Android自定义View实现分段选择按钮的实现代码

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

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