要实现的效果如下

考虑到关键是动画效果，所以直接继承View。不过CheckBox的超类CompoundButton实现了Checkable接口，这一点值得借鉴。

下面记录一下遇到的问题，并从源码的角度解决。

问题一: 支持 wrap_content

由于是直接继承自View，wrap_content需要进行特殊处理。

View measure流程的MeasureSpec：

 /**
  * A MeasureSpec encapsulates the layout requirements passed from parent to child.
  * Each MeasureSpec represents a requirement for either the width or the height.
  * A MeasureSpec is comprised of a size and a mode. 
  * MeasureSpecs are implemented as ints to reduce object allocation. This class
  * is provided to pack and unpack the <size, mode> tuple into the int.
  */
 public static class MeasureSpec {
  private static final int MODE_SHIFT = 30;
  private static final int MODE_MASK = 0x3 << MODE_SHIFT;

  /**
   * Measure specification mode: The parent has not imposed any constraint
   * on the child. It can be whatever size it wants.
   */
  public static final int UNSPECIFIED = 0 << MODE_SHIFT;

  /**
   * Measure specification mode: The parent has determined an exact size
   * for the child. The child is going to be given those bounds regardless
   * of how big it wants to be.
   */
  public static final int EXACTLY  = 1 << MODE_SHIFT;

  /**
   * Measure specification mode: The child can be as large as it wants up
   * to the specified size.
   */
  public static final int AT_MOST  = 2 << MODE_SHIFT;

  /**
   * Extracts the mode from the supplied measure specification.
   *
   * @param measureSpec the measure specification to extract the mode from
   * @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
   *   {@link android.view.View.MeasureSpec#AT_MOST} or
   *   {@link android.view.View.MeasureSpec#EXACTLY}
   */
  public static int getMode(int measureSpec) {
   return (measureSpec & MODE_MASK);
  }

  /**
   * Extracts the size from the supplied measure specification.
   *
   * @param measureSpec the measure specification to extract the size from
   * @return the size in pixels defined in the supplied measure specification
   */
  public static int getSize(int measureSpec) {
   return (measureSpec & ~MODE_MASK);
  }
 }

从文档说明知道android为了节约内存，设计了MeasureSpec，它由mode和size两部分构成，做这么多终究是为了从父容器向子view传达长宽的要求。

mode有三种模式：

      1、UNSPECIFIED:父容器不对子view的宽高有任何限制

      2、EXACTLY：父容器已经为子view指定了确切的宽高

      3、AT_MOST：父容器指定最大的宽高，子view不能超过

wrap_content属于AT_MOST模式。

来看一下大致的measure过程：

在View中首先调用measure(),最终调用onMeasure()

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
    getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
 }

setMeasuredDimension设置view的宽高。再来看看getDefaultSize()

public static int getDefaultSize(int size, int measureSpec) {
  int result = size;
  int specMode = MeasureSpec.getMode(measureSpec);
  int specSize = MeasureSpec.getSize(measureSpec);

  switch (specMode) {
  case MeasureSpec.UNSPECIFIED:
   result = size;
   break;
  case MeasureSpec.AT_MOST:
  case MeasureSpec.EXACTLY:
   result = specSize;
   break;
  }
  return result;
 }

由于wrap_content属于模式AT_MOST，所以宽高为specSize，也就是父容器的size，这就和match_parent一样了。支持wrap_content总的思路是重写onMeasure()具体点来说，模仿getDefaultSize()重新获取宽高。

 @Override
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  int widthMode = MeasureSpec.getMode(widthMeasureSpec);
  int widthSize = MeasureSpec.getSize(widthMeasureSpec);
  int heightMode = MeasureSpec.getMode(heightMeasureSpec);
  int heightSize = MeasureSpec.getSize(heightMeasureSpec);

  int width = widthSize, height = heightSize;

  if (widthMode == MeasureSpec.AT_MOST) {
   width = dp2px(DEFAULT_SIZE);
  }

  if (heightMode == MeasureSpec.AT_MOST) {
   height = dp2px(DEFAULT_SIZE);
  }
  setMeasuredDimension(width, height);
 }

问题二：Path.addPath()和PathMeasure结合使用

举例子说明问题：

 mTickPath.addPath(entryPath);
 mTickPath.addPath(leftPath);
 mTickPath.addPath(rightPath);
 mTickMeasure = new PathMeasure(mTickPath, false);
 // mTickMeasure is a PathMeasure

尽管mTickPath现在是由三个path构成，但是mTickMeasure此时的length和entryPath长度是一样的，到这里我就很奇怪了。看一下getLength()的源码：

 /**
  * Return the total length of the current contour, or 0 if no path is
  * associated with this measure object.
  */
 public float getLength() {
  return native_getLength(native_instance);
 }

从注释来看，获取的是当前contour的总长。

getLength调用了native层的方法，到这里不得不看底层的实现了。

通过阅读源代码发现，Path和PathMeasure实际分别对应底层的SKPath和SKPathMeasure。

查看native层的getLength()源码：

 SkScalar SkPathMeasure::getLength() {
  if (fPath == NULL) {
   return 0;
  }
  if (fLength < 0) {
   this->buildSegments();
  }
  SkASSERT(fLength >= 0);
  return fLength;
}

实际上调用的buildSegments()来对fLength赋值，这里底层的设计有一个很聪明的地方——在初始化SKPathMeasure时对fLength做了特殊处理：

SkPathMeasure::SkPathMeasure(const SkPath& path, bool forceClosed) {
 fPath = &path;
 fLength = -1; // signal we need to compute it
 fForceClosed = forceClosed;
 fFirstPtIndex = -1;

 fIter.setPath(path, forceClosed);
}

当fLength=-1时我们需要计算，也就是说当还没有执行过getLength()方法时，fLength一直是-1，一旦执行则fLength>=0，则下一次就不会执行buildSegments(),这样避免了重复计算.

