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Transfer of data using Intents (Part 2)

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Hi everyone!

In spite of trying hard, I couldn’t prevent the delay. I am again sorry for that. Let’s move on. In the last post, I introduced the concept of transfer of data between activities. I also described the code for declaring an Intent which could help us in accomplishing the task.

Now, it’s time to look at the code of SecondActivity.java, the second activity which will help us in adding new tasks to the list. As mentioned earlier, this activity will have an EditText to allow the user to input the task name and a Button, which when clicked, will take the user back to HelloWorldActivity.java and add the task to the List. The code for the click listener for this button looks as follows:

  1. String taskName = taskEdit.getText().toString();
  2. Intent intent = this.getIntent();
  3. intent.putExtra(“task”, taskName);
  4. setResult(RESULT_OK, intent);
  5. finish();

Here, taskEdit is an object of class EditText. The first line extracts the data input to the taskEdit, converts it into string and stores it in a variable. Second line is used to grab access to the intent which called this activity. The third line is the one which actually does the job of putting the data onto the intent. intent.putExtra function used in this line basically adds the information contained in the second parameter to the intent and the first parameter provides a way to access it. We will see the use of the first parameter in a greater detail later, when we will try to access this information in HelloWorldActivity.java. I hope that the fourth and fifth lines will be pretty easy to understand. If not, please refer to the last three posts on Intents.

The above code ensures that the clicking of the button takes us back to the initial activity with an intent which contains the name of the new task that is to be added to the list.

Clearly, the callback function described in Part 1 of this post will be used to access the information carried by the intent since this function will be automatically called when the control is given back to this activity via an intent. Straight away, let’s look at the code!

String extraData=data.getStringExtra(“task”);
taskText.append(extraData+”\n”);

I think it is self-explanatory. We are extracting the information from the variable data using the value of the first parameter of the function in Line 4 above, and saving it in a variable called extraData. The second line just appends this value to the list (referred by taskText).

In this way, we received the name of the task from a different activity and display it in our main activity. This provides a clean and user-friendly interface which is the basis of a useful app.

But here, we have not taken care of the situation when the user calls the intent to SecondActivity.java but wants to cancel it later. This is not perfect programming, though it can be dealt very easily. How?

In the next post, we will finish our discussion on intent and move on to explore some new concepts in Android App Development.

Till then, BYE!

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Intents-An Example(Part 2)

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Hi all!

Finally, I got some time to continue with the example. I am really very sorry for the long delay. From now on, I will be posting every two to three days. So, keep looking!

For a flashback,  we discussed the use of intents in achieving communication among activities. We also saw the basic code of the two activities which will communicate with each other in the last post. Without wasting more time, let’s discuss the final codes for both of these activities.

Remember! I told you that we will be playing with the onClickListeners of both the buttons in the respective activities to accomplish the task.

Let’s start with the code for SecondActivity.java. Remove the code for showing the toast and replace it with a function called finish(). Yes! That’s it.

On clicking the button on the second activity, we just need to finish this activity and move back to HelloWorldActivity.java. Since, we will be calling this activity only from HelloWorldActivity.java (How to call from others? We’ll see!), finishing this activity will automatically take us where we want. Hence, the code for SecondActivity.java is over.

How about HelloWorldActivity.java? It’s the place where we will be actually calling the intent.

So, replace the code for the toast in the onClickListener with the following lines of code:

Intent intent = new Intent(HelloWorldActivity.this, SecondActivity.class);
startActivity(intent);

I hope that you are able to understand it clearly. We are basically making an object of class Intent passing to it two parameters. The first parameter is a reference to the current activity. this is a keyword which always refers to the current object. Hence, HelloWorldActivity.this simply specifies the activity from which the intent is being called. The second parameter is the one which specifies the activity to which we want to move. Remember, we specify it as .class. Hence, we have specified the second parameter as SecondActivity.class. But, the first line declares an Intent object called intent. To actually accomplish the task,  we need to call a function called startActivity with the parameter as the Intent object, which transfers the control to the activity specified while declaring the Intent object.

So, just run the above application and check out what it does! You will see that you will be able to go back and forth every time you click the button on the screen.

