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The engine combines the entire tree into a renderable view and tells the operating system to display it. We just covered an entire framework in a couple of paragraphs. It was a lot. In fact, there are only two possible explanations. First, and most likely, you are just like every other human being: programming is hard, and learning takes time. Reread material, take a nap, and get back to it.

You will get it. The only other explanation is that I have done a poor job of making the material digestible, and I welcome you to berate me on your choice of platform. Widgets are simply Dart classes that describe their view.

A UI is created by composing several small widgets into complete widget trees. The good news is that Dart is also quite easy to pick up. For example, the command which dart in most Unix system terminals returns the file path to your Dart SDK.

Installation instructions can be found in appendix A. This is the first piece of the puzzle. Figure 2. Function name Return type void indicates no return value. The main function in Dart All Dart functions look like this, but main is special. Your program must contain a main function that the Dart compiler can find. Notice the word void in the example.

Types are a core part of writing Dart code. Every variable should have a type, and every function should return a type or void. But for now, just remember: all functions return a type or void. Next in this example is the line that contains the print function: print 'Hello, Dart!

On that same line, you have a String 'Hello, Dart! This is necessary at the end of every statement in Dart. This section will introduce the basics of setting up and running a Dart program. It will also introduce some common Dart syntax. In general, a lot of Dart syntax is similar to many languages. David Bowie! Hello, Dart! To start, refactor the old example to create a separate function that prints. Trying to call this function with anything other than exactly one argument, of the type String, is an error.

A List manages its own size and provides all the functional programming methods you expect for an array, like map and forEach. Another example would be Map. This sample calls helloDart without a string as an argument. We need to be passing each of those individual greetings in to the call to helloDart. It hits each member of the list once, in order, and exposes it as a variable in the code block.

This is done with interpolation. Here is the full example. Listing 2. This should work now. The user will be able to say who they want to greet. The first step is importing libraries. Some common libraries in the Dart SDK are dart:html, dart:async, and dart:math. Different libraries are available in different environments. This program asks for a name from the user on the command line and then greets that person. Everyone loves the Beatles, because the Beatles are great.

Every object inherits from the Object class. Even numbers are objects, not primitives. You cannot return a number from a function that declares it returns a string. And Dart is quite opinionated. In Dart, as in all programming languages, there are different ways of getting things done. But some ways are right and some are wrong.

There will almost never be a valid reason to stray from them. In human English, a language is typed if you, as the developer, can or must explicitly assign types to variables in your code.

A language is dynamic if the types are inferred at runtime. JavaScript, Python, and Ruby are dynamic languages. Under the hood, though, all languages are typed to some degree.

Types are used because they make your code safer. Importantly, in Dart, this type check is done at compile time. The biggest benefit of using a type system is that it reduces bugs. The type system is straightforward as far as type systems go. That said, it has to be briefly examined before I can talk about anything else.

I encourage you to circle back to this section at any time throughout the book if you need a type-system refresher. They have no concept of types to the developer. When I started writing Dart, I found using types to be the biggest hurdle. There are a few key places that you need to know about types for now, and the rest will be covered in time.

First, when declaring variables, you give them a type: String name; int age; The type always comes before the value it describes.

Try changing the type of the left hand side, or casting the right hand side to 'dart. Thanks, Dart team. Types ensure at compile time that your functions all get the right kind of data. This reduces the number of bugs you get at runtime. This is, in a nutshell, the value of type systems. It means the function returns nothing.

This works, but the type is still inferred. As for the var keyword, its usefulness is a matter of code style. The var keyword can only be used to define variables and cannot be used to define a type, unlike dynamic. So, the scope of where var can be used is small. This is almost always done in the bodies of functions, and not as class members. It's not convention to use block comments in Dart. Common programming concepts in Dart 33 Generally, documentation comments with three slashes are used for documenting your code.

Anything you think will stay in the code forever should use this style of comments. Inline comments are used for brief insights into something on a specific line. This is a variable definition: String name; That line simply tells your program that there will be a value called name, but the value is yet to be determined but, in this case, it will be a String.

All unassigned variables in Dart are null. The first two, final and const, are similar. You should use these keywords if you want to make a variable immutable in other words, if you never intended to change the value of the variable.

The difference in the two is subtle. Or, in English, a final variable is almost always a variable of a class that will be assigned in the constructor. Constants are variables that are always the same, no matter what, starting at compile time. In Flutter, there are special tools to help make your classes and widgets const. Lastly, there is a modifier called static.

Table 2. Interaction is largely handled by callbacks in Flutter, like onPressed. In all object-oriented programming, one of the hardest design issues is establishing relationships between your classes. There are two ways to create relationships between classes. For example, a Cowboy is a Human, and a Cowboy has a Voice. Inheritance tends to have you designing objects around what they are, and composition around what they do. Maybe you have classes like this: Human. You got to reuse the rideHorse method because cowboys and ranchers both, in fact, ride horses.

This could have been avoided by using composition from the beginning, rather than inheritance. You could have a HorseRiding class, which could be added as a member to any class.

