Difference between revisions of "Android Explicit Intents – A Kotlin Example"

Revision as of 19:05, 28 November 2017

The chapter entitled “An Overview of Android Intents” covered the theory of using intents to launch activities. This chapter will put that theory into practice through the creation of an example application.

The example Android Studio application project created in this chapter will demonstrate the use of an explicit intent to launch an activity, including the transfer of data between sending and receiving activities. The next chapter (“Android Implicit Intents – A Worked Example”) will demonstrate the use of implicit intents.

Creating the Explicit Intent Example Application

Launch Android Studio and create a new project, entering ExplicitIntent into the Application name field and ebookfrenzy.com as the Company Domain setting before clicking on the Next button.

On the form factors screen, enable the Phone and Tablet option and set the minimum SDK setting to API 14: Android 4.0 (IceCreamSandwich). Continue to proceed through the screens, requesting the creation of an Empty Activity named ActivityA with a corresponding layout named activity_a.

Click Finish to create the new project.

Designing the User Interface Layout for ActivityA

The user interface for ActivityA will consist of a ConstraintLayout view containing EditText (Plain Text), TextView and Button views named editText1, textView1 and button1 respectively. Using the Project tool window, locate the activity_a.xml resource file for ActivityA (located under app -> res -> layout) and double-click on it to load it into the Android Studio Layout Editor tool. Select and delete the default “Hello World!” TextView.

For this tutorial, Inference mode will be used to add constraints after the layout has been designed. Begin, therefore, by turning off the Autoconnect feature of the Layout Editor using the toolbar button indicated in Figure 50-1:





Drag a TextView widget from the palette and drop it so that it is centered within the layout and use the Attributes tool window to assign an ID of textView1.

Drag a Button object from the palette and position it so that it is centered horizontally and located beneath the bottom edge of the TextView. Change the text property so that it reads “Ask Question” and configure the onClick property to call a method named onClick().

Next, add an Plain Text object so that it is centered horizontally and positioned above the top edge of the TextView. Using the Attributes tool window, remove the “Name” string assigned to the text property and set the ID to editText1. With the layout completed, click on the toolbar Infer constraints button to add appropriate constraints:



Finally, click on the red warning button in the top right-hand corner of the Layout Editor window and use the resulting panel to extract the “Ask Question” string to a resource named ask_question.

Once the layout is complete, the user interface should resemble that illustrated in Figure 50-3:





Creating the Second Activity Class

When the “Ask Question” button is touched by the user, an intent will be issued requesting that a second activity be launched into which an answer can be entered by the user. The next step, therefore, is to create the second activity. Within the Project tool window, right-click on the com.ebookfrenzy.explicitintent package name located in app -> java and select the New -> Activity -> Empty Activity menu option to display the New Android Activity dialog as shown in Figure 50-4:





Enter ActivityB into the Activity Name and Title fields and name the layout file activity_b. Since this activity will not be started when the application is launched (it will instead be launched via an intent by ActivityA when the button is pressed), it is important to make sure that the Launcher Activity option is disabled before clicking on the Finish button.

Designing the User Interface Layout for ActivityB

The elements that are required for the user interface of the second activity are a Plain Text EditText, TextView and Button view. With these requirements in mind, load the activity_b.xml layout into the Layout Editor tool, turn off Autoconnect mode in the Layout Editor toolbar and add the views.

During the design process, note that the onClick property on the button view has been configured to call a method named onClick(), and the TextView and EditText views have been assigned IDs textView1 and editText1 respectively. Once completed, the layout should resemble that illustrated in Figure 50-5. Note that the text on the button (which reads “Answer Question”) has been extracted to a string resource named answer_question.

With the layout complete, click on the Infer constraints toolbar button to add the necessary constraints to the layout:





Reviewing the Application Manifest File

In order for ActivityA to be able to launch ActivityB using an intent, it is necessary that an entry for ActivityB be present in the AndroidManifest.xml file. Locate this file within the Project tool window (app -> manifests), double-click on it to load it into the editor and verify that Android Studio has automatically added an entry for the activity:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.ebookfrenzy.explicitintent">
 
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".ActivityA">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
 
                <category 
         android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".ActivityB"></activity>
    </application>
 
</manifest>

With the second activity created and listed in the manifest file, it is now time to write some code in the ActivityA class to issue the intent.

