置初始旋转角度
        look.setRotation((float) Math.random() * 180 - 90);
        // 设置旋转速度
        look.setRotationSpeed((float) Math.random() * 90 - 60);
        // 设置图片
        look.setBitmap(originalBitmap);
        return look;
	}

}

其中createDropLook(Context context, float xRange, Bitmap originalBitmap)的第一个参数代表着下落表情在x轴上的范围，第二个参数代表在y轴上的范围，第三个参数是表情的图片。在createDropLook方法中相信大家都看得懂，主要就是用随机数初始化DropLook的坐标及下落速度等。

好了，下面就是今天的重头戏DropLookView，先来看看onMeasure(): 

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@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    int widthSize = MeasureSpec.getSize(widthMeasureSpec);
    int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    if (widthMode == MeasureSpec.EXACTLY) {
        mWidth = widthSize;
    } else {
        mWidth = Tools.dip2px(getContext(),200);
    }
    int heightSize = MeasureSpec.getSize(heightMeasureSpec);
    int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    if (heightMode == MeasureSpec.EXACTLY) {
        mHeight = heightSize;
    } else {
        mHeight = Tools.dip2px(getContext(),200);
    }
    setMeasuredDimension(mWidth, mHeight);
    if (looks.size() == 0) {
        for (int i = 0; i < DEFAULT_DROP_LOOK_NUMS; ++i) {
            looks.add(DropLookFactory.createDropLook(mWidth, mHeight, mBitmap));
        }
        Log.i(TAG, "num = " + looks.size());
    }
}

onMeasure里主要是对View的测量，如果是wrap_content的话设置一个默认的宽高度200dp。然后就是初始化DropLook，looks是DropLook类的集合，用于管理DropLook。而DEFAULT_LOOK_NUMS是默认的looks集合的数量。

接下来就是最关键的onDraw():

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@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.save();
    long nowTime = System.currentTimeMillis();
    if (nowTime - startTime < 100) {
        try {
            Thread.sleep(100 + startTime - nowTime);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    for (int i = 0; i < DEFAULT_DROP_LOOK_NUMS; i++) {

        DropLook look = looks.get(i);
        mMatrix.setTranslate(-look.getWidth() / 2, -look.getHeight() / 2);
        mMatrix.postRotate(look.getRotation());
        mMatrix.postTranslate(look.getWidth() / 2 + look.getX(), look.getHeight() / 2 + look.getY());
        canvas.drawBitmap(look.getBitmap(), mMatrix, mPaint);

        look.setY(look.getY() + look.getSpeed());
        if (look.getY() > getHeight()) {
            look.setY((float) (0 - Math.random() * look.getHeight()));
        }

        look.setRotation(look.getRotation() + look.getRotationSpeed());
    }

    canvas.restore();
    startTime = System.currentTimeMillis();
    invalidate();
}

一开始判断时间间隔如果没有超过100ms，就让线程睡眠一会。然后就是用drawBitmap的方法把looks里面逐个绘制出来。并且再把look的y轴坐标加上下落速度等，旋转的角度也是如此。最后就是调用invalidate()不断地重绘。总体上并没有什么难点。

以下是DropLookView的完整代码：

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/**
 * 表情下落view
 */
public class DropLookView extends View {

    // 表情
    private Bitmap mBitmap;
    // 所有表情集合
    List<DropLook> looks = new ArrayList();
    // view开始时间
    private long startTime;
    // view宽度
    private int mWidth;
    // view高度
    private int mHeight;
    // 画笔
    private Paint mPaint;
    // 默认表情下落数
    private static final int DEFAULT_DROP_LOOK_NUMS = 35;

    private static final String TAG = "DropLookView";

    private Matrix mMatrix = new Matrix();

    public DropLookView(Context context) {
        this(context, null);
    }

    public DropLookView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public DropLookView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        // 图片
        mBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.d_5_xiaoku);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        if (widthMode == MeasureSpec.EXACTLY) {
            mWidth = widthSize;
        } else {
            mWidth = Tools.dip2px(getContext(),200);
        }
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        if (heightMode == MeasureSpec.EXACTLY) {
            mHeight = heightSize;
        } else {
            mHeight = Tools.dip2px(getContext(),200);
        }
        setMeasuredDimension(mWidth, mHeight);
        if (looks.size() == 0) {
            for (int i = 0; i < DEFAULT_DROP_LOOK_NUMS; ++i) {
                looks.add(DropLookFactory.createDropLook(mWidth, mHeight, mBitmap));
            }
            Log.i(TAG, "num = " + looks.size());
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.save();
        long nowTime = System.currentTimeMillis();
        if (nowTime - startTime < 100) {
            try {
                Thread.sleep(100 + startTime - nowTime);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        for (int i = 0; i < DEFAULT_DROP_LOOK_NUMS; i++) {

            DropLook look = looks.get(i);
            mMatrix.setTranslate(-look.getWidth() / 2, -look.getHeight() / 2);
            mMatrix.postRotate(look.getRotation());
            mMatrix.postTranslate(look.getWidth() / 2 + look.getX(), look.getHeight() / 2 + look.getY());
            canvas.drawBitmap(look.getBitmap(), mMatrix, mPaint);

            look.setY(look.getY() + look.getSpeed());
            if (look.getY() > getHeight()) {
                look.setY((float) (0 - Math.random() * look.getHeight()));
            }

            look.setRotation(look.getRotation() + look.getRotationSpeed());
        }

        canvas.restore();
        startTime = System.currentTimeMillis();
        invalidate();
    }

}

该讲的也差不多讲完了，其实并没有想象中的那么有难度，实现起来也比较容易。当然DropLookView也有需要改进的地方。比如说可以在布局文件中自定义表情下落的数量等。这些就需要自己根据需求来更改了，那今天就先这样吧。

下面是本Demo的完整代码：

DropLookView.rar

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