Android自定义View实现分段选择按钮

首先演示下效果，分段选择按钮，支持点击和滑动切换。

视图绘制过程中，要执行onMeasure、onLayout、onDraw等方法，这也是自定义控件最常用到的几个方法。
onMeasure：测量视图的大小，可以根据MeasureSpec的Mode确定父视图和子视图的大小。
onLayout：确定视图的位置
onDraw：绘制视图
这里就不做过多的介绍，主要介绍本控件涉及的到的部分。

1.1 获取item大小、起始位置

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

        if(isItemZero() || getMeasuredWidth() == 0)
            return;

        mHeight = getMeasuredHeight();
        int width = getMeasuredWidth();
        mItemWidth = (width - 2 * itemHorizontalMargin)/getCount();
        mStart = itemHorizontalMargin + mItemWidth * selectedItem;
        mEnd = width - itemHorizontalMargin - mItemWidth;
    }

1.2 绘制

绘制背景，所有的Item，以及选中项

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

        if(isItemZero())
            return;

        drawBackgroundRect(canvas);

        drawUnselectedItemsText(canvas);

        drawSelectedItem(canvas);

        drawSelectedItemsText(canvas);
    }

* 绘制背景区域

背景区域就是个带圆角的长方形

    /**
     * 画背景区域
     * @param canvas
     */
    private void drawBackgroundRect(Canvas canvas) {
        float r = cornersMode == Round?cornersRadius: mHeight >> 1;
        mPaint.setXfermode(null);
        mPaint.setColor(backgroundColor);
        mRectF.set(0, 0, getWidth(), getHeight());
        canvas.drawRoundRect(mRectF, r, r, mPaint);
    }

* 绘制所有未选中Item的文字

轮流绘制所有Item的文字

    /**
     * 画所有未选中Item的文字
     * @param canvas
     */
    private void drawUnselectedItemsText(Canvas canvas) {
        mTextPaint.setColor(textColor);
        mTextPaint.setXfermode(null);
        for (int i = 0; i< getCount(); i++){
            int start = itemHorizontalMargin + i * mItemWidth;
            float x = start + (mItemWidth >> 1) - mTextPaint.measureText(getName(i))/2;
            float y = (getHeight() >> 1) - (mTextPaint.ascent() + mTextPaint.descent())/2;
            canvas.drawText(getName(i), x, y, mTextPaint);
        }
    }

* 绘制选中项

    /**
     * 画选中项
     * @param canvas
     */
    private void drawSelectedItem(Canvas canvas) {
        float r = cornersMode == Round?cornersRadius: (mHeight >> 1) - itemVerticalMargin;
        mPaint.setColor(selectedItemBackgroundColor);
        mRectF.set(mStart, itemVerticalMargin, mStart + mItemWidth, getHeight() - itemVerticalMargin);
        canvas.drawRoundRect(mRectF, r, r, mPaint);
    }

* 绘制选中Item的文字

当选中项移动时，刚移动到下一个Item时，颜色应该是选中的颜色。这里在原来文字之上再画选中Item的文字颜色，就有了被选中的效果。

    /**
     * 画选中Item的文字
     * @param canvas
     */
    private void drawSelectedItemsText(Canvas canvas) {
        canvas.saveLayer(mStart, 0, mStart + mItemWidth, getHeight(), null, Canvas.ALL_SAVE_FLAG);
        mTextPaint.setColor(selectedItemTextColor);
        mTextPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));
        int begin = mStart/mItemWidth;
        int end = (begin + 2) < getCount()?begin+2:getCount();

        for (int i = begin; i< end; i++){
            int start = itemHorizontalMargin + i * mItemWidth;
            float x = start + (mItemWidth >> 1) - mTextPaint.measureText(getName(i))/2;
            float y = (getHeight() >> 1) - (mTextPaint.ascent() + mTextPaint.descent())/2;
            canvas.drawText(getName(i), x, y, mTextPaint);
        }
        canvas.restore();
    }

