iOS10.3 Label高度计算问题(UITableView+FDTemplateLayoutCell)

1.前言

今天有用户反馈说10.3的系统，有些文字显示不全，影响正式用户我哪里敢怠慢。急速的更新手机系统进行测试，发现真的是有问题，而且这个问题是UITableView+FDTemplateLayoutCell 引起的，计算的高度不准确引起的。

2.问题发现

经过测试发现问题出现在xib或者nib创建的cell 拉约束之后再用UITableView+FDTemplateLayoutCell 计算高度就会出现问题，用Masonry 配合UITableView+FDTemplateLayoutCell 使用没问题（至少我的是没有问题，当然如果你的有问题也可以看下我列举的解决办法，相信有适合你的）

3.解决问题

经过Gogle 发现目前为止很少有人提到这个问题（难道大家都没发现还是大家的都没问题，当然也有可能没用UITableView+FDTemplateLayoutCell 的),在UITableView+FDTemplateLayoutCell issues 和 Masonry issues 里面有很多提到这个问题的。究其原因好像iOS 10.3 会加一个宽一个高约束（Looks like iOS 10.3 has two additional constraints there for width/height),对Autolayout的约束有新的计算方式。

4.列举下解决问题的方法

1.设置 label的 preferredMaxLayoutWidth

这个方法亲测是可以的，但是有个问题nib 创建的cell 很多都不知道这个值到底是多少，就是知道也不能一个cell一个cell 的设置吧！至少我是不愿意 ，天啊几十种cell啊！当然有些人可能会图省事，随意设置一个吧，label.preferredMaxLayoutWidth = [UIScreen mainScreen].bounds.size.width - 80 这样你测试的是会发现确实换行了，但是真的行吗？label 的换行是要根据这个 preferredMaxLayoutWidth 类似计算文字高度的方法

- (CGRect)boundingRectWithSize:(CGSize)size options:(NSStringDrawingOptions)options attributes:(nullable NSDictionary *)attributes context:(nullable NSStringDrawingContext *)context NS_AVAILABLE(10_11, 7_0)

就像这个方法中size size的宽就是要设置label能够显示的宽，如果给的不对当然计算出的高度也会不对，如果给的小了计算的高度就高，给的大了计算的高度就低

2. 加 [cell layoutIfNeeded]

因为有时候我发现第一次label显示的是没问题的,但是刷新一下就不行了，所以我想到在刷新重新算高度之前刷新下约束，这样就可以知道label的最大宽度限制了,当然我也不想在所以的cell 里面处理 所以在UITableView+FDTemplateLayoutCell 里面做了些处理

- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier configuration:(void (^)(id))configuration
  {
   if (!identifier) {
    return 0;
   }

// Fetch a cached template cell for `identifier`.
UITableViewCell *cell = [self fd_templateCellForReuseIdentifier:identifier];

// Manually calls to ensure consistent behavior with actual cells (that are displayed on screen).
[cell prepareForReuse];

// Customize and provide content for our template cell.
if (configuration) {
    configuration(cell);
}

CGFloat contentViewWidth = CGRectGetWidth(self.frame);

// If a cell has accessory view or system accessory type, its content view's width is smaller
// than cell's by some fixed values.
    if (cell.accessoryView) {
    contentViewWidth -= 16 + CGRectGetWidth(cell.accessoryView.frame);
} else {
    static CGFloat systemAccessoryWidths[] = {
        [UITableViewCellAccessoryNone] = 0,
        [UITableViewCellAccessoryDisclosureIndicator] = 34,
        [UITableViewCellAccessoryDetailDisclosureButton] = 68,
        [UITableViewCellAccessoryCheckmark] = 40,
        [UITableViewCellAccessoryDetailButton] = 48
    };
    contentViewWidth -= systemAccessoryWidths[cell.accessoryType];
}

CGSize fittingSize = CGSizeZero;
// If auto layout enabled, cell's contentView must have some constraints.
BOOL autoLayoutEnabled = cell.contentView.constraints.count > 0 && !cell.fd_enforceFrameLayout;

   if (autoLayoutEnabled) {
    // Add a hard width constraint to make dynamic content views (like labels) expand vertically instead
    // of growing horizontally, in a flow-layout manner.
    if (IOS_VERSION > 10.2) {
        [cell layoutIfNeeded];
    }
    NSLayoutConstraint *tempWidthConstraint =
    [NSLayoutConstraint constraintWithItem:cell.contentView
                                 attribute:NSLayoutAttributeWidth
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:nil
                                 attribute:NSLayoutAttributeNotAnAttribute
                                multiplier:1.0
                                  constant:contentViewWidth];
    [cell.contentView addConstraint:tempWidthConstraint];
    // Auto layout engine does its math
    fittingSize = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
    [cell.contentView removeConstraint:tempWidthConstraint];

} else {

    // If not using auto layout, you have to override "-sizeThatFits:" to provide a fitting size by yourself.
    // This is the same method used in iOS8 self-sizing cell's implementation.
    // Note: fitting height should not include separator view.
    SEL selector = @selector(sizeThatFits:);
    BOOL inherited = ![cell isMemberOfClass:UITableViewCell.class];
    BOOL overrided = [cell.class instanceMethodForSelector:selector] != [UITableViewCell instanceMethodForSelector:selector];
    if (inherited && !overrided) {
        NSAssert(NO, @"Customized cell must override '-sizeThatFits:' method if not using auto layout.");
    }
    fittingSize = [cell sizeThatFits:CGSizeMake(contentViewWidth, 0)];
}

