First Android App – Part 5

My First Android App

 

Android is based on Java much like iOS is based on ObjectiveC.  If you are coming from an iOS background, itll be a bit jarring at first.  Even though ObjC “comes from” C, C formats are a bit different.  So I thought Id start with that first.

ObjC:

[myObject methodForDoingThis:someParameter];

is a method call which refers to this declared method:

-(void)methodForDoingThis: (id)someParameter{

//1. Take the passed in parameter

//2. Do something to with that parameter value

//3. Call some other method…

//4. Or if this were a method that returned an object instead of void

//4. Return a new object value

}

C:

myObject.methodForDoingThis(someParameter);

is a method call which refers to this declared method:

public methodForDoingThis void (id someParameter) {

//1.-4. Same as above

}

So this might throw you off a bit.  Add to that the fact that their IDE, be it Eclipse or AndroidStudio (or others), or rather their project files are a little less UI-friendly.  What I mean is that where in Xcode you have editable code files in Objective C and either a storyboard or NIBs to deal with UI stuff, in Android IDEs you have an AndroidManifest.xml, various layout.xml files and AndroidActivity.java files as well as a few other ones.

Ok, let’s get started with this Hello World App.

First Android App

First let’s create a New Project in Android Studio:

Here you can give it an Application Name in natural language, this means with spaces or what not.  This is the human readable name.  Then give it a Module Name which is recommended to not have spaces or strange characters other than alphanumerical characters.  Finally create a package name which is in reverse domain notation (com.something.app).  That something can you be company name or just your name.

In the above example we are actually creating an Android Glass project for Google Glass.  The idea is the same, but for this Android App, select API 15 as all three, Minimum, Target && Compile.  Leave the Theme as is and click on Next.

You will be asked about launch icons and just click Next.

You will get another screen about Activities, just leave Blank Activity.

Finally you might get this window:

Here make sure to make that one change, just to make things easier.  We must also remove the MainActivity.java fragment method later.

Once this is done, remember to go ahead to the MainActivity.java class and open it.  We are going to remove one method in here:

public static class PlaceholderFragment extends Fragment {

public PlaceholderFragment() {

}

@Override

public View onCreateView(LayoutInflater inflater, ViewGroup container,

Bundle savedInstanceState) {

View rootView = inflater.inflate(R.layout.fragment_main, container, false);

return rootView;

}

}

Finally you should see a screen much like this:

Let’s review these 10 top pointers:

1. Your Java Classes or Android Activities

2. Layout files in xml format

3. Value files for storing global settings of sorts

4. AndroidManifest is a sort of Central Registry

5. Top level build.gradle file

6. Low level build.gradle file

7. Tab bar for displayed files

8. Sync Gradle file button

9. Run button

10.Green = A-Ok button!

You can run the app now, it won’t do much, but its an app J.

1. MainActivity is selected and displayed in the editor window.  This contains your default, boilerplate/template created MainActivity Class required for any app.

1. The layout folder contains your activity_main.xml file.  If you look inside that file you will see an xml file with some parameters and a TextView element.  You will also see a graphical representation of it off to the far right.

1. The values folder contains more xml files.  Most importantly, the strings.xml which contains global references to string values.  These are used throughout the app to assign string values where needed.

1. The AndroidManifest.xml file contains some general settings elements for your app such as the package name, launcher icon, activity tags which contain your declared activities and in this case, if the activity has an intent, which is like an action, it must also be specified here.  In our case we will add a Voice Trigger intent filter if there isn’t one already.

1. Build.gradle files are not to be messed around with much.  This is a new M.O. used by Android Studio to organize files in a workspace.  You must keep in mind here that there is a Top level and Low level build.gradle file so make sure you know which one you are being told to put things in.

1. The tabs display whatever files you have double clicked on the File Window on the left.

1. The Sync Gradle button is that green-blue circle with a solid circle inside and a green arrow pointing down.  Go Figure!  Its basically a sort of I’ve-made-some-changes-to-the-AndroidManifest-and/or-other-project-wide-parameters-which-require-project-workspace-reindexing (phew) button!

