在autolayout中加入每一個view的weight

在autolayout中加入每一個view的weight，這樣相互之間的關係就更明確，好比：

　　double iconViewWeight = 1;
    double titleLabelWeight = 12;
    double timeLabelWeight = 3;
    
    double standardWeigth = iconViewWeight;
    double iconViewWeightRatio = iconViewWeight / standardWeigth;
    double titileLabelWightRatio = titleLabelWeight / standardWeigth;
    double timeLabelWightRatio = timeLabelWeight / standardWeigth;
    
    UIView *standardView = _iconView;
    
    NSDictionary *views = NSDictionaryOfVariableBindings(_iconView, _titileLabel, _timeLabel, bgImageView);
    NSLayoutConstraint *constraint = [NSLayoutConstraint constraintWithItem:_iconView
                                                                  attribute:NSLayoutAttributeWidth
                                                                  relatedBy:NSLayoutRelationEqual
                                                                     toItem:standardView
                                                                  attribute:NSLayoutAttributeWidth
                                                                 multiplier:iconViewWeightRatio
                                                                   constant:0.0f];
    [self addConstraint:constraint];
    
    constraint = [NSLayoutConstraint constraintWithItem:_titileLabel
                                              attribute:NSLayoutAttributeWidth
                                              relatedBy:NSLayoutRelationEqual
                                                 toItem:standardView
                                              attribute:NSLayoutAttributeWidth
                                             multiplier:titileLabelWightRatio
                                               constant:0.0f];
    [self addConstraint:constraint];
    constraint = [NSLayoutConstraint constraintWithItem:_timeLabel
                                              attribute:NSLayoutAttributeWidth
                                              relatedBy:NSLayoutRelationEqual
                                                 toItem:standardView
                                              attribute:NSLayoutAttributeWidth
                                             multiplier:timeLabelWightRatio
                                               constant:0.0f];
    [self addConstraint:constraint];
    
    constraints = [NSLayoutConstraint constraintsWithVisualFormat:@"|-[_iconView]-[_titileLabel]-[_timeLabel]-|"
                                                          options:NSLayoutFormatAlignAllCenterY
                                                          metrics:nil
                                                            views:views];
    [self addConstraints:constraints];

    constraint = [NSLayoutConstraint constraintWithItem:_iconView
                                              attribute:NSLayoutAttributeCenterY
                                              relatedBy:NSLayoutRelationEqual
                                                 toItem:self.contentView
                                              attribute:NSLayoutAttributeCenterY
                                             multiplier:1.0
                                               constant:0.0f];
    [self addConstraint:constraint];

代碼更加簡介，但這適用於一些特定的場合