Threads!

• Én tråd i Android!
   
• Kræsjer den, kræsjer appen
   
• Trege operasjoner fryser UI

Unntak(shåndtering)

• Ting kan treffe vifta
   
• Én tråd: UI
   
• Én exception = crash
   
• Bad reputation
   
• (Kort demo)

Løsninger

ANR

• Input event: 5 sekunder
• BroadcastReceiver: 10 sek.
   
• Threads!
   
• AsyncTask

"Has stopped"

• Try
   
• Catch
   
• Finally

LOGG & si fra til bruker!

Lett å lage!

new Thread(new Runnable() {
 public void run() {
  // code to run in background
 }
}).start();


// inside other thread
Activity.runOnUiThread(new Runnable() {
 public void run() {
  // code to run on UI thread
 }
});

Utfordringer med threads

• Kommunikasjon på tvers
   
• Kun UI-tråden kan "røre" UI
   
• Dermed:
  1. Start oppgaven
  2. (loop) Rapporter progress til UI thread
  3. Rapporter resultat til UI thread

Android: AsyncTask

• Magic?
   
• Har et ThreadFactory
  • Som kaller
    new Thread(...)
     
• (Åpne AsyncTask)

new AsyncTask<Params, Progress, Result>() {
 @Override
 protected void onPreExecute() {
  // Stuff to do on UI thread before starting
 }

 @Override
 protected <Result> doInBackground(
   <Params>... params
 ) {
  // Perform a long task, like an HTTP request
  return <Result>;
 }

 @Override
 protected void onPostExecute(
   <Result> result
 ) {
  // Stuff to do on UI thread after finishing
 }
}

Nettverk (HTTP)

Veldig vanlig case for apps:

1. Start appen
2. Hent data fra server
3. Vis frem data
4. (La bruker endre noe)
5. (Send data tilbake til server)

HTTP er enkelt i Java!

HttpURLConnection connection = (HttpURLConnection)
    new URL("http://a.page.xyz/whatever").openConnection();

InputStream in = connection.getInputStream();

Scanner scanner = new Scanner(in);

String body = "";

while (scanner.hasNextLine()) {
 body += scanner.nextLine();
}

// body = data from server as String

java.net.*

(Quick demo)

Permissions…

<uses-permission
    android:name="android.permission.INTERNET"/>

• Bare permission til å BRUKE internett
  • ACCESS_NETWORK_STATE for sjekk
     
• Ikke lenger nødvendig i Marshmallow
  • … men husk å spørre om de andre

http://developer.android.com/training/basics/network-ops/connecting.html

http://inthecheesefactory.com/blog/things-you-need-to-know-about-android-m-permission-developer-edition/en​

… men må gjøres i egen Thread

public class JustGetSomethingActivity extends Activity {
    private TextView resultTextView;

    // onCreate sets ^^^, calls vvv

     private void downloadAndDisplayData() {
        new AsyncTask<Void, Void, String>() {
            @Override
            protected String doInBackground(final Void... params) {
                try {
                    HttpURLConnection connection =
                       (HttpURLConnection) new URL("https://google.com").openConnection();
                    return inputStreamToString(connection.getInputStream()); // made up method
                } catch (IOException e) {
                    throw new RuntimeException(e); // I'm lazy
                }
            }

            @Override
            protected void onPostExecute(final String result) {
                super.onPostExecute(result);
                resultTextView.setText(result); // update the TextView
            }
        }.execute();
    }
}

(les: AsyncTask)

✓ JSON for data transfer

// Arrays

[0, 1, 2]
["Hello", "world"]

// Objects
{
 "key": "value",
 "anotherKey", "anotherValue"
}

// Object with array
{
 "name": "Liam",
 "modules": [
  "Physics",
  "Maths"
 ]
}

// Array of objects
[
 { "name": "Liam" },
 { "name": "Martin" }
]

"Fun"facts:
 

• More lightweight than XML
   
• JSON is valid JavaScript
   
• Easy to parse for JavaScript
   
• No schemas

http://beers.theneva.com/beers

Gson

• Googles forsøk på JSON-lib
   
• Veldig enkelt å bruke

// build.gradle

dependencies {
 // ...
 compile 'com.google.code.gson:gson:2.6.2'
}

// Serialization
Gson gson = new Gson();
gson.toJson(1);            // ==> 1
gson.toJson("abcd");       // ==> "abcd"

// Deserialization
int one = gson.fromJson("1", int.class);
Integer one = gson.fromJson("1", Integer.class);
Long one = gson.fromJson("1", Long.class);
Boolean false = gson.fromJson("false", Boolean.class);

Gson: liste av objekter

• java.lang.reflect <3

// Deserialization
Type collectionType = new TypeToken<Collection<Integer>>(){}.getType();
Collection<Integer> ints2 = gson.fromJson(json, collectionType);