1. The Run button is of course the one used to build and launch the app in the emulator or device.

1. The Green=AOk button tells you all file and project inconsistencies have been resolved and that the project will build and run.  Sometimes you have resolved coding issues and this box is still red.  Just tap on the Sync Gradle button mentioned before and Just Like That, Its Magic…AOk!

Layout Files

These determine where and how objects will be laid out onscreen.  Check out activity_main.xml and make sure it has this:

<?xml version=”1.0″ encoding=”utf-8″?>

<LinearLayout xmlns:android=”http://schemas.android.com/apk/res/android”

xmlns:tools=”http://schemas.android.com/tools”

android:layout_width=”match_parent”

android:layout_height=”match_parent”

android:orientation=”horizontal” >

</LinearLayout>

This simply tells us that we are using a Linear type layout.  And that the width and height should be stretched out to match whatever the parent is.

Now let’s actually add something onscreen by adding this code at the end of the last line but just before the </LinearLayout> closing tag:

<EditText android:id=”@+id/edit_message”

android:layout_width=”wrap_content”

android:layout_height=”wrap_content”

android:hint=”@string/edit_message” />

Ok, now we added an editable text field with id = edit_message which wraps its content and has a hint of placeholder text = whatever is in the edit_message string.

So what IS in the edit_message string?  Well, open your values folder and double click to open strings.xml.  You will see something like this:

<?xml version=”1.0″ encoding=”utf-8″?>

<resources>

<string name=”app_name”>My First App</string>

<string name=”edit_message”>Enter a message</string>

<string name=”button_send”>Send</string>

<string name=”action_settings”>Settings</string>

<string name=”title_activity_main”>MainActivity</string>

</resources>

As you can see here we have a string value for app_name, edit_message, button_send & action_settings.  You probably don’t have button_send but you will add it soon if you don’t.  This is a globally accessible xml values file.  Whenever some object looks for a string with an identifier, such as @string/xxx, it will fetch whatever is in that strings.xml file at that identifier location.

Nothing Magical here, just an app_name global identifier, a placeholder hint for the editable text field we added, one for a button and another for an action_settings action tool bar button.

So let’s add the button by going back to activity_main.xml and adding this code after the EditText block but again, before the closing </LinearLayout> tag:

<Button

        android:layout_width=”wrap_content”

        android:layout_height=”wrap_content”

        android:text=”@string/button_send” />

This should be pretty easy to understand.  This time we are adding a button with its text property set to the string value for button_send.  If you don’t have that button_send string in your strings.xml then add it in now.

Compile and run and you should see this:

As you can see they are ONLY as large as they have to be.  Both the EditText and the Button objects are only as large as their content is.

The only problem is that the text in the EditText field is, well, editable, thus it could grow longer or shrink.  We must make a small adjustment to account for this by modifying some properties of the EditText object:

<EditText

android:layout_weight=”1″

android:layout_width=”0dp”

… />

This means only add or edit the fields shown here (the rest, …, stays as is).  The layout_weight property states how much space the EditText field should take up relative to those objects around it.  So basically if EditText is 1 and Button is 1, the total is 2, out of which each will take half.

The final layout should look like this:

<?xml version=”1.0″ encoding=”utf-8″?>

<LinearLayout xmlns:android=”http://schemas.android.com/apk/res/android”

xmlns:tools=”http://schemas.android.com/tools”

android:layout_width=”match_parent”

android:layout_height=”match_parent”

android:orientation=”horizontal”>

<EditText android:id=”@+id/edit_message”

android:layout_weight=”1″

android:layout_width=”0dp”

android:layout_height=”wrap_content”

android:hint=”@string/edit_message” />

<Button

android:layout_width=”wrap_content”

android:layout_height=”wrap_content”

android:text=”@string/button_send” />

</LinearLayout>

Build & Run and see your app in all its splendor! Remember to check out Part 2 for a little user interaction before we move onto building Google Glass apps!