No matter how many objects need to ride a horse, you have an easy, decoupled way to add that functionality. Maybe the rancher learned how to fly a spaceship. So now how do we think about our objects that have these methods? This is what composition is. To make a button that says Decrement Counter, you pass in another widget Text that handles the responsibility of setting text. In Flutter, always favor composition over inheritance to create reusable and decoupled widgets. This is especially true for widgets like text blocks and dialogs, which are basically containers for content.

This is key! This makes it as flexible as possible. All it cares about is displaying a button and telling its parent when that button is pressed via the callback.

All you need to do is pass in an Icon instead of Text. You could kick that up a notch and pass in the color if you wanted to. Anyway, back in your app, you should now have a button that you can use to decrement the counter by one. It leaves that up to us. And often, we mix and match those systems to achieve the layout we want.

Widgets, as we now know, are high-level classes that describe a view. There are lower-level objects that know how to paint these widgets onto the screen. In practice, that means the layout system is abstracted away for the developer, which opens up the possibility of using several different paradigms together. There are widgets that use the flexible layout, commonly known as FlexBox on the web.

And there are widgets that allow us to explicitly place widget on the screen at given coordinates. In this section, I want to explore some of the most common layout widgets. Constraints are a core part of understanding layout. In a nutshell, though, constraints tell widgets how much space they can take up, and then the widgets decide what they will take up.

In section 3. You can use flex layouts with Column and Row widgets. The counter app already has a Column widget in it, as shown in the next listing. Listing 3.

It will take all its children and lay them out, one by one, next to each other, from left to the right. TIP In some languages as in speaking languages, not programming languages , words are written right-to-left. This is outside the scope of this chapter. I want to wrap the decrement button in Row in the example app so I can add a second button beside it. In that same code block, start by adding Row around RaisedButton. This is because flexible widgets try to take up as much space as they can on their main axis.

The Row widget expands as much as it can horizontally, which in this case is as wide as the whole screen, constrained by its parent the column. Flutter is, after all, mainly a UI library and a rendering engine. Understanding how widgets determine their sizes will save you headaches in the future.

These are layout-constraint errors. This is an error that can be a headache to correct, unless you know how constraints work. I need to take a conceptual aside to discuss how Flutter knows what pixels to paint on the screen, thanks to constraints. One of the most important to understand is RenderObject. This class is mainly used internally. Render objects are responsible for the actual painting to the screen done by Flutter.

They are made internally by the framework, and all the render objects make up the render tree, which is separate from the widget tree. The render tree is made up of classes that implement RenderObject.

And render objects have corresponding widgets. As developers, we write widgets, which provide data such as constraints to a render object. The render object has methods on it like performLayout and paint. These methods are responsible for painting the pixels on the screen. All styling and layout work done in widgets is largely an abstraction over the render objects. These render objects are also without any state or logic. Importantly, widgets build child widgets in their build method, which create more widgets, and so on down the tree until it bottoms out at a RenderObjectWidget or a collection of RenderObjectWidgets.

These are the widgets that create render objects that paint to the screen. Consider a Column widget, which would not be a leaf RenderObjectWidget in a widget tree. Text and colors are concrete objects that can be painted. The job of a column is to provide constraints, not to paint anything on the screen.

Size, on the other hand, is concerned with actual width and height. When a render box is given its constraints, it then decides how much of that allotted space it will actually take up its size. Different render objects behave differently. This has everything to do with the constraints that are passed to it, and the way its render object behaves. Sometimes the constraints that are given to a box are unbounded.

This happens when either the maxHeight or maxWidth given to a render box is double. Unbounded constraints are found in Row, Column, and widgets that are scrollable. That makes sense, because a row can be—in theory—infinitely wide depending on its children. They behave differently based on the constraints passed by their parents. If they have bounded constraints, they try to be as big as possible within those bounded constraints.

For example, a column full of images that has unbounded constraints will try to be as tall as the combined height of all the images. Let me try to make this more concrete with an example. The inner Column now has an child: Column unbounded constraint, so it children: [ will try to fit its children.

Expanded child: Text 'You have pushed the button this many times:', , , ], , ], , In this case, the inner column is going to try to be whatever size its child tries to be, and is unbounded by its own parent. A column will always try to be as wide as its parent, and a row will always try to be as tall as its parent. This often leads to that pesky error mentioned before. In general, though, if you know how flexible widgets behave, this problem is easier to tackle. To make that a little more pleasing to look at, we need to add an alignment to the Row.

Flexible widgets can be told how to space their children with a few different alignment options that can be passed to the mainAxisAlignment property: Row mainAxisAlignment: MainAxisAlignment.

Uses the spaceAround alignment option If you come from the web, spaceAround may look familiar see figure 3. Figure 3. The combination of diagrams, code examples, and annotations makes learning a snap. About the author Eric Windmill is a professional Dart developer and a contributor to open-source Flutter projects.

His work is featured on the Flutter Showcase page. Home Flutter in Action. The combination of diagrams, code examples, and annotations makes learning a snap. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment.

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