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) ofeach contours in a path.The iterator cleans up the segments along the way, removing degeneratesegments and adding close verbs where necessary. When the forceCloseargument is provided, each contour (as defined by a new startingmove command) will be completed with a close verb regardless of thecontour'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。