How to develop an android app
HTTP connection
JSON Parsing
Viewpager and fragment
I've released a new WeatherLib for Android that simplifies the app coding helping you in many app details. If you are interested give a look here
If you are interested there is another post explaing how to use Yahoo! Weather provider. In this first part it is expalined how to retrieve city id. Look here
This tutorial will describe how to develop an android application. In this tutorial we will explore what we need to create an android application and what are the bricks we can use to create our app. As example we will create a weather app. We talked about it some months ago, we will develop the weather app with weather current condition and weather forecast. You can give a look here to know how to retrieve current user position and the corresponding city name.To develop an android application we need at least:
- An activity
- a Layout
- HTTP connection
- AsyncTask (to not have ANR problems)
- JSON parsing
- Data model (that holds the JSON data)
App Layout
The first step is creating the layout. As you can see from the image below our layout is divided in two sections: one that holds the current weather condition and the other that holds the weather forecast. In the weather forecast we have to move among different days so we can use aViewPager. We can use in this case a LinearLayout:
Now we have our layout we can start working on each sections. In the first section (current weather) we can use a RelativeLayout to place every widget at the right place:
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".MainActivity"
android:layout_weight="1" >
<TextView
android:id="@+id/cityText"
style="?android:attr/textAppearanceMedium"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true">
</TextView>
<TextView
android:id="@+id/temp"
style="@style/tempStyle"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@id/cityText"
android:layout_centerHorizontal="true">
</TextView>
<TextView
android:id="@+id/unittemp"
android:layout_width="wrap_content"
style="?android:attr/textAppearanceMedium"
android:layout_height="wrap_content"
android:layout_below="@id/cityText"
android:layout_toRightOf="@id/temp"
android:layout_alignBaseline="@id/temp">
</TextView>
<TextView
android:id="@+id/skydesc"
android:layout_width="wrap_content"
style="?android:attr/textAppearanceMedium"
android:layout_height="wrap_content"
android:layout_below="@id/temp"
android:layout_alignStart="@id/temp"
android:layout_toRightOf="@id/temp">
</TextView>
<!-- Image weather condition -->
<ImageView android:id="@+id/condIcon"
android:layout_height="wrap_content"
android:layout_width="wrap_content"
android:layout_alignTop="@id/temp"
android:layout_toRightOf="@id/temp"/>
</RelativeLayout>
In the second section, as we said we have to show several day forecast. In this case we can use a ViewPager that helps us to swipe between pages, so we have:
<android.support.v4.view.ViewPagerThis is our layout. We have now to fill it we the weather data.
android:id="@+id/pager"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="6" >
<android.support.v4.view.PagerTitleStrip
android:id="@+id/pager_title_strip"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="top"
android:background="#E6E6E6"
android:paddingBottom="4dp"
android:paddingTop="4dp"
android:textColor="#fff" />
</android.support.v4.view.ViewPager>
HTTP, AsyncTask and json data parsing
The next step is retrieve data using HTTP connection with an asynctask to avoid a ANR problem. Once we have our data we will parse it using JSON parser. Let’s ask we our browser the weather forecast using this link:http://api.openweathermap.org/data/2.5/forecast/daily?q=Rome,IT&mode=json&units=metric&cnt=3
where cnt is the number of the days we want to have the weather condition. The result is shown below:
