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：
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/**
* 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。
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