Having discussed most of the basic concepts required to start the development of Android apps, let’s not wait any more! Let’s go straight to the development!
Today, we will start building the simplest version of the Hello World app (other than the one which is automatically generated! Remember? Refer to Let’s Code!) , in which we will focusing on the use of buttons and toasts(?? Don’t worry! I will discuss!) in Android applications. In the coming few posts, I will be building upon this tutorial to discuss the various concepts finally leading to the development of the app.
So, Let’s start! Open the Hello World Project that we created in the post titled Let’s Begin!.
Go to main.xml in the layout folder, drag a button from Form Widgets and place it onto the WYSIWYG screen. Go to the code! Is there any difference from the code discussed earlier?
Yes! We have a new tag named Button with various attributes! Let’s focus on each of them. Clearly, android:layout_width and layout_height are used to define the width and the height of the button respectively. By default, they are set to wrap_content. I hope you are familiar with this term. If not, please refer to the post titled A look at the code!.
Change the android:layout_width attribute of the Button tag to fill_parent in order to make the button stretched to the entire width of the screen, and leave the android:layout_height as it is. Save main.xml. Your screen in the WYSISYG editor should look as follows:
Refer to the code again. There are two more attributes for the tag Button. The android:text attribute sets the text which is displayed on the button. By default, it is set to Button and that’s why, the default text on the Button widget is Button.
You may also find a warning on this line. Can you guess why is it so? Basically, in the code that is generated automatically, android:text attribute has been set to the string Button whereas as per the correct syntax, one must declare a string variable (for example, button) whose value is Button and then, reference it by using @string/button in the value of this attribute. To fix it, simply click on the attribute, right click and choose Quick Fix. You will be provided with various options. Simply, select Extract String which will display a dialog box, in which set the R.string value to button and String to Button. Click OK and save the file. The text Button will replaced to @string/button and the warning will be gone.
We will look at the android:id attribute later. For now, simply run the project. What do you see?
You will find that there will be a button on the screen but, when you click it, nothing happens! Why?
In the next post, we will discuss the android:id attribute and it’s use in implementing the functioning of a button.
Till then, BYE!
So, it’s time to culminate our initial discussion on layouts. Till now, we have looked at the WYSIWYG editor, the code for the layout and a visual editor for playing with strings. Now, we shall come to the query raised in the last post, i.e., the value taken by the android:string attribute of the TextView which is described in the line:
For those who have been following this blog, it must be really easy now. But, still I should give the details for the sake of completeness.
Clearly, @string is a way to tell the compiler that the variable which will follow after a ‘/’ represents a string. This is necessary to distinguish the different types of variables. Yes! There are other kinds of variables which represent color, dimension, array, etc. Right now, I am not going to discuss how to use those variables. But, if you are one of those who really like to stay ahead, open the strings.xml and click on add. You will see the following dialog box.
Now, it’s on you to explore!
Also, the name that follows the ‘/’ is the name of the variable whose value is to be given to the android:text and thus, is to be displayed on the screen. Here, since the name is niktechs, the text that is displayed on the screen will be same as the value of the variable niktechs which is Developing Android Applications with Nikhil Gupta.
Now, I think I have laid the foundation of discussing the other way of changing the text on the screen. Remember, in the post titled Laying out the screen!, we saw how to use WYSIWYG to change the text on the screen. For a quick flashback, we used the edit text option and created a new R.string whose value was the text that we wanted to display.
The other way results from our discussion in the previous post. There, I created a new variable of type string named Blog. Now, instead of using the WYSIWYG editor, we can simply change niktechs (in the code describing android:text attribute) to Blog and save the file.
Now, look at the GUI (click on main.xml and then, choose Graphical Layout tab). What do you see? Yes, the text has been changed to Android Development which is the value of the variable Blog. Why does this happen? I hope that it’s now trivial for all. Since Blog is just another string type variable like niktechs (discussed in the previous post), replacing the two will result in replacing the values of the android:text attribute and hence, the text on the screen.
So, we have looked at the basic layout features and understood each and every concept related to it. But there are various attributes that can be used in the code for layout which can greatly vary the screen. It’s up to you to explore them. I may discuss a few of them, but only in the later posts. If you want to explore, go to a tag, type android: and press Ctrl+Space. You will see the entire options that are possible.
In the next few post, we will finally go into the JAVA coding. So, a lot more interesting stuff coming soon!
Till next time, BYE!
In the last post, we looked at the code for the layout of an android application. But we didn’t discuss discuss the strange value taken by the text attribute of the tag “TextView” and how it decides the text display by this TextView. Were you able to find any correlation?
In this post, I will discuss about strings in an android app. The concepts discussed in this post will be used to anwer the above query in the next post. So, let’s start!
Click on the values folder (It’s in the same directory as the layout folder). Do you find something which is related to our interest? Yes, a file named string.xml. Let’s open it! What do you see?
You can see that strings.xml opens up in an interactive editor. This editor is used to add Resources (to be discussed later) to our android application. For now, it’s sufficient to know that strings are treated as resources in an android app and hence, this editor allows us to make, modify or delete strings.
We can see three Resource Elements (as specified in the header), all being of type String as mentioned in the brackets beside the name. Thus, our android application has three resources, all of them being strings. The names that are visible are the names of the variables that are used to store the string type values (similar to the fact that x is a variable used to store integer type values when I write int x = 10 in C). Thus, hello and app_name are two string type variables which are automatically created during the creation of a project.
To look at the values stored by these variables, simply click on them. On clicking hello, you will see a panel in the right which displays the name of the variable and it’s value. Now, looking at the value of the variable hello, I hope that you can make out how the text displayed at the screen originally came about. Yes!, the value of this variable is Hello World, HelloWorldActivity!, the original text on the screen.
You must have seen another line of text in the screen of the app. Look at the value of the variable app_name and try to find out the place at which it is used. Comment below if you are able to find it!
Now, do you see why a variable named niktechs is there in the list? How it was created? Do you remember, while editing the text on the screen (Refer to the post Laying out the screen) , we made a new string variable? Yes, that variable is the same as displayed here. The value given to the field R.string there, is the name of the variable and the value is the same as given that time. Thus, when we changed the text displayed at the screen, we actually created a new string variable and gave its value to the TextView.
As mentioned earlier, we can create a variable by using this editor. Simply click on Add. A dialog box opens up. Don’t be afraid to see so many options. We will look at them as we proceed. For now, simply select String. In the resulting window, simply set the name (let’s set it to Blog) and the value (I am setting it to Android Development) as required and save the file. This creates a new String type variable.
We can modify a string by simply changing its value and saving the file. To delete a variable, simply select Remove.
Thus, we have seen how to play with strings in an Android Application. What is left to discuss is how to use these strings in the app. How these variables relate to the text displayed on the screen? How it answers the query raised at the beginning of the post?
All of this coming in the next post!
Till then, BYE!