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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