聊聊动画引擎 pop
iOS可以通过CADisplayLink实现自定义动画引擎，pop就是基于此实现的，而且比原生Core Animation更强大好用。譬如当ViewController侧滑返回的时候，系统会将Core Animation的动画会停止，而基于CADisplayLink实现的动画则不会停止，因而可以实现类似网易云音乐从播放页侧滑时hold住专辑封面图旋转的效果。
八一八魔性的pop
1、实用的宏
#define POP_ARRAY_COUNT(x) sizeof(x) / sizeof(x[0])
#define FB_PROPERTY_GET(stype, property, ctype) \
- (ctype)property { \
return ((stype *)_state)->property; \
}
#define FB_PROPERTY_SET(stype, property, mutator, ctype, ...) \
- (void)mutator (ctype)value { \
if (value == ((stype *)_state)->property) \
return; \
((stype *)_state)->property = value; \
__VA_ARGS__ \
}
#define FB_PROPERTY_SET_OBJ_COPY(stype, property, mutator, ctype, ...) \
- (void)mutator (ctype)value { \
if (value == ((stype *)_state)->property) \
return; \
((stype *)_state)->property = [value copy]; \
__VA_ARGS__ \
}
2、判定值的数据类型
pop定义了支持的值的数据类型
const POPValueType kPOPAnimatableSupportTypes[10] = {kPOPValueInteger, kPOPValueFloat, kPOPValuePoint, kPOPValueSize, kPOPValueRect, kPOPValueEdgeInsets, kPOPValueColor, kPOPValueSCNVector3, kPOPValueSCNVector4};
通过@encode指令，将给定类型编码的内部字符串与objcType对比，得到值的数据类型
static bool FBCompareTypeEncoding(const char *objctype, POPValueType type)
{
switch (type)
{
case kPOPValueFloat:
return (strcmp(objctype, @encode(float)) == 0
|| strcmp(objctype, @encode(double)) == 0
);
case kPOPValuePoint:
return (strcmp(objctype, @encode(CGPoint)) == 0
#if !TARGET_OS_IPHONE
|| strcmp(objctype, @encode(NSPoint)) == 0
#endif
);
case kPOPValueSize:
return (strcmp(objctype, @encode(CGSize)) == 0
#if !TARGET_OS_IPHONE
|| strcmp(objctype, @encode(NSSize)) == 0
#endif
);
case kPOPValueRect:
return (strcmp(objctype, @encode(CGRect)) == 0
#if !TARGET_OS_IPHONE
|| strcmp(objctype, @encode(NSRect)) == 0
#endif
);
case kPOPValueEdgeInsets:
#if TARGET_OS_IPHONE
return strcmp(objctype, @encode(UIEdgeInsets)) == 0;
#else
return false;
#endif
case kPOPValueAffineTransform:
return strcmp(objctype, @encode(CGAffineTransform)) == 0;
case kPOPValueTransform:
return strcmp(objctype, @encode(CATransform3D)) == 0;
case kPOPValueRange:
return strcmp(objctype, @encode(CFRange)) == 0
|| strcmp(objctype, @encode (NSRange)) == 0;
case kPOPValueInteger:
return (strcmp(objctype, @encode(int)) == 0
|| strcmp(objctype, @encode(unsigned int)) == 0
|| strcmp(objctype, @encode(short)) == 0
|| strcmp(objctype, @encode(unsigned short)) == 0
|| strcmp(objctype, @encode(long)) == 0
|| strcmp(objctype, @encode(unsigned long)) == 0
|| strcmp(objctype, @encode(long long)) == 0
|| strcmp(objctype, @encode(unsigned long long)) == 0
);
case kPOPValueSCNVector3:
#if SCENEKIT_SDK_AVAILABLE
return strcmp(objctype, @encode(SCNVector3)) == 0;
#else
return false;
#endif
case kPOPValueSCNVector4:
#if SCENEKIT_SDK_AVAILABLE
return strcmp(objctype, @encode(SCNVector4)) == 0;
#else
return false;
#endif
default:
return false;
}
}
3、将值的数据类型标准化为Vector
举个CGRect类型的例子：
case kPOPValueRect:
vec = Vector::new_cg_rect([value CGRectValue]);
Vector *Vector::new_cg_rect(const CGRect &r)
{
Vector *v = new Vector(4);
v->_values[0] = r.origin.x;
v->_values[1] = r.origin.y;
v->_values[2] = r.size.width;
v->_values[3] = r.size.height;
return v;
}
