Masonry 添加约束要注意顺序

Posted 2020-07-29 琼'

对一个视图添加约束，其依赖的约束必须先已经存在，不能依赖该代码后的约束，否则造成不可预料的结果，如下代码能达到预期效果

- (void)makeConstraints {
    __weak typeof(self) weakSelf = self;

    [self.photoMoreButton mas_makeConstraints:^(MASConstraintMaker *make) {
        make.right.equalTo(weakSelf.photoButton).with.offset(-6);
        make.centerY.equalTo(weakSelf.photoButton);
        make.width.and.height.equalTo(@16);
    }];
    
    [self.photoImageView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.centerY.equalTo(weakSelf.photoButton);
        make.right.equalTo(weakSelf.photoMoreButton.mas_left);
        make.width.and.height.equalTo(@64);
    }];
}

但是如果颠倒添加约束的顺序，如下

- (void)makeConstraints {
    __weak typeof(self) weakSelf = self;
 
    [self.photoImageView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.centerY.equalTo(weakSelf.photoButton);
        make.right.equalTo(weakSelf.photoMoreButton.mas_left);
        make.width.and.height.equalTo(@64);
    }];

     [self.photoMoreButton mas_makeConstraints:^(MASConstraintMaker *make) {
        make.right.equalTo(weakSelf.photoButton).with.offset(-6);
        make.centerY.equalTo(weakSelf.photoButton);
        make.width.and.height.equalTo(@16);
    }];
}

则得到的效果如下图

添加约束时 photoImageView 依赖于 photoMoreButton，而当时 photoMoreButton 的约束还未设置，所以导致后面不正确的结果，使用Masonry时要注意这点！