截取buildSegments()部分代码：

void SkPathMeasure::buildSegments() {
 SkPoint   pts[4];
 int    ptIndex = fFirstPtIndex;
 SkScalar  distance = 0;
 bool   isClosed = fForceClosed;
 bool   firstMoveTo = ptIndex < 0;
 Segment*  seg;

 /* Note:
 * as we accumulate distance, we have to check that the result of +=
 * actually made it larger, since a very small delta might be > 0, but
 * still have no effect on distance (if distance >>> delta).
 *
 * We do this check below, and in compute_quad_segs and compute_cubic_segs
 */
 fSegments.reset();
 bool done = false;
 do {
  switch (fIter.next(pts)) {
   case SkPath::kMove_Verb:
    ptIndex += 1;
    fPts.append(1, pts);
    if (!firstMoveTo) {
     done = true;
     break;
    }
    firstMoveTo = false;
    break;

   case SkPath::kLine_Verb: {
    SkScalar d = SkPoint::Distance(pts[0], pts[1]);
    SkASSERT(d >= 0);
    SkScalar prevD = distance;
    distance += d;
    if (distance > prevD) {
     seg = fSegments.append();
     seg->fDistance = distance;
     seg->fPtIndex = ptIndex;
     seg->fType = kLine_SegType;
     seg->fTValue = kMaxTValue;
     fPts.append(1, pts + 1);
     ptIndex++;
    }
   } break;

   case SkPath::kQuad_Verb: {
    SkScalar prevD = distance;
    distance = this->compute_quad_segs(pts, distance, 0, kMaxTValue, ptIndex);
    if (distance > prevD) {
     fPts.append(2, pts + 1);
     ptIndex += 2;
    }
   } break;

   case SkPath::kConic_Verb: {
    const SkConic conic(pts, fIter.conicWeight());
    SkScalar prevD = distance;
    distance = this->compute_conic_segs(conic, distance, 0, kMaxTValue, ptIndex);
    if (distance > prevD) {
     // we store the conic weight in our next point, followed by the last 2 pts
     // thus to reconstitue a conic, you'd need to say
     // SkConic(pts[0], pts[2], pts[3], weight = pts[1].fX)
     fPts.append()->set(conic.fW, 0);
     fPts.append(2, pts + 1);
     ptIndex += 3;
    }
   } break;

   case SkPath::kCubic_Verb: {
    SkScalar prevD = distance;
    distance = this->compute_cubic_segs(pts, distance, 0, kMaxTValue, ptIndex);
    if (distance > prevD) {
     fPts.append(3, pts + 1);
     ptIndex += 3;
    }
   } break;

   case SkPath::kClose_Verb:
    isClosed = true;
    break;

   case SkPath::kDone_Verb:
    done = true;
    break;
  }
 } while (!done);

 fLength = distance;
 fIsClosed = isClosed;
 fFirstPtIndex = ptIndex;

代码较长需要慢慢思考。fIter是一个Iter类型，在SKPath.h中的声明：

/* Iterate through all of the segments (lines, quadratics, cubics) of
each contours in a path.
The iterator cleans up the segments along the way, removing degenerate
segments and adding close verbs where necessary. When the forceClose
argument is provided, each contour (as defined by a new starting
move command) will be completed with a close verb regardless of the
contour's contents. /

从这个声明中可以明白Iter的作用是遍历在path中的每一个contour。看一下Iter.next()方法：

 Verb next(SkPoint pts[4], bool doConsumeDegerates = true) {
   if (doConsumeDegerates) {
    this->consumeDegenerateSegments();
   }
   return this->doNext(pts);
 }

返回值是一个Verb类型:

enum Verb {
 kMove_Verb,  //!< iter.next returns 1 point
 kLine_Verb,  //!< iter.next returns 2 points
 kQuad_Verb, //!< iter.next returns 3 points
 kConic_Verb, //!< iter.next returns 3 points + iter.conicWeight()
 kCubic_Verb, //!< iter.next returns 4 points
 kClose_Verb, //!< iter.next returns 1 point (contour's moveTo pt)
 kDone_Verb,  //!< iter.next returns 0 points
}

不管是什么类型的Path,它一定是由点组成，如果是直线，则两个点，贝塞尔曲线则三个点，依次类推。

doNext()方法的代码就不贴出来了，作用就是判断contour的类型并把相应的点的坐标取出传给pts[4]

当fIter.next()返回kDone_Verb时，一次遍历结束。

buildSegments中的循环正是在做此事，而且从case kLine_Verb模式的distance += d;不难发现这个length是累加起来的。在举的例子当中，mTickPath有三个contour（mEntryPath,mLeftPath,mRightPath），我们调用mTickMeasure.getLength()时，首先会累计获取mEntryPath这个contour的长度。

这就不难解释为什么mTickMeasure获取的长度和mEntryPath的一样了。那么想一想，怎么让buildSegments()对下一个contour进行操作呢？关键是把fLength置为-1

/** Move to the next contour in the path. Return true if one exists, or false if
 we're done with the path.
*/
bool SkPathMeasure::nextContour() {
 fLength = -1;
 return this->getLength() > 0;
}

与native层对应的API是PathMeasure.nextContour()

总结

以上就是Android开发之自定义CheckBox的全部内容，希望本文对大家开发Android有所帮助。