通过Vector的两个参数size_t _count;、CGFloat *_values;将给定的类型抽象出来，实现解耦。此外还有一个好处，当创建属性动画为kPOPLayerBounds，但toValue属性赋值的是一个NSNumber，得益于_values是数组指针，并不会引发数组越界导致的crash，只是动画效果不可预期。
4、基于NSRunLoop的动画更新机制
- (void)_scheduleProcessPendingList
{
// see WebKit for magic numbers, eg http://trac.webkit.org/changeset/166540
static const CFIndex CATransactionCommitRunLoopOrder = 2000000;
static const CFIndex POPAnimationApplyRunLoopOrder = CATransactionCommitRunLoopOrder - 1;
// lock
OSSpinLockLock(&_lock);
if (!_pendingListObserver) {
__weak POPAnimator *weakSelf = self;
_pendingListObserver = CFRunLoopObserverCreateWithHandler(kCFAllocatorDefault, kCFRunLoopBeforeWaiting | kCFRunLoopExit, false, POPAnimationApplyRunLoopOrder, ^(CFRunLoopObserverRef observer, CFRunLoopActivity activity) {
[weakSelf _processPendingList];
});
if (_pendingListObserver) {
CFRunLoopAddObserver(CFRunLoopGetMain(), _pendingListObserver, kCFRunLoopCommonModes);
}
}
// unlock
OSSpinLockUnlock(&_lock);
}
在主线程RunLoop中添加观察者，监听了kCFAllocatorDefault、kCFRunLoopBeforeWaiting、kCFRunLoopExit事件，在收到回调的时候，处理_pendingList里的动画。
5、更新动画的回调数组
static POPStaticAnimatablePropertyState _staticStates[] =
{
/* CALayer */
{kPOPLayerBackgroundColor,
^(CALayer *obj, CGFloat values[]) {
POPCGColorGetRGBAComponents(obj.backgroundColor, values);
},
^(CALayer *obj, const CGFloat values[]) {
CGColorRef color = POPCGColorRGBACreate(values);
[obj setBackgroundColor:color];
CGColorRelease(color);
},
kPOPThresholdColor
},
{kPOPLayerBounds,
^(CALayer *obj, CGFloat values[]) {
values_from_rect(values, [obj bounds]);
},
^(CALayer *obj, const CGFloat values[]) {
[obj setBounds:values_to_rect(values)];
},
kPOPThresholdPoint
},
...
封装不同的动画行为，实现类似模板模式，只需统一调用，即可更新动画
// write value
write(obj, currentVec->data());
6、动画插值的动态实现
switch (type) {
case kPOPAnimationSpring:
advanced = advance(time, dt, obj);
break;
case kPOPAnimationDecay:
advanced = advance(time, dt, obj);
break;
case kPOPAnimationBasic: {
advanced = advance(time, dt, obj);
computedProgress = true;
break;
}
case kPOPAnimationCustom: {
customFinished = [self _advance:obj currentTime:time elapsedTime:dt] ? false : true;
advanced = true;
break;
}
default:
break;
}
可以看出总共有四种动画插值的算法，以kPOPAnimationBasic为例：
bool advance(CFTimeInterval time, CFTimeInterval dt, id obj) {
// default timing function
if (!timingFunction) {
((POPBasicAnimation *)self).timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionDefault];
}
// solve for normalized time, aka progresss [0, 1]
CGFloat p = 1.0f;
if (duration > 0.0f) {
// cap local time to duration
CFTimeInterval t = MIN(time - startTime, duration) / duration;
p = POPTimingFunctionSolve(timingControlPoints, t, SOLVE_EPS(duration));
timeProgress = t;
} else {
timeProgress = 1.;
}
// interpolate and advance
interpolate(valueType, valueCount, fromVec->data(), toVec->data(), currentVec->data(), p);
progress = p;
clampCurrentValue();
return true;
}
依照给定的timingFunction，使用POPTimingFunctionSolve计算贝塞尔曲线的变化率，再通过混合计算#define MIX(a, b, f) ((a) + (f) * ((b) - (a)))，最终得到动画的插值。
小结
pop中还有很多有意思的地方，譬如TransformationMatrix里的矩阵操作，这里就暂且不挖WebCore底层了。简而言之，无论性能(c++混编)、易用、容错，pop都有着作为引擎该有的特性，而它所暴露的和Core Animation相似的接口也让人极易上手！
