这是《iOS Auto Layout开发秘籍》中的一个例子。这里使用Masonry改写了一下。

实现的效果如下：

2017-08-28 12_15_49.gif

代码如下：

#import "LockControl.h"
#import "Masonry.h"

@implementation LockControl

{
    UIImageView *lockView;
    UIImageView *trackView;
    UIImageView *thumbView;
}

#pragma mark - Layout

- (void) layoutConstraints {
    
    // Self
    [self mas_makeConstraints:^(MASConstraintMaker *make) {
        make.size.mas_equalTo(CGSizeMake(256, 256)).priority(UILayoutPriorityRequired);
    }];
    
    // Lock
    [lockView mas_makeConstraints:^(MASConstraintMaker *make) {
        
        make.center.mas_equalTo(self).centerOffset(CGPointMake(0, -38)).priority(UILayoutPriorityRequired);

    }];
    
    // Track
    [trackView mas_makeConstraints:^(MASConstraintMaker *make) {
        
        make.center.mas_equalTo(self).centerOffset(CGPointMake(0, 80)).priority(UILayoutPriorityRequired);
    }];
    
    // Thumb
    [thumbView mas_updateConstraints:^(MASConstraintMaker *make) {
        make.centerY.mas_equalTo(trackView.mas_centerY).priority(UILayoutPriorityRequired);
        
// 约束的关键!!!左边是大于等于，右边是小于等于
make.left.mas_greaterThanOrEqualTo(12).priorityHigh();
        make.right.mas_lessThanOrEqualTo(-12).priorityHigh();
        make.leading.mas_equalTo(trackView).priorityMedium();
    }];
}

- (void) buildView { 
    self.translatesAutoresizingMaskIntoConstraints = NO;
    self.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.3f];
    self.layer.cornerRadius = 32;
    self.layer.masksToBounds = YES;
    
    lockView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"lockClosed"]];
    [self addSubview:lockView];
    
    trackView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"track"]];
    hug(trackView, UILayoutPriorityRequired);
    [self addSubview:trackView];
    
    thumbView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"thumb"]];
    [trackView addSubview:thumbView];
    
    [self layoutConstraints];
    
}

#pragma mark - Creation

- (instancetype) initWithFrame:(CGRect)frame {
    
    self = [super initWithFrame:frame];
    
    if (self == nil) {
        return self;
    }
    
    self.backgroundColor = [UIColor clearColor];
    [self buildView];
    return self;
    
}

- (instancetype) init {
    return [self initWithFrame:CGRectZero];
}

#pragma mark - UIControl

- (BOOL) beginTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {
    
    // 测试开始时候的touch
    
    CGPoint touchPoint = [touch locationInView:self];
    CGRect largeTrack = CGRectInset(trackView.frame, -20.0f, -20.0f);
    if (!CGRectContainsPoint(largeTrack, touchPoint)) {
        return NO;
    }
    
    touchPoint = [touch locationInView:trackView];
    CGRect largeThumb = CGRectInset(thumbView.frame, -20.0f, -20.0f);
    if (!CGRectContainsPoint(largeThumb, touchPoint)) {
        return NO;
    }
    
    // 开始跟踪
    [self sendActionsForControlEvents:UIControlEventTouchDown];
    return YES;
}

- (BOOL) continueTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {
    
    CGPoint touchPoint = [touch locationInView:self];
    
    // 更新滑动的位置
    touchPoint = [touch locationInView:trackView];
    
    [UIView animateWithDuration:0.1f animations:^{
        
       
        [thumbView mas_updateConstraints:^(MASConstraintMaker *make) {
            
            make.leading.mas_equalTo(trackView).mas_offset(touchPoint.x).priorityMedium();
            
        }];
    }];
    return YES;
}

- (void) endTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {
    
    CGPoint touchPoint = [touch locationInView:trackView];
    
    if (touchPoint.x > trackView.frame.size.width * 0.75f) {
        // 默认为解锁
        self.userInteractionEnabled = NO;
        [self sendActionsForControlEvents:UIControlEventValueChanged];
        
        // 自我移除
        [UIView animateWithDuration:0.5f animations:^{
            self.alpha = 0.0f;
        } completion:^(BOOL finished) {
            [self removeFromSuperview];
        }];
    } else {
        // 约束的动画
        [UIView animateWithDuration:0.2f animations:^{
            [thumbView mas_updateConstraints:^(MASConstraintMaker *make) {
                make.leading.mas_equalTo(trackView).priorityMedium();
            }];
            // 动画的关键!!!
            [trackView layoutIfNeeded];
        }];
    }
    
    if (CGRectContainsPoint(trackView.bounds, touchPoint)) {
        [self sendActionsForControlEvents:UIControlEventTouchUpInside];
    } else {
        [self sendActionsForControlEvents:UIControlEventTouchUpOutside];
    }
}

- (void) cancelTrackingWithEvent:(UIEvent *)event {
    
    [self sendActionsForControlEvents:UIControlEventTouchCancel];
    
    [UIView animateWithDuration:0.2 animations:^{
        [thumbView mas_updateConstraints:^(MASConstraintMaker *make) {
            make.leading.mas_equalTo(trackView).priorityMedium();
        }];
        
        [trackView layoutIfNeeded];
    }];
}


#pragma mark - Helper

void hug(UIView *view, UILayoutPriority priority) {
    
    hug_H(view, priority);
    hug_V(view, priority);
}

void hug_H(UIView *view, UILayoutPriority priority){
    [view setContentHuggingPriority:(priority) forAxis:UILayoutConstraintAxisHorizontal];
}

void hug_V(UIView *view, UILayoutPriority priority) {
    [view setContentHuggingPriority:priority forAxis:UILayoutConstraintAxisVertical];
}
@end

素材：

1. lock

   
   
   lockClosed.png

2. thumb

   
   
   thumb.png

3. track

   
   
   track.png