自定义控件 - 切换开关：SwitchView

自定义控件一般的几个步骤：
1.初始化相关背景图片，布局文件，自定义属性
2.设置控件宽高OnMeasure()
3.布局或者排版OnLayout()
4.绘制控件OnDraw()
5.处理触摸事件OnTouchEvent()

public class SwitchView extends View implements View.OnTouchListener {

    //开关状态图片
    private Bitmap mSwitch_on, mSwitch_off, mSwitch_circle;

    //开关状态 默认关闭
    private boolean mCurrentState = false;

    //开关切换回调接口
    private OnSwitchChangedListener mOnSwitchChangedListener;

    //X轴按下坐标
    private int downX;
    //X轴移动时触点坐标
    private int moveX;
    //X轴偏移量
    private int left = 0;
    //最大可移动距离
    private int max;

    public SwitchView(Context context) {
        super(context);
        init();
    }

    public SwitchView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public SwitchView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    //1 初始化图片（加载）
    private void init() {
        Resources resr = getResources();
        mSwitch_on = BitmapFactory.decodeResource(resr, R.mipmap.switch_on);
        mSwitch_off = BitmapFactory.decodeResource(resr, R.mipmap.switch_off);
        mSwitch_circle = BitmapFactory.decodeResource(resr, R.mipmap.switch_circle);
        setOnTouchListener(this);
    }

    //2 设置控件宽高（测量）
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int widthSize = mSwitch_off.getWidth();
        int heightSize = mSwitch_off.getHeight();
        max = mSwitch_off.getWidth() - mSwitch_circle.getWidth();
        setMeasuredDimension(widthSize, heightSize);
    }

    //3 布局（排版）
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
    }

    //4 最后绘制控件
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Matrix m = new Matrix();
        Paint p = new Paint();
        if (mCurrentState) {
            canvas.drawBitmap(mSwitch_on, m, p);
        } else {
            canvas.drawBitmap(mSwitch_off, m, p);
        }
        canvas.drawBitmap(mSwitch_circle, left, 0, p);
    }

    //5 触摸事件
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                downX = (int) event.getX();//初始X轴按下坐标
                break;
            case MotionEvent.ACTION_MOVE:
                moveX = (int) event.getX();
                left = moveX - downX;
                if (!mCurrentState) {
                    //关闭状态边界处理
                    if (left < 0) {
                        left = 0;
                    } else if (left > max) {
                        left = max;
                    }
                } else {
                    //开启状态边界处理
                    left = moveX - downX;
                    if (left > 0) {//向右滑
                        left = max;
                    } else if (Math.abs(left) > max) {
                        left = 0;
                    } else {
                        left = max - Math.abs(left);
                    }
                }
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL://抬起时的判断
                int upX = (int) event.getX();
                boolean state = false; //滑动是否成功
                //1 关闭状态
                if (!mCurrentState) {
                    //滑动是否成功
                    //a 移动距离超过 1/2 的滑动宽度
                    //b 触摸点在滑动区域，开启按钮
                    state = (moveX - downX) >= max / 2 || upX >= max;
                    if (state) {
                        left = max;
                        mCurrentState = true;
                    } else {
                        left = 0;
                    }
                } else {
                    //2 开启状态，判断滑动是否成功
                    //a 移动距离超过 1/2 的滑动宽度
                    //b 触摸点在滑动区域，关闭按钮
                    state = (downX - moveX) >= max / 2 || upX <= max;
                    if (state) {
                        left = 0;
                        mCurrentState = false;
                    } else {
                        left = max;
                    }
                }
                //滑动成功 且 回调接口不能空 触发回调方法
                if (state && null != mOnSwitchChangedListener) {
                    mOnSwitchChangedListener.onSwitchChanged(mCurrentState);
                }
                break;
        }
        invalidate();
        return true;
    }

    //开关切换回调接口
    public interface OnSwitchChangedListener {
        void onSwitchChanged(boolean isOpen);
    }

    public void setOnSwitchChangedListener(OnSwitchChangedListener listener) {
        this.mOnSwitchChangedListener = listener;
    }

    public void setCurrentState(boolean isOpen) {
        mCurrentState = isOpen;
        left = isOpen ? max : 0;
        invalidate();
    }

}