Android Studio tutorial for beginners

quinta-feira, 5 de outubro de 2017

User Interaction

Unknown outubro 05, 2017 0 Comments

User Interaction

Now your list has function and style. What's it missing now? Try tapping or long pressing it. There's not much to thrill and delight the user.

What could you add here to make the user experience that much more satisfying? Well, when a user taps on a row, don't you think it'd be nice to show the full recipe, complete with instructions?

You’ll make use of AdapterView.onItemClickListener and a brand spanking new activity to do this with elegance.

Make a New Activity

This activity will display when the user selects an item in the list.

Right-click on java/com.raywenderlich.alltherecipes then select New > Activity > EmptyActivity to bring up a dialog. Fill in the Activity Name with RecipeDetailActivity. Leave the automatically populated fields as-is. Check that your settings match these:

Click Finish.

Open res/layout/activity_recipe_detail.xml and add a WebView by inserting the following snippet inside the RelativeLayout tag:

<WebView
      android:id="@+id/detail_web_view"
      android:layout_width="match_parent"
      android:layout_height="match_parent" />

WebView will be used to load and display a webpage containing the selected recipe's instructions.

Open up RecipeDetailActivity.java, and add a WebView reference as an instance variable by adding the following line within the class definition:

private WebView mWebView;

Head back to MainActivity.java and add the following to the bottom of the onCreate method:

final Context context = this;
mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

  @Override
  public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
    // 1
    Recipe selectedRecipe = recipeList.get(position);

    // 2
    Intent detailIntent = new Intent(context, RecipeDetailActivity.class);

    // 3
    detailIntent.putExtra("title", selectedRecipe.title);
    detailIntent.putExtra("url", selectedRecipe.instructionUrl);

    // 4
    startActivity(detailIntent);
  }

});

Note: Before you dive into the explanation, make sure you understand the four arguments that are provided by onItemClick; they work as follows:

parent: The view where the selection happens -- in your case, it's the ListView
view: The selected view (row) within the ListView
position: The position of the row in the adapter
id: The row id of the selected item

The first thing you do in the last snippet is set the OnItemClickListener object for the ListView. The OnItemClickListener class has a method named onItemClick that you override -- more on that in a moment.

At @Override, you override the existing onItemClick method and implement the following steps:

Get the recipe object for the row that was clicked
Create an intent to navigate to your RecipeDetailActivity to display more information
Let the RecipeDetailActivity know the title and URL of the recipe to display by passing that data via the Intent.
Launch the RecipeDetailActivity by passing the intent object you just created to the startActivity()method.

Note: To learn more about intents, check out the awesome Android: Intents Tutorial.

Once again, open RecipeDetailActivity.java and add the following snippet at the bottom of the onCreatemethod:

// 1
String title = this.getIntent().getExtras().getString("title");
String url = this.getIntent().getExtras().getString("url");

// 2
setTitle(title);

// 3
mWebView = (WebView) findViewById(R.id.detail_web_view);

// 4
mWebView.loadUrl(url);

You can see a few things happening here:

You retrieve the recipe data from the Intent passed by MainActivity by using the getExtras() method.
You set the title on the action bar of this activity to the recipe title.
You initialize mWebView to the web view defined in the XML layout.
You load the recipe web page by calling loadUrl() with the corresponding recipe's URL on the web view object.

Build and run. When you click on the first item in the list, you should see something like this:

Optimizing Performance

Whenever you scroll the ListView, its adapter's getView() method is called in order to create a row and display it on screen.

Now, if you look in your getView() method, you'll notice that each time this method is called, it performs a lookup for each of the row view's elements by using a call to the findViewById() method.

These repeated calls can seriously harm the ListView's performance, especially if your app is running on limited resources and/or you have a very large list. You can avoid this problem by using the View Holder Pattern.

Implement a ViewHolder Pattern

To implement the ViewHolder pattern, open RecipeAdapter.java and add the following after the getView()method definition:

private static class ViewHolder {
  public TextView titleTextView;
  public TextView subtitleTextView;
  public TextView detailTextView;
  public ImageView thumbnailImageView;
}

As you can see, you create a class to hold your exact set of component views for each row view. The ViewHolder object stores each of the row's subviews, and in turn is stored inside the tag field of the layout.

This means you can immediately access the row's subviews without the need to look them up repeatedly.

