版本记录

前言

如果你细看了我前面写的有关动画的部分，就知道前面介绍了CoreAnimation、序列帧以及LOTAnimation等很多动画方式，接下来几篇我们就以动画示例为线索，进行动画的讲解。相关代码已经上传至GitHub - 刀客传奇。感兴趣的可以看我写的前面几篇。
1. 动画示例（一） —— 一种外扩的简单动画
2. 动画示例（二） —— 一种抖动的简单动画
3. 动画示例（三） —— 仿头条一种LOTAnimation动画

功能要求

利用CAEmitterLayer实现下雪的动画。

功能实现

首先我们看一下这个类的API。

/* CoreAnimation - CAEmitterLayer.h

   Copyright (c) 2007-2017, Apple Inc.
   All rights reserved. */

/* Particle emitter layer.
 *
 * Each emitter has an array of cells, the cells define how particles
 * are emitted and rendered by the layer.
 *
 * Particle system is affected by layer's timing. The simulation starts
 * at layer's beginTime.
 *
 * The particles are drawn above the backgroundColor and border of the
 * layer. */

#import <QuartzCore/CALayer.h>

@class CAEmitterCell;

NS_ASSUME_NONNULL_BEGIN

CA_CLASS_AVAILABLE (10.6, 5.0, 9.0, 2.0)
@interface CAEmitterLayer : CALayer

/* The array of emitter cells attached to the layer. Each object must
 * have the CAEmitterCell class. */

@property(nullable, copy) NSArray<CAEmitterCell *> *emitterCells;

/* The birth rate of each cell is multiplied by this number to give the
 * actual number of particles created every second. Default value is one.
 * Animatable. */

@property float birthRate;

/* The cell lifetime range is multiplied by this value when particles are
 * created. Defaults to one. Animatable. */

@property float lifetime;

/* The center of the emission shape. Defaults to (0, 0, 0). Animatable. */

@property CGPoint emitterPosition;
@property CGFloat emitterZPosition;

/* The size of the emission shape. Defaults to (0, 0, 0). Animatable.
 * Depending on the `emitterShape' property some of the values may be
 * ignored. */

@property CGSize emitterSize;
@property CGFloat emitterDepth;

/* A string defining the type of emission shape used. Current options are:
 * `point' (the default), `line', `rectangle', `circle', `cuboid' and
 * `sphere'. */

@property(copy) NSString *emitterShape;

/* A string defining how particles are created relative to the emission
 * shape. Current options are `points', `outline', `surface' and
 * `volume' (the default). */

@property(copy) NSString *emitterMode;

/* A string defining how particles are composited into the layer's
 * image. Current options are `unordered' (the default), `oldestFirst',
 * `oldestLast', `backToFront' (i.e. sorted into Z order) and
 * `additive'. The first four use source-over compositing, the last
 * uses additive compositing. */

@property(copy) NSString *renderMode;

/* When true the particles are rendered as if they directly inhabit the
 * three dimensional coordinate space of the layer's superlayer, rather
 * than being flattened into the layer's plane first. Defaults to NO.
 * If true, the effect of the `filters', `backgroundFilters' and shadow-
 * related properties of the layer is undefined. */

@property BOOL preservesDepth;

/* Multiplies the cell-defined particle velocity. Defaults to one.
 * Animatable. */

@property float velocity;

/* Multiplies the cell-defined particle scale. Defaults to one. Animatable. */

@property float scale;

/* Multiplies the cell-defined particle spin. Defaults to one. Animatable. */

@property float spin;

/* The seed used to initialize the random number generator. Defaults to
 * zero. Each layer has its own RNG state. For properties with a mean M
 * and a range R, random values of the properties are uniformly
 * distributed in the interval [M - R/2, M + R/2]. */

@property unsigned int seed;

@end

/** `emitterShape' values. **/

CA_EXTERN NSString * const kCAEmitterLayerPoint
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerLine
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerRectangle
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerCuboid
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerCircle
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerSphere
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);

/** `emitterMode' values. **/

CA_EXTERN NSString * const kCAEmitterLayerPoints
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerOutline
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerSurface
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerVolume
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);

/** `renderMode' values. **/

CA_EXTERN NSString * const kCAEmitterLayerUnordered
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerOldestFirst
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerOldestLast
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerBackToFront
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);
CA_EXTERN NSString * const kCAEmitterLayerAdditive
    CA_AVAILABLE_STARTING (10.6, 5.0, 9.0, 2.0);

NS_ASSUME_NONNULL_END

下面我们就看一下代码

#import "ViewController.h"

@interface ViewController ()

@property (nonatomic, strong) CAEmitterLayer *fireEmitterLayer;

@end

@implementation ViewController

#pragma mark -  Override Base Function

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor blackColor];

    //下雪的动画
   [self beginSnow];
}

#pragma mark -  Object Private Function

- (void)beginSnow
{
    CAEmitterLayer *snowEmitterLayer = [CAEmitterLayer layer];
    snowEmitterLayer.frame = self.view.bounds;
    [self.view.layer addSublayer:snowEmitterLayer];
    
    //指定发射源的位置
    snowEmitterLayer.emitterPosition = CGPointMake(self.view.bounds.size.width / 2, -10);
    
    //指定发射源的大小
    snowEmitterLayer.emitterSize  = CGSizeMake(self.view.bounds.size.width, 0.0);
    
    //指定发射源的形状 和 模式
    snowEmitterLayer.emitterShape = kCAEmitterLayerLine;
    snowEmitterLayer.emitterMode  = kCAEmitterLayerOutline;
    
    //创建CAEmitterCell
    CAEmitterCell *snowCell = [CAEmitterCell emitterCell];
    
    //每秒多少个
    snowCell.birthRate = 3.0;
    
    //存活时间
    snowCell.lifetime = 100.0;
    
    //初速度 因为动画属于落体效果，所以我们只需要设置它在 y 方向上的加速度就行了。
    snowCell.velocity = 10;
    
    //初速度范围
    snowCell.velocityRange = 5;
    
    //y方向的加速度
    snowCell.yAcceleration = 3;
    
    snowCell.emissionRange = 0;
    
    snowCell.contents = (id)[[UIImage imageNamed:@"snow"] CGImage];
    
    //缩小
    snowCell.scale = 0.3;
    
    snowEmitterLayer.emitterCells = [NSArray arrayWithObject:snowCell];
}

@end

功能效果

下面我们就看一下功能效果。

后记

未完，待续~~~