//displayView
        UIView *displayView = [UIView new];
        displayView.backgroundColor = [UIColor blackColor];
        [self addSubview:displayView];
        
        //keybordView
        UIView *keybordView = [UIView new];
        [self addSubview:keybordView];
        
        [displayView mas_makeConstraints:^(MASConstraintMaker *make) {
            make.top.left.right.equalTo(self);
            make.height.equalTo(keybordView).multipliedBy(.3f);
        }];
        
        [keybordView mas_makeConstraints:^(MASConstraintMaker *make) {
            make.top.equalTo(displayView.mas_bottom);
            make.left.right.bottom.equalTo(self);
        }];
        
        //设置显示位置的数字为0
        UILabel *displayNum = [[UILabel alloc]init];
        [displayView addSubview:displayNum];
        displayNum.text = @"0";
        displayNum.font = [UIFont fontWithName:@"HeiTi SC" size:70];
        displayNum.textColor = [UIColor whiteColor];
        displayNum.textAlignment = NSTextAlignmentRight;
        [displayNum mas_makeConstraints:^(MASConstraintMaker *make) {
            make.left.and.right.equalTo(displayView).with.offset(-10);
            make.bottom.equalTo(displayView).with.offset(-10);
        }];
        
        UIView *testView = [UIView new];
        testView.backgroundColor = [UIColor redColor];
        [keybordView addSubview:testView];
        
        [testView mas_makeConstraints:^(MASConstraintMaker *make) {
            make.width.equalTo(keybordView.mas_width).multipliedBy(.25f);
            make.height.equalTo(keybordView.mas_height).multipliedBy(.2f);
            make.left.equalTo(keybordView.mas_left);
            make.baseline.equalTo(keybordView.mas_baseline).multipliedBy(0.2);
        }];
        
        //我的理解mas_baseline是view的底部距离顶部的距离。注意参照点是顶部
        //make.baseline.equalTo(keybordView.mas_baseline).multipliedBy(1);
        //等价testView的底部距离keybordView顶部的距离 = keybordView的底部距离距离顶部的距离 乘以 1

屏幕快照 2016-04-26 下午3.08.34.png

注解如下：

displayView 黑色区域
keybordView 图中白色区域

现在要把红色区域添加到白色区域

详细代码如下：

        UIView *testView = [UIView new];
        testView.backgroundColor = [UIColor redColor];
        [keybordView addSubview:testView];
        
        [testView mas_makeConstraints:^(MASConstraintMaker *make) {
            make.width.equalTo(keybordView.mas_width).multipliedBy(.25f);
            make.height.equalTo(keybordView.mas_height).multipliedBy(.2f);
            make.left.equalTo(keybordView.mas_left);
            make.baseline.equalTo(keybordView.mas_baseline).multipliedBy(0.2);
        }];

主要解释如下这句代码：

make.baseline.equalTo(keybordView.mas_baseline).multipliedBy(0.2);
        }];

• mas_baseline是view的底部距离顶部的距离。注意参照点是顶部

因为

make.height.equalTo(keybordView.mas_height).multipliedBy(.2f);

testView的高度等于keybordView的高度乘以0.2

所以

• make.baseline.equalTo(keybordView.mas_baseline).multipliedBy(.2f);

根据代码判断：

• 等价testView的底部距离keybordView顶部的距离 = keybordView的底部距离距离顶部的距离 乘以 0.2

刚好和高度吻合

所以testView显示的位置就刚好在keybordView的左上角的位置。顶部和keyboardView的顶部吻合。