Now, in getView(), replace everything above (but NOT including) this line:

Recipe recipe = (Recipe) getItem(position);

With:

ViewHolder holder;

// 1
if(convertView == null) {

  // 2
  convertView = mInflater.inflate(R.layout.list_item_recipe, parent, false);

  // 3
  holder = new ViewHolder();
  holder.thumbnailImageView = (ImageView) convertView.findViewById(R.id.recipe_list_thumbnail);
  holder.titleTextView = (TextView) convertView.findViewById(R.id.recipe_list_title);
  holder.subtitleTextView = (TextView) convertView.findViewById(R.id.recipe_list_subtitle);
  holder.detailTextView = (TextView) convertView.findViewById(R.id.recipe_list_detail);

  // 4
  convertView.setTag(holder);
}
else{
  // 5
  holder = (ViewHolder) convertView.getTag();
}

// 6
TextView titleTextView = holder.titleTextView;
TextView subtitleTextView = holder.subtitleTextView;
TextView detailTextView = holder.detailTextView;
ImageView thumbnailImageView = holder.thumbnailImageView;

Here's the play-by-play of what's happening above.

Check if the view already exists. If it does, there's no need to inflate from the layout and call findViewById() again.
If the view doesn't exist, you inflate the custom row layout from your XML.
Create a new ViewHolder with subviews initialized by using findViewById().
Hang onto this holder for future recycling by using setTag() to set the tag property of the view that the holder belongs to.
Skip all the expensive inflation steps and just get the holder you already made.
Get relevant subviews of the row view.

Finally, update the return statement of getView() with the line below.

return convertView;

Build and run. If your app was running a bit slow on the last build, you should see it running smoother now. :]

Building Adapters

Unknown outubro 05, 2017 0 Comments

Building Adapters

Okay, now that you've dabbled in theory, you can get on with building your very own adapter.

Create a new class by Right-clicking on java/com.raywenderlich.alltherecipes and selecting New > Java Class. Name it RecipeAdapter and define it with the following:

public class RecipeAdapter extends BaseAdapter {
}

You've made the skeleton of the adapter. It extends the BaseAdapter class, which requires several inherited methods you'll implement after taking care of one more detail.

Add the following snippet inside the RecipeAdapter class:

private Context mContext;
private LayoutInflater mInflater;
private ArrayList<Recipe> mDataSource;

public RecipeAdapter(Context context, ArrayList<Recipe> items) {
  mContext = context;
  mDataSource = items;
  mInflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}

In here, you've added the instance variables that will be associated with the adapter and defined a constructor for RecipeAdapter.

Your next step is to implement the adapter methods. Kick it off by placing the following code at the bottom of RecipeAdapter.

//1
@Override
public int getCount() {
  return mDataSource.size();
}

//2
@Override
public Object getItem(int position) {
  return mDataSource.get(position);
}

//3
@Override
public long getItemId(int position) {
  return position;
}

//4
@Override
public View getView(int position, View convertView, ViewGroup parent) {
  // Get view for row item
  View rowView = mInflater.inflate(R.layout.list_item_recipe, parent, false);

  return rowView;
}

Here's a step-by-step breakdown:

getCount() lets ListView know how many items to display, or in other words, it returns the size of your data source.
getItem() returns an item to be placed in a given position from the data source, specifically, Recipe objects obtained from mDataSource.
This implements the getItemId() method that defines a unique ID for each row in the list. For simplicity, you just use the position of the item as its ID.
Finally, getView() creates a view to be used as a row in the list. Here you define what information shows and where it sits within the ListView. You also inflate a custom view from the XML layout defined in res/layout/list_item_recipe.xml -- more on this in the next section.

Defining the Layout of the ListView's Rows

You probably noticed that the starter project comes with the file res/layout/list_item_recipe.xml that describes how each row in the ListView should look and be laid out.

Below is an image that shows the layout of the row view and its elements:

Your task is to populate each element of the row view with the relevant recipe data, hence, you’ll define what text goes in the "title" element, the "subtitle" element and so on.

In the getView() method, add the following code snippet just before the return statement:

// Get title element
TextView titleTextView = 
  (TextView) rowView.findViewById(com.raywenderlich.alltherecipes.R.id.recipe_list_title);

// Get subtitle element
TextView subtitleTextView = 
  (TextView) rowView.findViewById(com.raywenderlich.alltherecipes.R.id.recipe_list_subtitle);

// Get detail element
TextView detailTextView = 
  (TextView) rowView.findViewById(com.raywenderlich.alltherecipes.R.id.recipe_list_detail);

// Get thumbnail element
ImageView thumbnailImageView = 
  (ImageView) rowView.findViewById(com.raywenderlich.alltherecipes.R.id.recipe_list_thumbnail);

This obtains references to each of the elements (or subviews) of the row view, specifically the title, subtitle, detail and thumbnail.

