之前做项目的时候，有个图片编辑页面，用到拖动条添加编辑效果。老板很喜欢Instagram那种拖动条的效果，0点从中间开始，左减右增，也要做成这样。Android原生的SeekBar，是从左边开始，一直往右边增加的，不能将起点设在中间。然后在GitHub上找了几遍，也没有找到符合要求的，只能自己实现了。

先看一下效果图：

ccc.gif

思路：

1、一根线，一个活动圆

2、线分三段，活动圆未到达的前后两段，活动圆扫过的中间段

3、监听触摸事件，刷新活动圆的位置，和三段线

具体实现

1、资源初始化

 private void init(Context context, AttributeSet attrs, int defStyleAttr) {
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.MiddleSeekBar, defStyleAttr, R.style.MiddleSeekBar);
        mThumbRadius = a.getDimensionPixelSize(R.styleable.MiddleSeekBar_MB_ThumbRadius, 0);
        mThumbColor = a.getColor(R.styleable.MiddleSeekBar_MB_ThumbColor, 0);
        mTextColor = a.getColor(R.styleable.MiddleSeekBar_MB_TextColor, 0);
        mProgressColor = a.getColor(R.styleable.MiddleSeekBar_MB_ProgressColor, 0);
        mProgressBackgroundColor = a.getColor(R.styleable.MiddleSeekBar_MB_ProgressBackgroundColor, 0);
        mTextSize = a.getDimensionPixelSize(R.styleable.MiddleSeekBar_MB_TextSize, 0);
        mProgressWidth = a.getDimensionPixelOffset(R.styleable.MiddleSeekBar_MB_ProgressWidth, 0);
        a.recycle();

        //初始化提示文字画笔
        mTextPaint.setTextSize(mTextSize);
        mTextPaint.setColor(mTextColor);
        //初始化进度条画笔
        mProgressPaint.setStrokeCap(Paint.Cap.ROUND);
        mProgressPaint.setStyle(Paint.Style.STROKE);
        mProgressPaint.setStrokeWidth(mProgressWidth);
        //初始化活动圆画笔
        mThumbShadowRadius = (int) (getResources().getDisplayMetrics().density * THUMB_SHADOW);
        mYOffset = (int) (getResources().getDisplayMetrics().density * Y_OFFSET);
        if(!isInEditMode()) {
            ViewCompat.setLayerType(this, ViewCompat.LAYER_TYPE_SOFTWARE, mThumbPaint);
            mThumbPaint.setShadowLayer(mThumbShadowRadius, 0, mYOffset, KEY_SHADOW_COLOR);
        }
        mThumbPaint.setColor(mThumbColor);
    }

2、测量

  @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        Paint.FontMetrics fontMetrics = mTextPaint.getFontMetrics();
        //suggestHeight算出拖动条的高度
        int suggestHeight = (int) (3 * mThumbRadius + mThumbShadowRadius - fontMetrics.top + fontMetrics.bottom + getPaddingTop() + getPaddingBottom());
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                resolveSizeAndState(suggestHeight, heightMeasureSpec, 0));
    }

3、绘制

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        //设置最左边的起始点
        mStartPoint.set(getPaddingLeft() + mThumbRadius, h - getPaddingBottom() - mThumbRadius - mThumbShadowRadius);
        mProgressLength = w - getPaddingLeft() - getPaddingRight() - 2 * mThumbRadius;
        //设置活动圆作用区域的长方形
        mThumbRect.set(mStartPoint.x + mProgressLength * mThumbRatio - HIT_SCOPE_RATIO * mThumbRadius, mStartPoint.y - HIT_SCOPE_RATIO * mThumbRadius,
                mStartPoint.x + mProgressLength * mThumbRatio + HIT_SCOPE_RATIO * mThumbRadius, mStartPoint.y + HIT_SCOPE_RATIO * mThumbRadius);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        int y = mStartPoint.y;
        int firstX = mStartPoint.x;//最左边点
        int secondX = mStartPoint.x + mProgressLength/2;//中间点
        int thirdX = (int) mThumbRect.centerX();//活动圆所在点

        //draw progress将进度条分成三段，前段没有到达时显示背景色，中段显示进度颜色，后端没有到达时显示背景色
        mProgressPaint.setColor(mProgressBackgroundColor);
        canvas.drawLine(firstX, y, secondX<thirdX?secondX:thirdX, y, mProgressPaint);
        mProgressPaint.setColor(mProgressColor);
        if (secondX < thirdX){
            canvas.drawLine(secondX, y, thirdX, y, mProgressPaint);
        }else {
            canvas.drawLine(thirdX, y, secondX, y, mProgressPaint);
        }
        mProgressPaint.setColor(mProgressBackgroundColor);
        canvas.drawLine(thirdX>secondX?thirdX:secondX, y, firstX + mProgressLength, y, mProgressPaint);

        //画活动圆
        canvas.drawCircle(thirdX, y, mThumbRadius, mThumbPaint);

        //画提示文字
        mPromptString = ratio2String(mThumbRatio);
        mTextPaint.setTextAlign(Paint.Align.CENTER);
        canvas.drawText(mPromptString, mThumbRect.centerX(), mThumbRect.top - mThumbRadius, mTextPaint);
    }

4、触摸事件

 @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                //判断触摸点在活动圆方形作用域内
                if (mThumbRect.contains(x, y)) {
                    selectInfo.pointerId = event.getPointerId(0);
                    selectInfo.isCaptured = true;
                }
                break;
            case MotionEvent.ACTION_MOVE:
                if (selectInfo.isCaptured) {
                    if (moveThumb(x)) return false;
                }
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                selectInfo.invalid();
                break;
        }
        if (selectInfo.isCaptured) {
            if(mOnSeekBarChangeListener!=null){
                mOnSeekBarChangeListener.onProgressChanged(this, mThumbRatio);
            }
            invalidate();
        }
        //让父View的 onInterceptTouchEvent(MotionEvent)失效
        if(ViewCompat.isAttachedToWindow(this)){
            getParent().requestDisallowInterceptTouchEvent(selectInfo.isCaptured);
        }
        return selectInfo.isCaptured;
    }


    private boolean moveThumb(float x) {
        if (x > mStartPoint.x) {
            mThumbRatio = (x - mStartPoint.x) / mProgressLength;
            if (mThumbRatio > 1) {
                mThumbRatio = 1;
                return true;
            }
            float offsetX = x - mThumbRect.centerX();
            //活动圆位置进行偏移调整
            mThumbRect.offset(offsetX, 0);
        }
        return false;
    }

总结

就这样，一个中间为起点可以两边拖动的拖动条实现了。
完整代码GitHub