自定义控件类

public class FloatingCilcleView extends View {

    // 控件上的文字
    private String text = "点击隐藏";
    // 控件的半径
    private int radius = dp2px(60);
    // 控件的宽
    private int width = radius * 2;
    // 控件的高
    private int height = radius * 2;

    private Paint textPaint;
    private Paint cilclePaint;

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

    public FloatingCilcleView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public FloatingCilcleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        obtainStyleAttrs(context, attrs);
    }

    private void obtainStyleAttrs(Context context, AttributeSet attrs) {
        textPaint = new Paint();
        textPaint.setColor(0xAAFF0000);
        textPaint.setTextSize(sp2px(20));
        textPaint.setAntiAlias(true);
        textPaint.setDither(true);
        textPaint.setFakeBoldText(true);
        textPaint.setStyle(Paint.Style.FILL);

        cilclePaint = new Paint();
        cilclePaint.setColor(0x55D2D2D2);
        cilclePaint.setDither(true);
        cilclePaint.setAntiAlias(true);
    }

    @Override
    protected synchronized void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(width, height);
    }

    @Override
    protected synchronized void onDraw(Canvas canvas) {
        // 画圆
        canvas.drawCircle(width / 2, height / 2, radius, cilclePaint);

        int x = (int) (width / 2 - textPaint.measureText(text) / 2);
        // 基线
        Paint.FontMetricsInt fontMetrics = textPaint.getFontMetricsInt();
        int dy = (fontMetrics.bottom - fontMetrics.top) / 2 - fontMetrics.bottom;
        int baseLine = getHeight() / 2 + dy;
        // 绘制文字
        canvas.drawText(text, x, baseLine, textPaint);
    }

    private int dp2px(int value) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, value,
                getResources().getDisplayMetrics());
    }

    private int sp2px(int value) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, value,
                getResources().getDisplayMetrics());
    }

}

WindowManager管理器

public class FloatingViewManager {

    // 悬浮控件移动的X值
    private float startX = 0f;
    // 悬浮控件移动的Y值
    private float startY = 0f;
    // 用于记录解决OnTouch与OnClick事件冲突的X值
    private float startX1 = 0f;
    // 用于记录解决OnTouch与OnClick事件冲突的Y值
    private float startY1 = 0f;
    // 悬浮控件
    private FloatingCilcleView cilcleView;
    
    private Context context;
    private WindowManager wm;
    private WindowManager.LayoutParams params;

    private static FloatingViewManager instance;

    public static FloatingViewManager getInstance(Context context) {
        if (instance == null) {
            synchronized (FloatingViewManager.class) {
                if (instance == null) {
                    instance = new FloatingViewManager(context);
                }
            }
        }
        return instance;
    }

    private View.OnTouchListener touchListener = new View.OnTouchListener() {
        @Override
        public boolean onTouch(View v, MotionEvent event) {
            switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN:
                    startX = startX1 = event.getRawX();
                    startY = startY1 = event.getRawY();
                    break;
                case MotionEvent.ACTION_MOVE:
                    float x = event.getRawX();
                    float y = event.getRawY();
                    float dx = x - startX;
                    float dy = y - startY;
                    params.x += dx;
                    params.y += dy;
                    wm.updateViewLayout(cilcleView, params);
                    startX = x;
                    startY = y;
                    break;
                case MotionEvent.ACTION_UP:
                    float endX = event.getRawX();
                    float endY = event.getRawY();
                    int width = getWindowWidth();
                    if (endX > (float) (width / 2)) {       // 记录悬浮控件最后停在屏幕的左右哪一边
                        params.x = width - cilcleView.getWidth();
                    } else {
                        params.x = 0;
                    }
                    params.y = (int) endY;
                    wm.updateViewLayout(cilcleView, params);
                    // 解决OnTouch与OnClick事件冲突
                    if ((Math.abs((endX - startX1)) > 4) || (Math.abs((endY - startY1)) > 4)) {
                        return true;
                    } else {
                        return false;
                    }
                default:
                    break;
            }
            return false;
        }
    };

    private View.OnClickListener clickListener = new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            wm.removeView(cilcleView);      // 移除悬浮控件
        }
    };

    private FloatingViewManager(Context context) {
        this.context = context;
        wm = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        cilcleView = new FloatingCilcleView(context);
        cilcleView.setOnTouchListener(touchListener);
        cilcleView.setOnClickListener(clickListener);
    }

    public void showFloatingCilcleView() {
        params = new WindowManager.LayoutParams();
        params.width = cilcleView.getWidth();
        params.height = cilcleView.getHeight();
        params.gravity = Gravity.LEFT | Gravity.TOP;
        params.x = 0;       // x、y是相对于params.gravity的偏移量
        params.y = 0;
        params.format = PixelFormat.RGBA_8888;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            params.type = WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY;
        } else {
            params.type = WindowManager.LayoutParams.TYPE_PHONE;
        }
        params.flags = WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE | WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL;
        wm.addView(cilcleView, params);
    }

    // 获取屏幕的宽度
    private int getWindowWidth() {
        return wm.getDefaultDisplay().getWidth();
    }

}

使用Service来显示悬浮控件

public class FloatingService extends Service {

    @Override
    public void onCreate() {
        FloatingViewManager manager = FloatingViewManager.getInstance(this);
        manager.showFloatingCilcleView();
        super.onCreate();
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

}

在Activity中开启Service

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        requestPermission();
    }

    // 按钮点击事件
    public void open(View view) {
        Intent intent = new Intent(this, FloatingService.class);
        startService(intent);
        finish();
    }

    private void requestPermission() {
        // 判断权限是否授权
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.SYSTEM_ALERT_WINDOW) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[] { Manifest.permission.SYSTEM_ALERT_WINDOW }, 200);
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        switch (requestCode) {
            case 200:
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    Log.i("xxx", "已授权权限申请");
                } else {
                    Log.i("xxx", "未授权权限申请");
                }
                break;
        }
    }

}