Now that you've got the references sorted out, you need to populate each element with relevant data. To do this, add the following code snippet under the previous one but before the return statement:

// 1
Recipe recipe = (Recipe) getItem(position);

// 2
titleTextView.setText(recipe.title);
subtitleTextView.setText(recipe.description);
detailTextView.setText(recipe.label);

// 3
Picasso.with(mContext).load(recipe.imageUrl).placeholder(R.mipmap.ic_launcher).into(thumbnailImageView);

Here's what you're doing in the above snippet:

Getting the corresponding recipe for the current row.
Updating the row view's text views so they are displaying the recipe.
Making use of the open-source Picasso library for asynchronous image loading -- it helps you download the thumbnail images on a separate thread instead of the main thread. You're also assigning a temporary placeholder for the ImageView to handle slow loading of images.

Note: You should never perform long-running tasks on the main thread. When you do, you expose yourself to the risk of blocking the UI, and that would make scrolling your lists a nightmare!

Now open up MainActivity.java so that you can get rid of the old adapter. In onCreate, replace everything below (but not including) this line:

final ArrayList<Recipe> recipeList = Recipe.getRecipesFromFile("recipes.json", this);

With:

RecipeAdapter adapter = new RecipeAdapter(this, recipeList);
mListView.setAdapter(adapter);

You just replaced the rather simple ArrayAdapter with your own RecipeAdapter to make the list more informative.

Build and run and you should see something like this:

Now you're cooking for real! Look at those recipes -- thumbnails and descriptions sure make a big difference.

Styling

Now that you've got function under wraps, it's time to turn your attention to the finer things in life. In this case, your finer things are elements that make your app more snazzy, such as compelling colors and fancy fonts.

Start with the fonts. Look for some custom fonts under assets/fonts. You’ll find three font files: JosefinSans-Bold.ttf, JosefinSans-SemiBoldItalic.ttf and Quicksand-Bold.otf.

Open RecipeAdapter.java and go to the getView() method. Just before the return statement, add the following:

Typeface titleTypeFace = Typeface.createFromAsset(mContext.getAssets(), "fonts/JosefinSans-Bold.ttf");
titleTextView.setTypeface(titleTypeFace);

Typeface subtitleTypeFace = 
  Typeface.createFromAsset(mContext.getAssets(), "fonts/JosefinSans-SemiBoldItalic.ttf");
subtitleTextView.setTypeface(subtitleTypeFace);

Typeface detailTypeFace = Typeface.createFromAsset(mContext.getAssets(), "fonts/Quicksand-Bold.otf");
detailTextView.setTypeface(detailTypeFace);

In here, you're assigning a custom font to each of the text views in your rows' layout. You access the font by creating a Typeface, which specifies the intrinsic style and typeface of the font, by using createFromAsset(). Next you call setTypeface() for the corresponding TextView to set the custom font.

Now build and run. Your result should look like this:

On to sprucing up the colors, which are defined in res/values/colors.xml. Open up RecipeAdapter.java and add the following below the instance variable declarations and above the constructor:

private static final HashMap<String, Integer> LABEL_COLORS = new HashMap<String, Integer>() {{
  put("Low-Carb", R.color.colorLowCarb);
  put("Low-Fat", R.color.colorLowFat);
  put("Low-Sodium", R.color.colorLowSodium);
  put("Medium-Carb", R.color.colorMediumCarb);
  put("Vegetarian", R.color.colorVegetarian);
  put("Balanced", R.color.colorBalanced);
}};

You've created a hash map that pairs a recipe detail label with the resource id of a color defined in colors.xml.

Now go to the getView() method, and add this line just above the return statement:

detailTextView.setTextColor(ContextCompat.getColor(mContext, LABEL_COLORS.get(recipe.label)));

Working from the inside out:

Here you get the resource id for the color that corresponds to the recipe.label from the LABEL_COLORShash map.
getColor() is used inside of ContextCompat to retrieve the hex color associated with that resource id.
Then you set the color property of the detailTextView to the hex color.

Build and run. Your app should look like this:

Android ListView Tutorial

Unknown outubro 05, 2017 0 Comments

Android ListView Tutorial

How many times have you needed an app to display a group of related items in a list? How about all the time. :]

Displaying a specific list is essential to the function of almost any app that queries a set of data and returns a list of results, so many apps need to do this at one point or another. For instance, maybe you have a chat app that queries a certain social platform’s database to find your friends, and then want to display them in a list that lets you select which friends to connect with.