{
"cod": "200",
"message": 0.0192,
"city": {
"id": "3169070",
"name": "Rome",
"coord": {
"lon": 12.4958,
"lat": 41.903
},
"country": "Italy",
"population": 0
},
"cnt": 7,
"list": [
{
"dt": 1377774000,
"temp": {
"day": 26.83,
"min": 16.41,
"max": 29.12,
"night": 16.41,
"eve": 24.81,
"morn": 26.83
},
"pressure": 1007.2,
"humidity": 72,
"weather": [
{
"id": 800,
"main": "Clear",
"description": "sky is clear",
"icon": "01d"
}
],
"speed": 0.71,
"deg": 146,
"clouds": 0
},
{
"dt": 1377860400,
"temp": {
"day": 29.84,
"min": 16.8,
"max": 29.84,
"night": 16.8,
"eve": 24.81,
"morn": 17.93
},
"pressure": 1006.68,
"humidity": 50,
"weather": [
{
"id": 800,
"main": "Clear",
"description": "sky is clear",
"icon": "02d"
}
],
"speed": 2.2,
"deg": 235,
"clouds": 8
},
{
"dt": 1377946800,
"temp": {
"day": 29.14,
"min": 14.11,
"max": 29.14,
"night": 14.11,
"eve": 24.08,
"morn": 18.32
},
"pressure": 1005.17,
"humidity": 45,
"weather": [
{
"id": 801,
"main": "Clouds",
"description": "few clouds",
"icon": "02d"
}
],
"speed": 1.56,
"deg": 320,
"clouds": 20
},
{
"dt": 1378033200,
"temp": {
"day": 28.43,
"min": 12.11,
"max": 28.95,
"night": 14.28,
"eve": 23.89,
"morn": 12.11
},
"pressure": 1005.23,
"humidity": 39,
"weather": [
{
"id": 800,
"main": "Clear",
"description": "sky is clear",
"icon": "01d"
}
],
"speed": 1.55,
"deg": 289,
"clouds": 0
},
{
"dt": 1378119600,
"temp": {
"day": 29.83,
"min": 16.02,
"max": 29.83,
"night": 18.86,
"eve": 25.35,
"morn": 16.02
},
"pressure": 1006.57,
"humidity": 32,
"weather": [
{
"id": 800,
"main": "Clear",
"description": "sky is clear",
"icon": "01d"
}
],
"speed": 3.3,
"deg": 292,
"clouds": 0
},
{
"dt": 1378206000,
"temp": {
"day": 27.76,
"min": 19.68,
"max": 27.76,
"night": 19.68,
"eve": 25,
"morn": 20.25
},
"pressure": 1014.89,
"humidity": 0,
"weather": [
{
"id": 800,
"main": "Clear",
"description": "sky is clear",
"icon": "01d"
}
],
"speed": 4.05,
"deg": 298,
"clouds": 0
},
{
"dt": 1378292400,
"temp": {
"day": 27.85,
"min": 19.91,
"max": 27.85,
"night": 19.91,
"eve": 24.78,
"morn": 20.13
},
"pressure": 1017.2,
"humidity": 0,
"weather": [
{
"id": 800,
"main": "Clear",
"description": "sky is clear",
"icon": "01d"
}
],
"speed": 1.91,
"deg": 252,
"clouds": 0
}
]
}Analyzing the result we find out that the information we are looking for are inside the list tag and this an array. To hold this information we can create two classes on that hold the daily forecast and the other that hold the daily forecast classes:public class DayForecast {
private static SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
public Weather weather = new Weather();
public ForecastTemp forecastTemp = new ForecastTemp();
public long timestamp;
public class ForecastTemp {
public float day;
public float min;
public float max;
public float night;
public float eve;
public float morning;
}
public String getStringDate() {
return sdf.format(new Date(timestamp));
}
}and the other one:public class WeatherForecast {
private List<DayForecast> daysForecast = new ArrayList<DayForecast>();
public void addForecast(DayForecast forecast) {
daysForecast.add(forecast);
System.out.println("Add forecast ["+forecast+"]");
}
public DayForecast getForecast(int dayNum) {
return daysForecast.get(dayNum);
}
}Now we have our data model and we have simply parse the JSON data using a simple class like the one used in the previous post (JSONWeatherParser).public static WeatherForecast getForecastWeather(String data) throws JSONException {
WeatherForecast forecast = new WeatherForecast();
// We create out JSONObject from the data
JSONObject jObj = new JSONObject(data);
JSONArray jArr = jObj.getJSONArray("list"); // Here we have the forecast for every day
// We traverse all the array and parse the data
for (int i=0; i < jArr.length(); i++) {
