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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