Any time you need to display a lot of data and make it easy to navigate, you’ve got a job for Android’s ListView, which handily creates scrollable lists.

By working through this tutorial, you’ll become familiar with a ListView, and you’ll do so by creating a recipe list app. Specifically, you’ll learn:

How to construct and populate a ListView
How to customize the layout
How to style and beautify a ListView
How to optimize a ListView’s performance

You’re welcome to up your game in the kitchen by learning the recipes too, but maybe wait until you’ve built the app, okay?

Note: If you’re new to Android Development, it’s highly recommended that you start with the Android Tutorial for Beginners to learn your way around the basic tools and concepts.

Getting Started

Download the starter project and open Android Studio.

In the Welcome to Android Studio dialog, select Import project (Eclipse ADT, Gradle, etc.).

In the following dialog, select the top-level directory of the starter project AllTheRecipes-Starter and click OK.

Inside the imported project, you’ll find some assets and resources that you’ll use to create your app, such as strings, colors, XML layout files, and fonts. Additionally, there’s some boilerplate code modeling a Recipe object and a bare bones MainActivity class.

Build and run. You should see something like this:

Are you ready to get cracking on this list thing? Awesome!

Add Your First ListView

The first order of business is to add a ListView to MainActivity.

Open res/layout/activity_main.xml. As you may know, this is the file that describes the layout of MainActivity. Add a ListView to MainActivity by inserting the following code snippet inside the RelativeLayout tag:

  <ListView
      android:id="@+id/recipe_list_view"
      android:layout_height="match_parent"
      android:layout_width="match_parent">
  </ListView>

Open MainActivity.java and add an instance variable for your ListView with the following line:

private ListView mListView;

Add the following snippet below the existing code inside the onCreate method:

mListView = (ListView) findViewById(R.id.recipe_list_view);
// 1
final ArrayList<Recipe> recipeList = Recipe.getRecipesFromFile("recipes.json", this);
// 2
String[] listItems = new String[recipeList.size()];
// 3
for(int i = 0; i < recipeList.size(); i++){
  Recipe recipe = recipeList.get(i);
  listItems[i] = recipe.title;
}
// 4
ArrayAdapter adapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, listItems);
mListView.setAdapter(adapter);

Here's a breakdown of what's happening in there:

This loads a list of Recipe objects from a JSON asset in the app. Notice that the starter project contains a Recipe class that models and stores the information about the recipes that will be displayed.
This creates an array of strings that'll contain the text to be displayed in the List View.
This populates the ListView's data source with the titles of the recipes loaded in section one.
This creates and sets a simple adapter for the ListView. The ArrayAdapter takes in the current context, a layout file specifying what each row in the list should look like, and the data that will populate the list as arguments.

Enough talk! Your ListView has all that it needs to function. Build and run the project. You should see something like this:

Adapters: Servants of the ListView

Your recipe app is starting to look functional, but not all that appetizing...yet.

In the previous section, you successfully built a list of recipe titles. It works, but it's nothing to get excited about. What if you needed to show more than just the titles? More than just text? Maybe even add some screen-licking worthy thumbnails?

For these cases, the simple ArrayAdapter you just used won't cut it. You'll have to take matters into your own hands and write your own adapter. Well, you won't actually write your own adapter, per se; you'll simply extend a regular adapter and make some tweaks.

What Exactly is an Adapter?

An adapter loads the information to be displayed from a data source, such as an array or database query and creates a view for each item. Then it inserts the views into the ListView.

Adapters not only exist for ListViews, but for other kinds of views as well; ListView is a subclass of AdapterView, so you can populate it by binding it to an adapter.

The adapter acts as the middle man between the ListView and data source, or its provider. It works kind of like this:

The ListView asks the adapter what it should display, and the adapter jumps into action:

It fetches the items to be displayed from the data source

It decides how they should be displayed

It passes this information on to the ListView

In short, The ListView isn't very smart, but when given the right i

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quinta-feira, 5 de outubro de 2017

User Interaction

User Interaction

Make a New Activity

Optimizing Performance

Implement a ViewHolder Pattern

Building Adapters

Building Adapters

Defining the Layout of the ListView's Rows

Styling

Android ListView Tutorial

Android ListView Tutorial

Getting Started

Add Your First ListView

Adapters: Servants of the ListView

What Exactly is an Adapter?

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