All language subtitles for 3. How to Use Container Widgets

Now in this lesson, I want to talk to you about one of the most fundamental widgets that we're going
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to be using all the time, to lay out our apps.
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And that is a container widget.
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Now a container widget, for those you guys who have done a little bit of Web Development or Web Design,
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is very similar to a Div.
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It's essentially just a layout box.
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You can position it and move it around on the screen, and then you can give it a child to display such
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as an image or a piece of text.
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Now whenever you're using a new widget from Flutter, my top tip is to always start with the documentation.
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As I said previously, Flutter's documentation is really good. And all you have to do to find it, is head
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over to flutter.dev and we're going to go over to the widget catalog. And again, you'll find this
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URL to get here directly, in the course resources.
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Now once you're here, you can see that the team has organized all of the widgets based on what they do.
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And we've already seen some of the images and assets widgets, but now we're going to be looking at the
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layout section. And these other widgets that allows you to arrange other widgets. So how to position things
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on screen. And under the layout widget,
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you can see that they are split by single-child widgets and multi-child widgets. A widget that can contain
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a single widget, say a piece of text or an image, or lots of different widgets, such as five images or an
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image and a piece of text.
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And in this module, we're going to be exploring both single child, and multi-channel widgets to learn
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how to lay out your screen, exactly the way that you want it to look.
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The most commonly use single child widget is probably going to be a container.
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And if we click on it, we get taken to the container class. And this is a widget that allows us to position
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other widgets and size other widgets.
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And if you read through this, you'll learn a lot about exactly how it works.
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And one of the things that it tells you is that, containers with no children try to be as big as possible.
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And this is exactly what we're seeing over here.
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We have a container that is currently trying to be as big as possible.
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How do we know that?
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Well let's give it a color.
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Let's see it for ourselves.
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So let's go ahead and change this back to teal, and instantly, we have our background using hot reload.
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And now inside the container,
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so inside these round brackets, I'm gonna hit ENTER and I'm going to change its color. And here we get
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to specify a color for our container.
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Let's change our container to have a color of white.
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So now when I hit save and hot reload, you can see that my entire screen changes to white, even though
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we know that the background color is still teal.
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And the reason for this is because our container is taking up all of the space that's available, exactly
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as described here.
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Containers with no children
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try to be as big as possible. Our container currently doesn't have anything inside the child property.
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So that's why it tries to be as big as possible.
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But what if we did give it a child.
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What if we gave it a child widget
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that's a piece of text, say that just says 'Hello.'
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Right.
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Let's hit save, and immediately our container shrinks to the size of the child widget. And we could have
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known this by reading the documentation. Containers with children size themselves to their children's
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size. And our child in this case, is the piece of text and its size is determined by the font size of
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that piece of text.
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Now at the moment, you can see that all piece of text is kind of hidden away a bit by the bezel of our
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iPhone screen, and on Android depending on which device you're simulating it on, it might also have a
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notch like this.
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So how do we keep our content that we put on screen, safe from all of these bordering elements like the
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top bar where we've got our time and our signal and our notch? Well Flutter actually comes with a really
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simple widget that does exactly this.
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It's called a safe area. And anything that you put inside the safe area, will be inside the parts which
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don't have a notch or an interactive area like this part on the iPhone here.
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In order to embed our container inside a safe area, we can hold down OPTION or ALT and hit ENTER, to pull
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up our intention actions here, our little menu. And I'm going to wrap our container, which is what's currently
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selected, inside a new widget. And all that does, is it puts our previous container inside a new widget,
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and it sets it as the child of that widget.
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So now, we get to change this widget and I'm gonna call it a Safe Area, with a capital S and a capital
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A.
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Now as soon as I hit save, you'll see my container moves in to the safe area and it now only takes up
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the space where it's visible to the user.
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Whenever you're creating something for the user interface that you don't want to bleed all the way to
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the edges but instead you want it to be fully visible to the user,
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then you're gonna be using your safe area.
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What else can you do with your container?
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Well we can also change its size, such as its width and its height.
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So if we just go ahead and add a new property called height, and let's set it to maybe a 100 pixels,
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100 points. And let's change the width also to a 100.
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So now we should have a 100 by 100, so a square container, as soon as I hit save.
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And there you go.
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We've got our square container, with a single child that is the piece of text that says 'Hello.' Now what
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if I wanted my container to be a little bit offset from the edges of the screen.
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What if I wanted to set a margin for my container?
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Well, I can use the margin property. And to specify the margin property, we have to use something called
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edge insets.
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And you can see there's different types of insetting an edge. You can use the all,
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which means that whatever you enter here, let's say 20 pixels, is going to be set for all four sides of
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your container.
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So there's now a little margin of 20, for the left, the top, the right and the bottom.
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Now what if we didn't want that.
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What if we wanted to only set it for the top and the bottom?
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Well, then you can use symmetric.
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So now you can say that for the vertical axes, I want it to have a 50, but for the horizontal axis, I only
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want it to have 10 for the margin.
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So now if we hit save, you can see that it's offset from the top by 50 and from the left and right it's
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only 10.
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And you can always view this easily by going to the Flutter Inspector and clicking on this button here.
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And now, you can see the margin for our container at the top and bottom being 50, and the left and right
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being 10. And you can also see the safe area which is this box over here as well. Now instead of symmetric
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where you provide the same value for the top, bottom and the same value for left and right, you can also
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give it different values for each side. So you can either say, 'from LTRB'
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So this is left, top, right and bottom. Left,
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let's say it's 30, top is 10,
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right
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is 50, bottom is 20. Every single side
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I've given a different margin. And now, when I hit save, you can see how our box changes right.
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You can see that the left is 30, the top is 10, the right is 50 and the bottom is 20.
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So this box here, is what our margins are doing to our screen. But if you didn't want to have four values
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specified, you can use the 'only'. So saying 'only' gonna set a margin for the left, which is 30. And now everything
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else is left as zero and only the left gets that margin.
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So there's all sorts of ways that you can do this. Now,
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what if you wanted to have the child of the container to be a little bit more offset from the edge of
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the container.
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In this case, you wouldn't be using the margin and you would instead be using a padding.
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So if you've ever worked with CSS or Web Design, then this will all feel very intuitive to you.
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And remembering that the Flutter team actually came from a background of Web Development,
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this is one of the reasons why it might be so familiar and so intuitive to work with.
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So for the padding, it also expects a edge inset.
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So let's go ahead and provide one.
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And this allows us to set basically different values for the padding, for all four edges of whatever
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it is that we're setting it to, which in this case is our container.
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Let's add a padding for all sides,
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that's 20.
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And when I hit save, you'll see that I now have a little box inside my container, that's limiting the
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child to this area right here.
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And if I go ahead and remove all of these guidelines, then you can see that this is what I end up with.
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I've got a container that's pushed slightly away from the edges of the screen, and I've got its content
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pushed away from the edges of the container.
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So margin is for the outside of your widget, and padding is for the inside of your widget.
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And remember that a container can only have one child.
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Right.
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So for example, I can't have a piece of text and maybe say an image. That won't work. Because remember
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the widget itself, can only have a single child.
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So we're only laying out a single child. In the next lesson, we're going to look at some of the ways for
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laying out multiple children namely, using widgets such as columns and rows.
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So all of that and more, I'll see on the next lesson.