iOS snow animation

//
//  CTSnowAnimationView.h
//  CTMyCtrip
//
//  Created by alex on 16/1/4.
//  Copyright © 2016年 ctrip. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface CTSnowAnimationView : UIView
-(id)initWithFrame:(CGRect)frame;
-(void)startSnowWithFlakesNumber:(int)num duration:(float)duration;
-(void)stopSnowAnimation;
-(void)removeSnowAnimation;
@end

//
//  CTSnowAnimationView.m
//  CTMyCtrip
//
//  Created by alex on 16/1/4.
//  Copyright © 2016年 ctrip. All rights reserved.
//

#import "CTSnowAnimationView.h"

#define REPEATSNOW @"geneRepeatSnow"

@interface CTSnowAnimationView()
@property (strong, nonatomic) NSMutableArray<UIImageView*> *snowFlakes;
@end
@implementation CTSnowAnimationView

-(id)initWithFrame:(CGRect)frame{
    self = [super initWithFrame:frame];
    if (self) {
        return self;
    }
    return [[CTSnowAnimationView alloc] init];
}

-(void)removeSnowAnimation{
    for (UIView *flake in self.snowFlakes) {
        [flake removeFromSuperview];
    }
    [[NSNotificationCenter defaultCenter] removeObserver:self name:REPEATSNOW object:nil];
    self.snowFlakes = nil;
}

-(void)startSnowWithFlakesNumber:(int)num duration:(float)duration{
    dispatch_async(dispatch_get_main_queue(), ^{
        if ([self.snowFlakes count] != num) {
            if ([self.snowFlakes count] != 0) {
                for (UIView *subview in self.snowFlakes) {
                    [subview removeFromSuperview];
                }
            }
            self.snowFlakes = [NSMutableArray array];
            for (int i = 0; i < num; i++) {
                UIImageView *tmpView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"myctrip_snowflake.png"]];
                tmpView.frame = CGRectZero;
                tmpView.userInteractionEnabled = FALSE;
                [self addSubview:tmpView];
                [self.snowFlakes addObject:tmpView];
            }
        }
        [self showFlakes:YES];
        
        [[NSNotificationCenter defaultCenter] removeObserver:self name:REPEATSNOW object:nil];
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveRepeatNotification:) name:REPEATSNOW object:nil];
        for (UIView *flake in self.snowFlakes) {
            [flake.layer removeAllAnimations];
        }
        for (int i = 0; i < [self.snowFlakes count]; i++) {
            [self singleFlakeSnow:i withDuration:duration];
        }
    });
}

-(void)stopSnowAnimation{
    for (UIView *flake in self.snowFlakes) {
        [flake.layer removeAllAnimations];
    }
    [self showFlakes:false];
}


- (float)randomFloatBetween:(float)min and:(float)max {
    float diff = max - min;
    return (((float) (arc4random() % ((unsigned)RAND_MAX + 1)) / RAND_MAX) * diff) + min;
}

-(void)showFlakes:(BOOL)need{
    for (UIImageView *flake in self.snowFlakes) {
        flake.hidden = !need;
    }
}

-(void)singleFlakeSnow:(int)index withDuration:(float)duration{
    UIView *flake = self.snowFlakes[index];
    int size = [self randomFloatBetween:8.0 and:14.0];
    CGRect flakeFrame = flake.frame;
    [flake.layer removeAllAnimations];
    flake.layer.opacity = 0.1;
    flakeFrame.origin.x = arc4random_uniform(CTScreenWidth);
    flakeFrame.origin.y = -size;
    flakeFrame.size.width = size;
    flakeFrame.size.height = size;
    flake.frame = flakeFrame;
    float delay = arc4random_uniform(5.0);
    [self layoutIfNeeded];
    [UIView animateWithDuration:duration delay:delay options:UIViewAnimationOptionCurveEaseInOut animations:^{
        flake.frame = CGRectMake(flakeFrame.origin.x + 40 - arc4random_uniform(80), self.bounds.size.height, size, size);
        flake.layer.opacity = 1.0;
        [self layoutIfNeeded];
    } completion:^(BOOL finished) {
        if (finished) {
            [[NSNotificationCenter defaultCenter] postNotificationName:REPEATSNOW object:nil userInfo:@{@"index":[NSNumber numberWithInt:index],
                                                                                                        @"duration":[NSNumber numberWithFloat:duration]
                                                                                                        }];
        }
    }];
}

-(void)receiveRepeatNotification:(NSNotification *)notification{
    if ([[notification name] isEqualToString:REPEATSNOW]) {
        int index = [((NSNumber*)[notification.userInfo objectForKey:@"index"]) intValue];
        float duration = [((NSNumber*)[notification.userInfo objectForKey:@"duration"]) floatValue];
        [self singleFlakeSnow:index withDuration:duration];
    }
}


@end

Using AutoLayout:

 

- (float)randomFloatBetween:(float)min and:(float)max {
    float diff = max - min;
    return (((float) (arc4random() % ((unsigned)RAND_MAX + 1)) / RAND_MAX) * diff) + min;
}

-(void)showFlakes:(BOOL)need{
    for (UIImageView *flake in self.snowFlakes) {
        flake.hidden = !need;
    }
}

-(void)singleFlakeSnow:(int)index{
    UIView *flake = self.snowFlakes[index];
    NSLayoutConstraint *x = self.flakesHorizonalPosition[index];
    NSLayoutConstraint *y = self.flakesVerticalPosition[index];
    int size = [self randomFloatBetween:8.0 and:14.0];
    ((NSLayoutConstraint*)self.flakesWidth[index]).constant = size;
    ((NSLayoutConstraint*)self.flakesHeight[index]).constant = size;
    [flake.layer removeAllAnimations];
    flake.layer.opacity = 0.1;
    x.constant = arc4random_uniform(CTScreenWidth);
    y.constant = -size;
    float delay = arc4random_uniform(5.0);
    [self layoutIfNeeded];
    [UIView animateWithDuration:7.0 delay:delay options:UIViewAnimationOptionCurveEaseInOut animations:^{
        x.constant = x.constant + 40 - arc4random_uniform(80);
        y.constant = self.bounds.size.height;
        flake.layer.opacity = 1.0;
        [self layoutIfNeeded];
    } completion:^(BOOL finished) {
        if (finished) {
            [[NSNotificationCenter defaultCenter] postNotificationName:REPEATSNOW object:nil userInfo:@{@"index":[NSNumber numberWithInt:index]}];
        }
    }];
}

-(void)snowAnimation:(BOOL)need{
    if (need ) {
        dispatch_async(dispatch_get_main_queue(), ^{
            [self showFlakes:YES];
            self.snowGround.hidden = FALSE;
            self.snowBackground.hidden = FALSE;
            [[NSNotificationCenter defaultCenter] removeObserver:self name:REPEATSNOW object:nil];
            [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveRepeatNotification:) name:REPEATSNOW object:nil];
            for (UIView *flake in self.snowFlakes) {
                [flake.layer removeAllAnimations];
            }
            for (int i = 0; i < [self.snowFlakes count]; i++) {
                [self singleFlakeSnow:i];
            }
        });
    }
    else{
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] removeObserver:self name:REPEATSNOW object:nil];
            for (UIView *flake in self.snowFlakes) {
                [flake.layer removeAllAnimations];
            }
            [self showFlakes:NO];
        });
    }
}

-(void)receiveRepeatNotification:(NSNotification *)notification{
    if ([[notification name] isEqualToString:REPEATSNOW]) {
        int index = [((NSNumber*)[notification.userInfo objectForKey:@"index"]) intValue];
        [self singleFlakeSnow:index];
    }
}