Flutter 布局基础教程
欢迎来到 Flutter 布局 codelab!你将在这里学到如何构建 Flutter UI,更棒的是这一切都不需要安装 Flutter 或者 Dart!
Welcome to the Flutter layout codelab, where you learn how to build a Flutter UI without downloading and installing Flutter or Dart!
Flutter 与其他框架有着明显的差异,原因在于它使用代码来构建 UI,而不是 XML 或其他东西。其中,widget 是构建 Flutter UI 的基本单元。当你逐渐深入这个 codelab,你将会发现在 Flutter 中几乎所有的东西都是 widget。 widget 是一个不会改变的对象,它是 UI 中一个特定部分的描述。你还会学到 Flutter 的 widget 非常容易组合,这意味着你能够通过组合已有的 widgets 来创造更多复杂的 widgets。到这篇文章的最后,你会运用这里所学的知识构建一个显示名片的 Flutter UI。
Flutter is different from other frameworks because its UI is built in code, not (for example) in an XML file or similar. widgets are the basic building blocks of a Flutter UI. As you progress through this codelab, you’ll learn that almost everything in Flutter is a widget. A widget is an immutable object that describes a specific part of a UI. You’ll also learn that Flutter widgets are composable, meaning, that you can combine existing widgets to make more sophisticated widgets. At the end of this codelab, you’ll get to apply what you’ve learned into building a Flutter UI that displays a business card.
本 codelab 的预期完成时间约为 45 - 60 分钟
Estimated time to complete this codelab: 45-60 minutes.
Row 和 Column 类
Row and Column classes
Row
和 Column
是两个用来容纳和布局 widgets 的类。在它们内部的 widgets 我们称为 children,
Row
和 Column
就作为它们的父级。
Row
将会让 widgets 水平排列,而 Column 则会让其竖直排列。
Row
and Column
are classes that contain and lay out widgets.
widgets inside of a Row
or Column
are called children,
and Row
and Column
are referred to as parents.
Row
lays out its widgets horizontally,
and Column
lays out its widgets vertically.
样例:创建一个 Column
Example: Creating a Column
轴大小和对齐方式
Axis size and alignment
至此,BlueBox
widget 已经在一起被压扁了 (在界面的左边或者上面 )。你可以通过轴大小和对齐属性来改变 BlueBox
widget 的间距。
So far, the BlueBox
widgets have been squished together
(either to the left or at the top of the UI Output).
You can change how the BlueBox
widgets are spaced
out using the axis size and alignment properties.
mainAxisSize 属性
mainAxisSize property
Row
和 Column
分别占据了不同的主轴。Row
的主轴是水平的。
mainAxisSize
决定了 Row
和 Column
能够在主轴上占据多大空间。
mainAxisSize
有两个可选属性:
Row
and Column
occupy different main axes.
A Row
’s main axis is horizontal,
and a Column
’s main axis is vertical.
The mainAxisSize
property determines how much
space a Row
and Column
can occupy on their main axes.
The mainAxisSize
property has two possible values:
MainAxisSize.max
Row
和 Column
占据它们主轴上所有空间。如果子 widget 的总宽度小于主轴上的空间,它们就会充满剩余的空间。
MainAxisSize.max
Row
and Column
occupy all of the space on their main axes.
If the combined width of their children is
less than the total space on their main axes,
their children are laid out with extra space.
MainAxisSize.min
Row
和 Column
仅占据它的 children 在主轴上所需的空间,它的 children 在主轴之间将没有额外空间。
MainAxisSize.min
Row
and Column
only occupy enough space on their main axes
for their children. Their children are laid out without extra space
and at the middle of their main axes.
样例:自定义轴大小
Example: Modifying axis size
mainAxisAlignment 属性
mainAxisAlignment property
当 mainAxisSize
被设为 MainAxisSize.max
,
Row
和 Column
将会使用额外空间来对齐它的 children。
mainAxisAlignment
属性决定了 Row
和 Column
将会在额外空间中如何对齐它的 children。
mainAxisAlignment
有以下六个可选属性:
When mainAxisSize
is set to MainAxisSize.max
,
Row
and Column
might lay out their children with extra space.
The mainAxisAlignment
property determines how Row
and Column
can position their children in that extra space.
mainAxisAlignment
has six possible values:
MainAxisAlignment.start
将其 children 从主轴起点处开始对齐。 (Row
的起点在左边,Column
的起点在顶部 )
MainAxisAlignment.start
Positions children near the beginning of the main axis.
(Left for Row
, top for Column
)
MainAxisAlignment.end
将其 children 从主轴终点处开始对齐。 (Row
的终点在右边,Column
的终点在底部 )
MainAxisAlignment.end
Positions children near the end of the main axis.
