要實現的效果如下
考慮到關鍵是動畫效果，所以直接繼承View。不過CheckBox的超類CompoundButton實現了Checkable接口，這一點值得借鑒。
下面記錄一下遇到的問題，并從源碼的角度解決。
問題一: 支持 wrap_content
由于是直接繼承自View，wrap_content需要進行特殊處理。
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); } }從文檔說明知道android為了節約內存，設計了MeasureSpec，它由mode和size兩部分構成，做這么多終究是為了從父容器向子view傳達長寬的要求。
mode有三種模式：
1、UNSPECIFIED:父容器不對子view的寬高有任何限制
2、EXACTLY：父容器已經為子view指定了確切的寬高
3、AT_MOST：父容器指定最大的寬高，子view不能超過
wrap_content屬于AT_MOST模式。
來看一下大致的measure過程：
在View中首先調用measure(),最終調用onMeasure()
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); }setMeasuredDimension設置view的寬高。再來看看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屬于模式AT_MOST，所以寬高為specSize，也就是父容器的size，這就和match_parent一樣了。支持wrap_content總的思路是重寫onMeasure()具體點來說，模仿getDefaultSize()重新獲取寬高。
@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); }問題二：Path.addPath()和PathMeasure結合使用
舉例子說明問題：
mTickPath.addPath(entryPath); mTickPath.addPath(leftPath); mTickPath.addPath(rightPath); mTickMeasure = new PathMeasure(mTickPath, false); // mTickMeasure is a PathMeasure
盡管mTickPath現在是由三個path構成，但是mTickMeasure此時的length和entryPath長度是一樣的，到這里我就很奇怪了。看一下getLength()的源碼：
/** * 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); }從注釋來看，獲取的是當前contour的總長。
getLength調用了native層的方法，到這里不得不看底層的實現了。
通過閱讀源代碼發現，Path和PathMeasure實際分別對應底層的SKPath和SKPathMeasure。
查看native層的getLength()源碼：
SkScalar SkPathMeasure::getLength() { if (fPath == NULL) { return 0; } if (fLength < 0) { this->buildSegments(); } SkASSERT(fLength >= 0); return fLength;}實際上調用的buildSegments()來對fLength賦值，這里底層的設計有一個很聰明的地方――在初始化SKPathMeasure時對fLength做了特殊處理：
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);}當fLength=-1時我們需要計算，也就是說當還沒有執行過getLength()方法時，fLength一直是-1，一旦執行則fLength>=0，則下一次就不會執行buildSegments(),這樣避免了重復計算.
截取buildSegments()部分代碼：
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;代碼較長需要慢慢思考。fIter是一個Iter類型，在SKPath.h中的聲明：
/* 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. /
從這個聲明中可以明白Iter的作用是遍歷在path中的每一個contour。看一下Iter.next()方法：
Verb next(SkPoint pts[4], bool doConsumeDegerates = true) { if (doConsumeDegerates) { this->consumeDegenerateSegments(); } return this->doNext(pts); }返回值是一個Verb類型:
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}不管是什么類型的Path,它一定是由點組成，如果是直線，則兩個點，貝塞爾曲線則三個點，依次類推。
doNext()方法的代碼就不貼出來了，作用就是判斷contour的類型并把相應的點的坐標取出傳給pts[4]
當fIter.next()返回kDone_Verb時，一次遍歷結束。
buildSegments中的循環正是在做此事，而且從case kLine_Verb模式的distance += d;不難發現這個length是累加起來的。在舉的例子當中，mTickPath有三個contour（mEntryPath,mLeftPath,mRightPath），我們調用mTickMeasure.getLength()時，首先會累計獲取mEntryPath這個contour的長度。
這就不難解釋為什么mTickMeasure獲取的長度和mEntryPath的一樣了。那么想一想，怎么讓buildSegments()對下一個contour進行操作呢？關鍵是把fLength置為-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;}與native層對應的API是PathMeasure.nextContour()
總結
以上就是Android開發之自定義CheckBox的全部內容，希望本文對大家開發Android有所幫助。
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