// Add 1px extra space for separator line if needed, simulating default UITableViewCell.
if (self.separatorStyle != UITableViewCellSeparatorStyleNone) {
    fittingSize.height += 1.0 / [UIScreen mainScreen].scale;
}

if (autoLayoutEnabled) {
    [self fd_debugLog:[NSString stringWithFormat:@"calculate using auto layout - %@", @(fittingSize.height)]];
} else {
    [self fd_debugLog:[NSString stringWithFormat:@"calculate using frame layout - %@", @(fittingSize.height)]];
}
  return fittingSize.height;
   }

*重点在这里

1.png

当然如果这个能满足你也是很好的，但是却满足不了我的工程，这个是可以完全解决nib 创建的cell 的高度问题，但是纯代码用Masonry加约束的却出现了问题，反而不能换行了，所以这个方法也不适合我，此路不通我再想他法继续往下看

3.给cell.contentView 加左右约束

在这里我这样理解的，既然xcode自动帮我们加的左右约束有问题，那我不用他的了， 我自己加

2.png

在同样的地方替换掉layoutIfNeeded，换成加左右约束, 记得后面加上priorityLow ，这样是避免跟cell 里面手动加的约束起冲突，这样就技能满足nib cell 也能满足纯代码cell ，也不用一个cell 一个cell 的改。至此这个问题完美的解决了（我的问题是解决了，你的解决了吗？欢迎留言共同探讨，小牛路过，不喜勿喷！）

什么,你还懒得敲，要我发源码！好吧忍不了你了

- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier configuration:(void (^)(id))configuration
 {
   if (!identifier) {
    return 0;
   }

// Fetch a cached template cell for `identifier`.
UITableViewCell *cell = [self fd_templateCellForReuseIdentifier:identifier];

// Manually calls to ensure consistent behavior with actual cells (that are displayed on screen).
[cell prepareForReuse];

// Customize and provide content for our template cell.
if (configuration) {
    configuration(cell);
}

CGFloat contentViewWidth = CGRectGetWidth(self.frame);

// If a cell has accessory view or system accessory type, its content view's width is smaller
// than cell's by some fixed values.
if (cell.accessoryView) {
    contentViewWidth -= 16 + CGRectGetWidth(cell.accessoryView.frame);
} else {
    static CGFloat systemAccessoryWidths[] = {
        [UITableViewCellAccessoryNone] = 0,
        [UITableViewCellAccessoryDisclosureIndicator] = 34,
        [UITableViewCellAccessoryDetailDisclosureButton] = 68,
        [UITableViewCellAccessoryCheckmark] = 40,
        [UITableViewCellAccessoryDetailButton] = 48
    };
    contentViewWidth -= systemAccessoryWidths[cell.accessoryType];
}

CGSize fittingSize = CGSizeZero;
// If auto layout enabled, cell's contentView must have some constraints.
BOOL autoLayoutEnabled = cell.contentView.constraints.count > 0 && !cell.fd_enforceFrameLayout;

if (autoLayoutEnabled) {
    // Add a hard width constraint to make dynamic content views (like labels) expand vertically instead
    // of growing horizontally, in a flow-layout manner.
    if (IOS_VERSION > 10.2) {
        [cell.contentView mas_makeConstraints:^(MASConstraintMaker *make) {
            make.left.mas_equalTo(0).priorityLow();
            make.right.mas_equalTo(0).priorityLow();
        }];
    }
    NSLayoutConstraint *tempWidthConstraint =
    [NSLayoutConstraint constraintWithItem:cell.contentView
                                 attribute:NSLayoutAttributeWidth
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:nil
                                 attribute:NSLayoutAttributeNotAnAttribute
                                multiplier:1.0
                                  constant:contentViewWidth];
    [cell.contentView addConstraint:tempWidthConstraint];
    // Auto layout engine does its math
    fittingSize = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
    [cell.contentView removeConstraint:tempWidthConstraint];

} else {

    // If not using auto layout, you have to override "-sizeThatFits:" to provide a fitting size by yourself.
    // This is the same method used in iOS8 self-sizing cell's implementation.
    // Note: fitting height should not include separator view.
    SEL selector = @selector(sizeThatFits:);
    BOOL inherited = ![cell isMemberOfClass:UITableViewCell.class];
    BOOL overrided = [cell.class instanceMethodForSelector:selector] != [UITableViewCell instanceMethodForSelector:selector];
    if (inherited && !overrided) {
        NSAssert(NO, @"Customized cell must override '-sizeThatFits:' method if not using auto layout.");
    }
    fittingSize = [cell sizeThatFits:CGSizeMake(contentViewWidth, 0)];
}

// Add 1px extra space for separator line if needed, simulating default UITableViewCell.
if (self.separatorStyle != UITableViewCellSeparatorStyleNone) {
    fittingSize.height += 1.0 / [UIScreen mainScreen].scale;
}

if (autoLayoutEnabled) {
    [self fd_debugLog:[NSString stringWithFormat:@"calculate using auto layout - %@", @(fittingSize.height)]];
} else {
    [self fd_debugLog:[NSString stringWithFormat:@"calculate using frame layout - %@", @(fittingSize.height)]];
}

    return fittingSize.height;
 }