android 倒计时动画

要在 Android 上实现倒计时动画，可以参考以下步骤：

1. 创建一个自定义 View，并在其中实现倒计时动画的绘制逻辑。

2. 在 View 中添加一个计时器，每隔一段时间更新倒计时的数值，并调用 invalidate() 方法刷新界面。

3. 在绘制逻辑中，根据当前的倒计时数值计算出圆环的弧度，并使用 Canvas 绘制圆环和数字。

4. 可以添加一些动画效果，例如圆环的渐变填充色、数字的放大缩小等等，增强视觉效果。

以下是一个简单的示例代码，仅供参考：

public class CountDownView extends View {
    private static final long COUNT_DOWN_INTERVAL = 1000; // 倒计时间隔，默认为 1 秒
    private static final int DEFAULT_COUNT_DOWN_TIME = 60; // 默认倒计时时间为 60 秒

    private Paint mPaint;
    private RectF mRectF;
    private float mSweepAngle;
    private int mCountDownTime;
    private long mStartTime;
    private boolean mIsCountingDown;

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

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

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

        // 初始化画笔
        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeWidth(10);

        // 初始化圆弧矩形
        mRectF = new RectF();

        // 初始化倒计时时间
        mCountDownTime = DEFAULT_COUNT_DOWN_TIME;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        // 计算圆弧矩形
        int width = MeasureSpec.getSize(widthMeasureSpec);
        int height = MeasureSpec.getSize(heightMeasureSpec);
        int size = Math.min(width, height);
        int padding = 10;
        mRectF.set(padding, padding, size - padding, size - padding);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // 绘制圆弧
        mPaint.setColor(Color.GRAY);
        canvas.drawArc(mRectF, 0, 360, false, mPaint);
        mPaint.setColor(Color.BLUE);
        canvas.drawArc(mRectF, -90, mSweepAngle, false, mPaint);

        // 绘制数字
        mPaint.setColor(Color.BLACK);
        mPaint.setTextSize(50);
        String text = String.valueOf(mCountDownTime);
        Rect textBounds = new Rect();
        mPaint.getTextBounds(text, 0, text.length(), textBounds);
        canvas.drawText(text, mRectF.centerX() - textBounds.width() / 2,
                mRectF.centerY() + textBounds.height() / 2, mPaint);
    }

    public void startCountDown() {
        if (mIsCountingDown) {
            return;
        }

        mIsCountingDown = true;
        mStartTime = System.currentTimeMillis();
        post(countDownRunnable);
    }

    public void stopCountDown() {
        mIsCountingDown = false;
        removeCallbacks(countDownRunnable);
    }

    private Runnable countDownRunnable = new Runnable() {
        @Override
        public void run() {
            long elapsedTime = System.currentTimeMillis() - mStartTime;
            int remainingTime = mCountDownTime - (int) (elapsedTime / COUNT_DOWN_INTERVAL);

            if (remainingTime >= 0) {
                mSweepAngle = (float) remainingTime / mCountDownTime * 360;
                invalidate();
                postDelayed(this, COUNT_DOWN_INTERVAL);
            } else {
                stopCountDown();
            }
        }
    };
}

使用时可以调用 startCountDown() 方法开始倒计时，调用 stopCountDown() 方法停止倒计时。