所有这些自动布局更新方法有什么区别?都是必须的吗? [英] What is the difference between all these Auto Layout update methods? Are all necessary?

问题描述

在下面的代码中，调用了这四个方法进行布局推理.不过，我有点困惑为什么需要所有这些，以及它们彼此之间的不同之处.它们用于通过自动布局使单元格的高度动态化.(取自 这个存储库 来自 这个问题.)

In the code below, these four methods are called for layout reasoning. I'm a little confused why all of them are needed, though, and what they do differently from one another. They're used in the process to make a cell's height be dynamic with Auto Layout. (Taken from this repository from this question.)

[cell setNeedsUpdateConstraints];
[cell updateConstraintsIfNeeded];
[cell.contentView setNeedsLayout];
[cell.contentView layoutIfNeeded];

它来自这个单元格高度的代码块:

And it's from this block of code for the cell's height:

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{

    RJTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    [cell updateFonts];

    NSDictionary *dataSourceItem = [self.model.dataSource objectAtIndex:indexPath.row];
    cell.titleLabel.text =  [dataSourceItem valueForKey:@"title"];
    cell.bodyLabel.text = [dataSourceItem valueForKey:@"body"];

    cell.bodyLabel.preferredMaxLayoutWidth = tableView.bounds.size.width - (kLabelHorizontalInsets * 2.0f);

    [cell setNeedsUpdateConstraints];
    [cell updateConstraintsIfNeeded];
    [cell.contentView setNeedsLayout];
    [cell.contentView layoutIfNeeded];

    CGFloat height = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;

    return height;
}

但他们的做法有何不同?为什么都需要它们?

But what are they doing differently? Why are they all needed?

推荐答案

Layout

假设您将视图逻辑封装在一个 UIView 子类中，并将其命名为 SomeView.这意味着SomeView应该知道如何自己布局，也就是如何在其中放置一些其他的视图(你也可以创建一个不使用任何子视图来绘制自己的视图)，但这超出了普通开发人员的需求).

Layout

Suppose you encapsulate your view logic in a UIView subclass, and call it SomeView. This means that SomeView should know how to layout itself, that is, how to position some other views inside it (you can also create a view that draws itself without using any subviews, but that's beyond the needs of an average developer).

此布局由[SomeView layoutSubviews] 完成.您可以选择覆盖它:

This layout is done by [SomeView layoutSubviews]. You have an option of overriding it:

subview.frame = CGRectMake(100 + shiftX, 50 + shiftY, 250 + shiftX - paddingRight, ...
// Oh no, I think I didn't do it right.

但您很少需要这样做.在 Cocoa Touch 的黑暗时代，这种手动布局很普遍，但现在我想说 99% 的布局都可以用自动布局覆盖.

but you rarely need to do so. In the dark ages of Cocoa Touch, this manual layout was widespread, but now I'd say 99% of layouts can be covered with Auto Layout.

系统需要知道什么时候应该调用[UIView layoutSubviews].它显然是在您第一次需要绘制视图时完成的，但它也可能在父视图框架更改时调用.这里有详细说明.

The system needs to know when it should call [UIView layoutSubviews]. It's obviously done the first time you need to draw a view, but it may be also called whenever the superview frame changes. Here's a detailed explanation.

所以系统经常调用[view layoutIfNeeded].你也可以在任何时候调用它，但只有当有一些事件调用了[view setNeedsLayout]或者你手动调用它时才会有效果，就像在这种情况下一样.

So the system often calls [view layoutIfNeeded]. You can also call it at any time, but this will have an effect only if there is some event that has called [view setNeedsLayout] or if you have called it manually, as in this case.

自动布局(在 文档) 被这样调用是因为您将 [SomeView layoutSubviews] 保留为它继承自 UIView 和用约束来描述你的子视图的位置.

The Auto Layout (capitalized this way in the documentation) is called like that because you're leaving [SomeView layoutSubviews] as it is inherited from UIView and describe the position of your subviews instead in terms of constraints.

使用自动布局时，系统将在每次布局传递时调用[view updateConstraintsIfNeeded].但是，仅当设置了标志 [view setNeedsUpdateConstraints]; 时，该方法才会调用 -updateConstraints(真正起作用).

When using Auto Layout, system will perform calls to [view updateConstraintsIfNeeded] at each layout pass. However, only if the flag [view setNeedsUpdateConstraints]; is set, the method calls into -updateConstraints (which does the real job).

如果您不使用自动布局，则这些方法不相关.

If you don't use Auto Layout, those methods are not relevant.

您可以像在本例中那样实现它.

很少需要直接调用 -layoutIfNeeded 和 -updateConstraintsIfNeeded，因为 UI 引擎会在每次布局时自动执行此操作.然而，在这种情况下，作者选择立即打电话给他们；这是因为现在需要生成的高度，而不是将来的某个时候.

It's rarely necessary to call -layoutIfNeeded and -updateConstraintsIfNeeded directly, because UI engine will do that automatically at each layout pass. However, in this case the author has chosen to call them immediately; this is because the resulting height is needed right now, not at some point in the future.

这种更新单元格高度的方法似乎是正确的.请注意，cell 可能是新创建的单元格，因此尚未添加到视图层次结构中；这不会影响其自身布局的能力.

This method of updating the cell's height seems right. Note that cell could be a newly created cell and thus not added into view hierarchy yet; this does not impact its ability to layout itself.

在您的自定义视图中使用以下选项，从最通用"到最自定义":

In your custom view go with the following options, starting from the most 'universal' to most 'customized':

1. 在视图创建过程中创建约束(手动或在 IB 中)
2. 如果您稍后需要更改约束，请覆盖 -updateConstraints.
3. 如果有无法通过上述方式描述的复杂布局，请覆盖 -layoutSubviews.

在更改某些内容可能使您的视图的约束发生变化的代码中，调用

In the code that changes something that could make your view's constraints change, call

[view setNeedsUpdateConstraints];

如果您需要立即获得结果，也请致电

If you need results immediately, call also

[view updateConstraintsIfNeeded]; 

如果代码改变了视图的框架使用

If the code changes view's frame use

[view setNeedsLayout]; 

最后，如果您想要立即获得结果，请致电

and finally if you want the results immediately, call

[view layoutIfNeeded];

这就是为什么在这种情况下需要所有四个调用.

This is why all four calls are required in this case.

看文章中的详细解释高级自动布局工具箱，objc.io issue #3

Take a look at detailed explanation in the article Advanced Auto Layout Toolbox, objc.io issue #3

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