JSONObject jDayForecast = jArr.getJSONObject(i);
// Now we have the json object so we can extract the data
DayForecast df = new DayForecast();
// We retrieve the timestamp (dt)
df.timestamp = jDayForecast.getLong("dt");
// Temp is an object
JSONObject jTempObj = jDayForecast.getJSONObject("temp");
df.forecastTemp.day = (float) jTempObj.getDouble("day");
df.forecastTemp.min = (float) jTempObj.getDouble("min");
df.forecastTemp.max = (float) jTempObj.getDouble("max");
df.forecastTemp.night = (float) jTempObj.getDouble("night");
df.forecastTemp.eve = (float) jTempObj.getDouble("eve");
df.forecastTemp.morning = (float) jTempObj.getDouble("morn");
// Pressure & Humidity
df.weather.currentCondition.setPressure((float) jDayForecast.getDouble("pressure"));
df.weather.currentCondition.setHumidity((float) jDayForecast.getDouble("humidity"));
// ...and now the weather
JSONArray jWeatherArr = jDayForecast.getJSONArray("weather");
JSONObject jWeatherObj = jWeatherArr.getJSONObject(0);
df.weather.currentCondition.setWeatherId(getInt("id", jWeatherObj));
df.weather.currentCondition.setDescr(getString("description", jWeatherObj));
df.weather.currentCondition.setCondition(getString("main", jWeatherObj));
df.weather.currentCondition.setIcon(getString("icon", jWeatherObj));
forecast.addForecast(df);
}
return forecast;
} Well done! What do we need more?…Well we have to handle the ViewPager and create the adapter to hold every day forecast.ViewPager, Fragments and the adapter
The first thing we have to create our adapter that handles each fragment that shows the weather daily details. In this case to create our adapter we have to extend FragmentPagerAdapter:public class DailyForecastPageAdapter extends FragmentPagerAdapter {
private int numDays;
private FragmentManager fm;
private WeatherForecast forecast;
private final static SimpleDateFormat sdf = new SimpleDateFormat("E, dd-MM");
public DailyForecastPageAdapter(int numDays, FragmentManager fm, WeatherForecast forecast) {
super(fm);
this.numDays = numDays;
this.fm = fm;
this.forecast = forecast;
}
// Page title
@Override
public CharSequence getPageTitle(int position) {
// We calculate the next days adding position to the current date
Date d = new Date();
Calendar gc = new GregorianCalendar();
gc.setTime(d);
gc.add(GregorianCalendar.DAY_OF_MONTH, position);
return sdf.format(gc.getTime());
}
@Override
public Fragment getItem(int num) {
DayForecast dayForecast = (DayForecast) forecast.getForecast(num);
DayForecastFragment f = new DayForecastFragment();
f.setForecast(dayForecast);
return f;
}
/*
* Number of the days we have the forecast
*/
@Override
public int getCount() {
return numDays;
}Two methods are important: one that “create” the fragment and another one that “create” the page title. The first one instantiate a fragment that shows the daily forecat passing the dayForecast data and the other one create the page tile using GregorianCalendar.The last step is coding the fragment:
public class DayForecastFragment extends Fragment {
...
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.dayforecast_fragment, container, false);
TextView tempView = (TextView) v.findViewById(R.id.tempForecast);
TextView descView = (TextView) v.findViewById(R.id.skydescForecast);
tempView.setText( (int) (dayForecast.forecastTemp.min - 275.15) + "-" + (int) (dayForecast.forecastTemp.max - 275.15) );
descView.setText(dayForecast.weather.currentCondition.getDescr());
iconWeather = (ImageView) v.findViewById(R.id.forCondIcon);
// Now we retrieve the weather icon
JSONIconWeatherTask task = new JSONIconWeatherTask();
task.execute(new String[]{dayForecast.weather.currentCondition.getIcon()});
return v;
}
...
}In the onCreateView we inflate our layout and retrieve with (AsyncTask) the weather icon.Source code available @github
0 comments:
Post a Comment