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iOS:autolayout,多个label如何设置居中显示?

刚学autolayout,关于这个问题,一直没搞清楚!

疑惑一:多个view(label)如何设置居中显示?
backview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[lab_before[lab_after]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(lab_before,lab_middle,lab_after)]];
枚举类型参数中:

typedef NS_OPTIONS(NSUInteger, NSLayoutFormatOptions) {
    NSLayoutFormatAlignAllLeft = (1 << NSLayoutAttributeLeft),
    NSLayoutFormatAlignAllRight = (1 << NSLayoutAttributeRight),
    NSLayoutFormatAlignAllTop = (1 << NSLayoutAttributeTop),
    NSLayoutFormatAlignAllBottom = (1 << NSLayoutAttributeBottom),
    NSLayoutFormatAlignAllLeading = (1 << NSLayoutAttributeLeading),
    NSLayoutFormatAlignAllTrailing = (1 << NSLayoutAttributeTrailing),
    NSLayoutFormatAlignAllCenterX = (1 << NSLayoutAttributeCenterX),
    NSLayoutFormatAlignAllCenterY = (1 << NSLayoutAttributeCenterY),
    NSLayoutFormatAlignAllBaseline = (1 << NSLayoutAttributeBaseline),
    NSLayoutFormatAlignAllLastBaseline = NSLayoutFormatAlignAllBaseline,
    NSLayoutFormatAlignAllFirstBaseline NS_ENUM_AVAILABLE_IOS(8_0) = (1 << NSLayoutAttributeFirstBaseline),

    NSLayoutFormatAlignmentMask = 0xFFFF,

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a123456678 2016-07-27 17:09:04 3127 0
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  • 层次结构是:

    - view
        -backview
            -label1
            -label2
            -label3
    // 设置水平布局
        [backview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[lab_before]-[lab_middle]-[lab_after]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(lab_before,lab_middle,lab_after)]];
    // 设置backview宽度
        [view addConstraint:[NSLayoutConstraint constraintWithItem:backview attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:dynaContentView attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:before_width+middle_width+after_width]];
    
        // 设置backview居中显示
        [view addConstraint:[NSLayoutConstraint constraintWithItem:backview attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:dynaContentView attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0.0]];
    
        // 设置backview的高度
        [view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[backview(=
    2019-07-17 20:00:54
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