Autolayout实现基本动画（使用Masonry库）

在使用自动布局之前一直对自动布局的动画实现有怀疑。

有文章说不断更改布局带来的内存消耗很大。于是做了个测试动画的demo，发现完全木有问题啊

原生的约束写起来很麻烦，还好有Masonry和UIView-Autolayout这些库。

另外看了这位大神对使用Masonry介绍的文章，加上Masonry的demo，很快就会用了，多亏有这些大神们的分享精神。

#import "MASExampleUpdatingFrequentlyTest.h"
@interface MASExampleUpdatingFrequentlyTest()<UIScrollViewDelegate>
{
    UIScrollView *scrollview;
    UIView *runningScreen;
    UILabel *offsetLabel;
}

@end

@implementation MASExampleUpdatingFrequentlyTest
-(instancetype)init
{
    if (self = [super init]) {
        scrollview = [[UIScrollView alloc]init];
        scrollview.backgroundColor = [UIColor grayColor];
        scrollview.delegate = self;
        [self addSubview:scrollview];
        scrollview.contentSize = CGSizeMake([UIScreen mainScreen].bounds.size.width, 1000);

        runningScreen = [UIView new];
        runningScreen.backgroundColor = [UIColor blackColor];
        [self addSubview:runningScreen];

        offsetLabel = [UILabel new];
        offsetLabel.textAlignment = NSTextAlignmentCenter;
        offsetLabel.text [email protected]"running boy!";
        offsetLabel.preferredMaxLayoutWidth = 300;
        offsetLabel.backgroundColor = [UIColor orangeColor];
        [runningScreen addSubview:offsetLabel];

        [scrollview mas_makeConstraints:^(MASConstraintMaker *make){
            make.edges.equalTo(self);
        }];
        [runningScreen mas_makeConstraints:^(MASConstraintMaker *make){
            make.width.equalTo(scrollview);
            make.height.equalTo(50);
            make.top.equalTo(self);
            make.leading.equalTo(self);
        }];
        [offsetLabel mas_makeConstraints:^(MASConstraintMaker *make){
            make.center.equalTo(runningScreen);
        }];
    }
    return self;
}

-(void)scrollViewDidScroll:(UIScrollView *)aScrollView
{
    CGFloat offsetY = aScrollView.contentOffset.y;
    NSLog(@"%f",offsetY);
    [runningScreen mas_updateConstraints:^(MASConstraintMaker *make){
        make.top.equalTo(offsetY);
    }];
    offsetLabel.text = [NSString stringWithFormat:@"You have run %.f ",offsetY];
    // tell constraints they need updating
    [runningScreen setNeedsUpdateConstraints];

    // update constraints now so we can animate the change
    [runningScreen updateConstraintsIfNeeded];
    [runningScreen updateConstraints];
}

思路主要是改变约束的top属性达到移动的效果。

其实这个库的demo列的情况已经很全了，都试着用一遍基本的应该没啥问题了