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！