Creating the Intent

The objective for ActivityA is to create and start an intent when the user touches the “Ask Question” button. As part of the intent creation process, the question string entered by the user into the EditText view will be added to the intent object as a key-value pair. When the user interface layout was created for ActivityA, the button object was configured to call a method named onClick() when “clicked” by the user. This method now needs to be added to the ActivityA class ActivityA.kt source file as follows:

package com.ebookfrenzy.explicitintent
 
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.content.Intent
 
import kotlinx.android.synthetic.main.activity_a.*
 
class ActivityA : AppCompatActivity() {
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_a)
    }
 
    fun onClick(view: View) {
 
        val i = Intent(this, ActivityB::class.java)
 
        val myString = editText.text.toString()
        i.putExtra("qString", myString)
        startActivity(i)
    }
} 

The code for the onClick() method follows the techniques outlined in “An Overview of Android Intents”. First, a new Intent instance is created, passing through the current activity and the class name of ActivityB as arguments. Next, the text entered into the EditText object is added to the intent object as a key-value pair and the intent started via a call to startActivity(), passing through the intent object as an argument.

Compile and run the application and touch the “Ask Question” button to launch ActivityB and the back button (located in the toolbar along the bottom of the display) to return to ActivityA.

Extracting Intent Data

Now that ActivityB is being launched from ActivityA, the next step is to extract the String data value included in the intent and assign it to the TextView object in the ActivityB user interface. This involves adding some code to the onCreate() method of ActivityB in the ActivityB.kt source file:

package com.ebookfrenzy.explicitintent
 
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.content.Intent
 
import kotlinx.android.synthetic.main.activity_b.*
 
class ActivityB : AppCompatActivity() {
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_b)
        val extras = intent.extras ?: return
 
        val qString = extras.getString("qString")
        textView1.text = qString
    }
}

Compile and run the application either within an emulator or on a physical Android device. Enter a question into the text box in ActivityA before touching the “Ask Question” button. The question should now appear on the TextView component in the ActivityB user interface.

Launching ActivityB as a Sub-Activity

In order for ActivityB to be able to return data to ActivityA, ActivityB must be started as a sub-activity of ActivityA. This means that the call to startActivity() in the ActivityA onClick() method needs to be replaced with a call to startActivityForResult(). Unlike the startActivity() method, which takes only the intent object as an argument, startActivityForResult() requires that a request code also be passed through. The request code can be any number value and is used to identify which sub-activity is associated with which set of return data. For the purposes of this example, a request code of 5 will be used, giving us a modified ActivityA class that reads as follows:

package com.ebookfrenzy.explicitintent
 
import android.content.Intent
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.View
 
import kotlinx.android.synthetic.main.activity_a.*
 
class ActivityA : AppCompatActivity() {
 
    private val request_code = 5
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_a)
    }
 
    fun onClick(view: View) {
 
        val i = Intent(this, ActivityB::class.java)
 
        val myString = editText.text.toString()
        i.putExtra("qString", myString)
        startActivityForResult(i, request_code)
 
    }
}

When the sub-activity exits, the onActivityResult() method of the parent activity is called and passed as arguments the request code associated with the intent, a result code indicating the success or otherwise of the sub-activity and an intent object containing any data returned by the sub-activity. Remaining within the ActivityA class source file, implement this method as follows:

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent) {
    if ((requestCode == request_code) &&
            (resultCode == RESULT_OK)) {
 
        if (data.hasExtra("returnData")) {
                val returnString = data.extras.getString("returnData")
                textView1.text = returnString
        }
    }
}

The code in the above method begins by checking that the request code matches the one used when the intent was issued and ensuring that the activity was successful. The return data is then extracted from the intent and displayed on the TextView object.

Returning Data from a Sub-Activity

ActivityB is now launched as a sub-activity of ActivityA, which has, in turn, been modified to handle data returned from ActivityB. All that remains is to modify ActivityB.kt to implement the finish() method and to add code for the onClick() method, which is called when the “Answer Question” button is touched. The finish() method is triggered when an activity exits (for example when the user selects the back button on the device):

fun onClick(view: View) {
    finish()
}
 
override fun finish() {
    val data = Intent()
 
    val returnString = editText1.text.toString()
    data.putExtra("returnData", returnString)
 
    setResult(RESULT_OK, data)
    super.finish()
}

All that the finish() method needs to do is create a new intent, add the return data as a key-value pair and then call the setResult() method, passing through a result code and the intent object. The onClick() method simply calls the finish() method.

Testing the Application

Compile and run the application, enter a question into the text field on ActivityA and touch the “Ask Question” button. When ActivityB appears, enter the answer to the question and use either the back button or the “Submit Answer” button to return to ActivityA where the answer should appear in the text view object.

Summary

Having covered the basics of intents in the previous chapter, the goal of this chapter was to work through the creation of an application project in Android Studio designed to demonstrate the use of explicit intents together with the concepts of data transfer between a parent activity and sub-activity.

The next chapter will work through an example designed to demonstrate implicit intents in action.