Android通过自定义view实现刮刮乐效果详解

pathPaint.setXfermode(new PorterDuffXfermode((PorterDuff.Mode.DST_IN)));

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mBitmapBackground = getBitmap(mBitmapBackground, w, h);
        mBitmapFront = Bitmap.createBitmap(mBitmapBackground.getWidth(),
                mBitmapBackground.getHeight(), Bitmap.Config.ARGB_8888);
        mCanvas.setBitmap(mBitmapFront);
        drawText(mCanvas, w, h);
    }

    private void drawText(Canvas canvas, int mWidth, int mHeight) {
        String text = "赶紧刮开吧";
        canvas.drawColor(Color.GRAY);
        Paint.FontMetrics fm = mPaintText.getFontMetrics();
        int mTxtWidth = (int) mPaintText.measureText(text, 0, text.length());
        int mTxtHeight = (int) Math.ceil(fm.descent - fm.ascent);
        int x = mWidth / 2 - mTxtWidth / 2; //文字在画布中的x坐标
        int y = mHeight / 2 + mTxtHeight / 4; //文字在画布中的y坐标
        canvas.drawText(text, x, y, mPaintText);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                path.reset();
                path.moveTo(event.getX(), event.getY());//原点移动至手指的触摸点
                break;
            case MotionEvent.ACTION_MOVE:
                path.lineTo(event.getX(), event.getY());
                break;
        }
        mCanvas.drawPath(path, pathPaint);
        invalidate();
        return true;
    }

import Android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
 
import androidx.annotation.Nullable;
 
import com.example.androidprogressbar.R;
 
public class ScratchCard extends View {
 
    private Bitmap mBitmapBackground;
    private Bitmap mBitmapFront;
    private Canvas mCanvas;
    private Paint pathPaint;
    private Path path;
    private Paint mPaintText;
 
    public ScratchCard(Context context) {
        super(context);
        init();
    }
 
    public ScratchCard(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        init();
    }
 
    public ScratchCard(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }
 
    private void init() {
        pathPaint = new Paint();
        pathPaint.setAlpha(0);
        pathPaint.setStyle(Paint.Style.STROKE);
        pathPaint.setStrokeWidth(70);
        pathPaint.setXfermode(new PorterDuffXfermode((PorterDuff.Mode.DST_IN)));//混合模式
        pathPaint.setStrokeJoin(Paint.Join.ROUND);//线段之间连接处的样式
        pathPaint.setStrokeCap(Paint.Cap.ROUND);//设置画笔的线冒样式
        path = new Path();
        mBitmapBackground = BitmapFactory.decodeResource(getResources(), R.drawable.card);
        mCanvas = new Canvas();
        mPaintText = new Paint();
        mPaintText.setColor(Color.WHITE);
        mPaintText.setTextSize(100);
        mPaintText.setStrokeWidth(20);
    }
 
    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawBitmap(mBitmapBackground, 0, 0, null);
        canvas.drawBitmap(mBitmapFront, 0, 0, null);
    }
 
    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mBitmapBackground = getBitmap(mBitmapBackground, w, h);
        mBitmapFront = Bitmap.createBitmap(mBitmapBackground.getWidth(),
                mBitmapBackground.getHeight(), Bitmap.Config.ARGB_8888);
        mCanvas.setBitmap(mBitmapFront);
        drawText(mCanvas, w, h);
    }
 
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                path.reset();
                path.moveTo(event.getX(), event.getY());//原点移动至手指的触摸点
                break;
            case MotionEvent.ACTION_MOVE:
                path.lineTo(event.getX(), event.getY());
                break;
        }
        mCanvas.drawPath(path, pathPaint);
        invalidate();
        return true;
    }
 
    private void drawText(Canvas canvas, int mWidth, int mHeight) {
        String text = "赶紧刮开吧";
        canvas.drawColor(Color.GRAY);
        Paint.FontMetrics fm = mPaintText.getFontMetrics();
        int mTxtWidth = (int) mPaintText.measureText(text, 0, text.length());
        int mTxtHeight = (int) Math.ceil(fm.descent - fm.ascent);
        int x = mWidth / 2 - mTxtWidth / 2; //文字在画布中的x坐标
        int y = mHeight / 2 + mTxtHeight / 4; //文字在画布中的y坐标
        canvas.drawText(text, x, y, mPaintText);
    }
 
    public Bitmap getBitmap(Bitmap bm, int newWidth, int newHeight) {
        // 获得图片的宽高
        int width = bm.getWidth();
        int height = bm.getHeight();
        // 计算缩放比例
        float scaleWidth = ((float) newWidth) / width;
        float scaleHeight = ((float) newHeight) / height;
        // 取得想要缩放的matrix参数
        Matrix matrix = new Matrix();
        matrix.postScale(scaleWidth, scaleHeight);
        // 得到新的图片
        Bitmap newbm = Bitmap.createBitmap(bm, 0, 0, width, height, matrix, true);
        return newbm;
    }
 
}