(Right for Row
, bottom for Column
)
MainAxisAlignment.center
将其 children 置于主轴中心。
MainAxisAlignment.center
Positions children at the middle of the main axis.
MainAxisAlignment.spaceBetween
在 children 之间平均分配额外空间。
MainAxisAlignment.spaceBetween
Divides the extra space evenly between children.
MainAxisAlignment.spaceEvenly
在 children 之间,以及第一个 children 之前和最后一个 children 之后,平均分配额外空间。
MainAxisAlignment.spaceEvenly
Divides the extra space evenly between children
and before and after the children.
MainAxisAlignment.spaceAround
与 MainAxisAlignment.spaceEvenly
相似,但在第一个 child 之前以及最后一个孩子之后减少了一半的空间,让其 children 之间宽度缩减一半。
MainAxisAlignment.spaceAround
Similar to MainAxisAlignment.spaceEvenly
,
but reduces half of the space before the first
child and after the last child
to half of the width between the children.
样例:自定义主轴对齐方式
Example: Modifying main axis alignment
crossAxisAlignment 属性
crossAxisAlignment property
crossAxisAlignment
属性决定了 Row
和 Column
能够如何在其横轴上定位 children。
Row
的横轴是竖直的,而 Column
则是水平的,
crossAxisAlignment
属性有五个可选属性:
The crossAxisAlignment
property determines
how Row
and Column
can position their children
on their cross axes.
A Row
’s cross axis is vertical,
and a Column
’s cross axis is horizontal.
The crossAxisAlignment
property has five possible values:
CrossAxisAlignment.start
将其 children 横轴顶部对齐。 (顶部是 Row
,左侧是 Column
)
CrossAxisAlignment.start
Positions children near the start of the cross axis. (Top for Row
, Left for Column
)
CrossAxisAlignment.end
将其 children 横轴底部对齐。 (底部是 Row
,右侧是 Column
)
CrossAxisAlignment.end
Positions children near the end of the cross axis. (Bottom for Row
, Right for Column
)
CrossAxisAlignment.center
将其 children 横轴中心对齐。 (中间是 Row
,中心是 Column
)
CrossAxisAlignment.center
Positions children at the middle of the cross axis. (Middle for Row
, Center for Column
)
CrossAxisAlignment.stretch
沿横轴延伸 children。 (在 Row
中是从顶至底,Column
则是从左至右 )
CrossAxisAlignment.stretch
Stretches children across the cross axis.
(Top-to-bottom for Row
, left-to-right for Column
)
CrossAxisAlignment.baseline
根据 children 的基线对子节点。
(仅限Text
类,并要求 textBaseline
属性设置为
TextBaseline.alphabetic
。在 Text class 小节中查看样例。
CrossAxisAlignment.baseline
Aligns children by their character baselines.
(Text
class only, and requires that the
textBaseline
property is set to
TextBaseline.alphabetic
. See the
Text widget section for an example.)
样例:自定义横轴对齐方式
Example: Modifying cross axis alignment
Flexible widget
正如你所看到,mainAxisAlignment
和 crossAxisAlignment
属性决定了 Row
和 Column
在各个轴上如何布局 widget。
Row
和 Column
首先布置固定大小的 widget。固定大小的小部件被认为是 不灵活的 因为它们布局后无法自我调整大小。
As you’ve seen, the mainAxisAlignment
and
crossAxisAlignment
properties determine
how Row
and Column
position widgets along both axes.
Row
and Column
first lay out widgets of a fixed size.
Fixed size widgets are considered inflexible because
they can’t resize themselves after they’ve been laid out.
Flexible
widget 包裹一个 widget 让这个 widget 变得可以调整大小。当 Flexible
widget 包裹 widget 时,这个 widget 就成为 Flexible
widget 的子节点,并被视为 flexible 的。在布置固定大小的 widget 后,
Flex 的 widget 根据其 flex
和 fit
属性调整大小:
The Flexible
widget wraps a widget,
so the widget becomes resizable.
When the Flexible
widget wraps a widget,
the widget becomes the Flexible
widget’s child
and is considered flexible.
After inflexible widgets are laid out,
the widgets are resized according to their
flex
and fit
properties.:
flex
将自身的 flex
因子与其他的比较,以决定自身占剩余空间的比例。
flex
Compares itself against other flex
properties before determining what fraction of the
total remaining space each Flexible
widget receives.
fit
决定 Flexible
的 widget 是否能够填充所有剩余空间。
fit
Determines whether a Flexible
widget
fills all of its extra space.
样例:改变 fit 属性
Example: Changing fit properties
样例:测试 flex 值
Example: Testing flex values
Expanded widget
Expanded
widget 能够包裹一个 widget 并强制其填满剩余空间,与 Flexible
非常相似。
Similar to Flexible
, the Expanded
widget can
wrap a widget and force the widget to fill extra space.
