Library

We have been learning the basic concept in the pass lesson,but the knowledges are used in one file. if we want to use the other file in dart,we need the library.

How to use a library

If you want use an exist library,you must use the keyword import,the example is beneath.

import 'dart:math';

void main() {
  var a = 1;
  var b = 2;
  var m = min(a, b);
  print("m = $m"); //print  `m = 1`
}

Or Like this:

import 'package:flutter_test/flutter_test.dart';

void main() {
  var tP = TestPointer(1);
  print("isDown = ${tP.isDown}");
}

As we can see,the syntax of using a library is import URL or import special library like 'dart:math',
We import a library math here, for the library has been defined named ‘dart.math’ that we can use the name directly. If the library is not defined a name,then we can use this:

import 'package:io/io.dart';

void main() {
  copyPath("/home/nb/Workshop/FirstFlutter/second_flutter/test",
      "/home/nb/Workshop/FirstFlutter/second_flutter/tests");
}

The io library has not a name,so you can use them the URL of a file in the packages.the syntax is import 'package: xxxLib/xxxLib.dart';.Then it can be imported the xxxLib into the code that you can use it. If you want to use the part of a library you can use the show or hide keyword,the show means that only the appointed part can be used.On the opposite side,hide means that the designation part is hided.the syntax like this import 'package:lib_1/lib_1.dart' show A,B; or import 'package:lib_2/lib_2.dart' hide C,D;,Examples are following:

Show

If you call the copyPath,it is fine,but if you call the copyPathSync,there is an error The function 'copyPathSync' isn't defined.

import 'package:io/io.dart' show copyPath;

void main() {
  copyPath("/home/nb/Workshop/FirstFlutter/second_flutter/test",
      "/home/nb/Workshop/FirstFlutter/second_flutter/tests");
  copyPathSync("/home/nb/Workshop/FirstFlutter/second_flutter/test",
      "/home/nb/Workshop/FirstFlutter/second_flutter/tests");
}

Hide

If you write like below,it will say there is an error The function 'copyPath' isn't defined.

import 'package:io/io.dart' hide copyPath;

void main() {
  copyPath("/home/nb/Workshop/FirstFlutter/second_flutter/test",
      "/home/nb/Workshop/FirstFlutter/second_flutter/tests");
}

How to define a library

You can use the existing library above, but sometimes, you need custom a library by yourself. In the path of ./animal ,we create a file named Animal.dart,contents are below:

void printAnimal() {
  print("This is an animal.");
}

Use them like this:

import 'animal/Animal.dart';

void main() {
  printAnimal(); //This is an animal.
}

If you want a prefix for the library,just like following:

import 'animal/Animal.dart' as animal;

void main() {
  animal.printAnimal(); //This is an animal.
}

Packages

We learn to use the library above,but the library is already load in the packages,if you do not load it,how to use a package,the most important file you need to know is the pubspec.yaml file,which is in the root directory of a project.Let’s see example in flutter project ‘pubspec.yaml’ file.

name: second_flutter
description: A new Flutter application.

dependencies:
  flutter:
    sdk: flutter

  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^0.1.2

If you want to add a package,you need add it below the dependencies as the flutter,for example,if you want to css_colors package, you just need add like following:

name: second_flutter
description: A new Flutter application.

dependencies:
  flutter:
    sdk: flutter

  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^0.1.2
  css_colors: ^1.0.0

Then click the packages get in the android studio ,or use the command line pub get in pure dart program or the flutter packages get in flutter. If the package is installed completely,you can use them as a library ,which are used as the same as the above topics. At present,we often use the Git tool to develop, the open sources of libraries are most in the Github,so we often need to load a library from the Github directly,the using is bellow.

Source of using packages

End

The serials basic lessons about the dart are complete,if you follow this in the pass times,you will learn some base knowledges, you can take some real projects that write with the dart to go further.The next time I will talk about the using of dart flutter,which is a cross platform developed by Google and used to develop the Android and Ios apps.if you interest it,you can look forward to it.Just wait and see,you will have a surprising for the next trip by following me.Above all,I hope the articles are useful for you.Thanks for your reading.

The End!