1.3 添加手势事件

手势分为三种，ACTION_DOWN、ACTION_MOVE、ACTION_UP，对应动作就是按下，滑动，按起。
当按下时确定按下位置，是在当前Item，则不做处理，当按下位置为其它Item位置，就滑动到其它Item位置。
当手势滑动时，计算相对滑动值，通过改变mStart，改变选中项的位置。
当手势按起时，根据按下位置、速度和方向，判断是否可用移动到下一个Item。

    @Override
    public boolean onTouchEvent(MotionEvent event) {

        if(!isEnabled() || !isInTouchMode() || getCount() == 0)
            return false;

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(event);

        int action = event.getActionMasked();
        if(action == MotionEvent.ACTION_DOWN){
            x = event.getX();
            onClickDownPosition = -1;
            final float y = event.getY();
            if(isItemInside(x, y)){
                return scrollSelectEnabled;
            }else if(isItemOutside(x, y)){
                if(!mScroller.isFinished()){
                    mScroller.abortAnimation();
                }
                onClickDownPosition = (int) ((x - itemHorizontalMargin)/ mItemWidth);
                startScroll(positionStart(x));
                return true;
            }
            return false;
        }else if(action == MotionEvent.ACTION_MOVE){
            if(!mScroller.isFinished() || !scrollSelectEnabled){
                return true;
            }
            float dx = event.getX() - x;
            if(Math.abs(dx) > MIN_MOVE_X){
                mStart = (int) (mStart + dx);
                mStart = Math.min(Math.max(mStart, itemHorizontalMargin), mEnd);
                postInvalidate();
                x = event.getX();
            }
            return true;
        }else if(action == MotionEvent.ACTION_UP){

            int newSelectedItem;
            float offset = (mStart - itemHorizontalMargin)%mItemWidth;
            float itemStartPosition = (mStart - itemHorizontalMargin) * 1.0f/ mItemWidth;

            if(!mScroller.isFinished() && onClickDownPosition != -1){
                newSelectedItem = onClickDownPosition;
            }else{
                if(offset == 0f){
                    newSelectedItem = (int)itemStartPosition;
                }else {
                    VelocityTracker velocityTracker = mVelocityTracker;
                    velocityTracker.computeCurrentVelocity(VELOCITY_UNITS, mMaximumFlingVelocity);
                    int initialVelocity = (int) velocityTracker.getXVelocity();

                    float itemRate = offset/mItemWidth;
                    if (isXVelocityCanMoveNextItem(initialVelocity, itemRate)){
                        newSelectedItem = initialVelocity > 0?(int)itemStartPosition+1:(int)itemStartPosition;
                    }else {
                        newSelectedItem = Math.round(itemStartPosition);
                    }
                    newSelectedItem = Math.max(Math.min(newSelectedItem, getCount() - 1), 0);
                    startScroll(getXByPosition(newSelectedItem));
                }
            }
            onStateChange(newSelectedItem);
            mVelocityTracker = null;
            onClickDownPosition = -1;
            return true;
        }
        return super.onTouchEvent(event);
    }

1.4 保存状态

当手机屏幕方向转换或者内存不足等情况下， 视图会重新加载，这样就会导致状态丢失。使用onSaveInstanceState和onRestoreInstanceState方法保存并恢复状态。

    @Override
    public Parcelable onSaveInstanceState() {
        Parcelable parcelable = super.onSaveInstanceState();
        SelectedItemState pullToLoadState = new SelectedItemState(parcelable);
        pullToLoadState.setSelectedItem(selectedItem);
        return pullToLoadState;
    }

    @Override
    public void onRestoreInstanceState(Parcelable state) {
        if(!(state instanceof SelectedItemState))
            return;
        SelectedItemState pullToLoadState = ((SelectedItemState)state);
        super.onRestoreInstanceState(pullToLoadState.getSuperState());
        selectedItem = pullToLoadState.getSelectedItem();
        invalidate();
    }

想要学习的同学，建议还是直接看项目源码。项目源码地址：https://github.com/danledian/SegmentedControl