But, there is no practical use of this communication. Intents become more meaningful when we need to transfer some data from one activity to another.

In the next post, we will be looking at an application which involves some useful transfer of data.

Till then, BYE!

 

Intents – An Example (Part 1)

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Hi everyone!

Sorry for the delay in the post this time! Let’s go ahead!

In the last post, I gave an overview of something called Intents in Android. If you have not gone through it, please refer to Communication between Activities using Intents! first and then, come back here!

This time, I am going to discuss the simplest app which can demonstrate the use of Intents. Even though, the app in itself is of no use but, concepts discussed during its development will help us in our future endeavors. I will be building on our previous HelloWorld App which (till now) simply shows a Toast when we touch a  button on the screen.

First of all, we will be needing two activities in our project which will be communicating with one another. So, simply go to the project folder in the HelloWorldApp that we have been developing upon, right click, select New and the, click on Class. Fill the fields as per the following figure and select Finish.

A new activity called SecondActivity.java has been created. But, you will see that there is no onCreate function. Just copy the code from below. It will be exactly the same as the one automatically generated when you create a new project and hence, you don’t need to remember it.

public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.second);
 }

Note the change in the value of the parameter of setContentView. Yes, it refer to a new layout file called second.xml. Simply make a new file called second.xml in the layout folder and copy the contents from main.xml to this file as we want the similar layout for this activity too. To create the new xml file, simply right click on the layout folder and select New->File. Name the file as second.xml and select Finish. Remember to give a different ID to the button of second.xml to avoid conflicts. Just change the value of android:id attribute of Button tag to something that was not used for main.xml. Let me change it to button2. Also, change the text of this button so that we could distinguish between the two activites.

Now, we need to play with the onClick Listeners of the two buttons (present in the two activities) to start the communication. In this app, we simply want to switch between the activites when the button is pressed. When we click the button on HelloWorldActivity.java (Default Activity of our app), we want to switch to SecondActivity.java and when we click the button on SecondActivity.java, we want to return to HelloWorldActivity.java.

Try to search and write the code for the same yourself based on our discussion in the previous post. In the next post, I will discuss it and some more concepts in the Android app development.

Till then, BYE!

Let’s Code!

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Hi all!

So, I am back again. This time, we will delve into the coding arena and try to understand the code that is automatically generated when you create a new project (Remember! when we had run the project in the post titled Let’s Begin! , an app was launched in the emulator in spite of the fact that we had not written any code ourselves).

How did that happen? We have already seen that the layout which is automatically created when we make a new project is exactly same to the screen of that app. What about the rest of the its functioning? Who tells Android to set the screen to that particular layout? Let’s see!

If you open the project, click on the package name (in our case, it was com.hello.world) and open the source file (which is HelloWorldActivity.java in our HelloWorld project discussed in previous posts), you will find a few lines of code. The code which does the task mentioned above. But, the question is How?

The first line of code is simply to tell the package name to which this class belongs to(Class? For now, it is sufficient to understand that it is the most basic component of a JAVA code, which encapsulates other functions and variables . We will look at them in a greater depth in the coming posts). It’s the same as selected while making the project. The second line of code simply imports the android classes that are already present in the Android framework and provide us the functions required in the code.

Now, you can see we have declared a class with an access modifier called public which poses no restriction on the access (For those who are unfamiliar with it, we will come to that later). The name of the class is same as the name of the java file. Now, the code says that this class extends an Activity. So, what is an Activity? An activity is the top level component ( a pre-built class) which controls what you see on the screen. It may either cover the entire screen or a part of it, but only one activity runs at a time in Android. So, even if the activity covers just a part of the screen, the remaining part will be inactive till the time that activity is running.

So, the third line of code basically declares a class with no access restrictions, which extends an Activity (because of which we call this class an Activity as well!) so that it can interact with the screen, which means that this class can use all the functions and variables provided by the Activity class.

The rest of the code simply defines a function which is automatically called when this activity is first created. We don’t have to remember the syntax as this function is automatically generated. There are two things happening in this function. We don’t need to discuss the first one.

What about the line setContentView (R.layout.main)? Can you guess what this line of code does?

In the next post, we will discuss this line of code and also, dive deeper into the coding world of Android App Development.

Till then, BYE!