样例:填补额外空间
Example: Filling extra space
SizedBox widget
SizedBox
widget 的两种用途之一就是创建精确的尺寸。当 SizedBox
包裹了一个 widget 时,它会使用 height
和 width
调整其大小。如果它没有包裹 widget,它可以使用height
和width
属性创造空的空间。
The SizedBox
widget can be used in one of two ways when
creating exact dimensions.
When SizedBox
wraps a widget, it resizes the widget
using the height
and width
properties.
When it doesn’t wrap a widget,
it uses the height
and width
properties to
create empty space.
样例:调整一个 widget
Example: Resizing a widget
样例:创建空间
Example: Creating space
Spacer widget
与 SizedBox
相似,Spacer
widget 也能在 widgets 之间创建空间。
Similar to SizedBox
, the Spacer
widget also
can create space between widgets.
Example:创建更多空间
Example: Creating more space
Text widget
Text
widget 不仅能够显示文字,并能够配置不同的字体,大小和颜色。
The Text
widget displays text and can be configured
for different fonts, sizes, and colors.
样例:文字对齐
Example: Aligning text
Icon widget
Icon
widget 能够显示图形符号,这代表了 UI 的一个方面。
Flutter 将会为 Material 和 Cupertino
的应用提前加载 icon packages。
The Icon
widget displays a graphical symbol
that represents an aspect of the UI.
Flutter is preloaded with icon packages for
Material and Cupertino applications.
样例:创建一个 Icon
Example: Creating an Icon
Image widget
Image
widget 显示了一张图片。你还能够直接引用图片 URL,或是你的应用 package 中的图片。但是由于 DartPad 无法引用包图片,所以下面的样例将会使用网络上的图片。
The Image
widget displays an image.
You either can reference images using a URL,
or you can include images inside your app package.
Since DartPad can’t package an image,
the following example uses an image from the network.
样例:显示一张图片
Example: Displaying an image
综合练习
Putting it all together
你就要完成这个 codelab 了!如果你想要检验你刚学的知识,为何不讲这些结合起来,构建一个显示名片的 Flutter UI 呢!
You’re almost at the end of this codelab. If you’d like to test your knowledge of the techniques that you’ve learned, why not apply those skills into building a Flutter UI that displays a business card!
你将会把 Flutter 的布局分解成几个部分,这就是如何在实际开发中构建 Flutter UI 方式!
You’ll break down Flutter’s layout into parts, which is how you’d create a Flutter UI in the real world.
在第一部分, 你将会实现包含姓名和标题的 Column
。然后你将会在 Column
包裹一个含有 icon 的 Row
,它将会被放在姓名和标题的左边。
In Part 1,
you’ll implement a Column
that contains the name and title.
Then you’ll wrap the Column
in a Row
that contains the icon,
which is positioned to the left of the name and title.
在第二部分中,你将会在 Row
外包裹一个 Column
,所以你的代码中就包含了一个 Column (Row (Column) )。然后你将调整最外面的Column
的布局,所以它看起来不错。最后,您将添加联系信息到最外面的Column
的 children 中,所以它将显示在名称,标题和图标下方。
In Part 2, you’ll wrap the Row
in a Column
,
so the code contains a Column
within a Row
within a Column
.
Then you’ll tweak the outermost Column
’s layout,
so it looks nice.
Finally, you’ll add the contact information
to the outermost Column
’s list of children,
so it’s displayed below the name, title, and icon.
在第三部分,你将会完成添加了更多图标的名片,它们会被放在联系信息的下方。
In Part 3, you’ll finish building the business card display by adding four more icons, which are positioned below the contact information.
第一部分
Part 1
练习:创建 name 和 title
Exercise: Create the name and title
练习:在 Column 外包裹一个 Row
Exercise: Wrap the Column in a Row
第二部分
Part 2
练习:调整布局
Exercise: Tweak the layout
练习:输入联系信息
Exercise: Enter contact information
第三部分
Part 3
练习:添加四个图标
Exercise: Add four icons
下一步是什么?
What’s next?
恭喜你,已经完成了这个 codelab!如果你想要了解关于 Flutter 的更多信息,这里有些值得探索的资源要推荐给你:
Congratulations, you’ve finished this codelab! If you’d like to know more about Flutter, here are a few suggestions for resources worth exploring:
-
在 Building layouts 页面中学习关于 Flutter 的布局。
Learn more about layouts in Flutter by visiting the Building layouts page.
-
查看 sample apps。
Check out the sample apps.
-
访问 Flutter’s YouTube channel,你将能够观看大量专注于独立的 widget 以及开发者如何构建应用的视频。
Visit Flutter’s YouTube channel, where you can watch a variety of videos, from videos that focus on individual widgets to videos of developers building apps.
你可以在 install 页面中下载 Flutter。
You can download Flutter from the install page.