I've been doing a bunch of research and looking over the documentation for ASyncTask in Android, but I just can't seem to wrap my head around it. I simply want to run some methods while a fragment is visible in an application, but in order to more easily do that, I think I should be using ASyncTask. My example code is as follows:
private class syncExample extends ASyncTask <Void, Void, Void>
{
#Override
protected void onPreExecute()
{
}
#Override
protected void doInBackground(Void... voids)
{
}
#Override
protected void onProgressUpdate()
{
}
#Override
protected void onPostExecute()
{
}
}
Now my questions are as follows:
In the angle brackets, I have Void, Void, Void. What exactly do those represent and how do I know what's correct to place in there?
For each method within the class, I have the each method called as void. When should they be different than void (like boolean, String, long, etc.)?
For the doInBackground method, I have Void... voids in the parenthesis. What exactly should I be putting in there? What do they represent?
Thank you for your help. The documentation on this is not very clear for a beginner like myself.
AsyncTask uses parameterized types (java generics) so that you can specify the types it uses when you define your own AsyncTask. Perhaps it's easier to explain in this form:
public abstract class AsyncTask<Params, Progress, Result> {
...
protected abstract Result doInBackground(Params... params);
protected abstract void onProgressUpdate(Progress... progress);
protected abstract void onPostExecute(Result result);
...
}
There are no classes named Params, Progress, or Result. These are instead generic types. They are just placeholders for types you wish to use when you define your own AsyncTask subclass. The above could equally be written as such:
public abstract class AsyncTask<A, B, C> {
...
protected abstract C doInBackground(A... params);
protected abstract void onProgressUpdate(B... progress);
protected abstract void onPostExecute(C result);
...
}
Suppose I were defining an AsyncTask that takes a list of Strings representing URLs, and it will ping each one to see if it's reachable, then return the number that were reachable. Meanwhile, with each test, it will update a ProgressBar as each test completes. It might look something like this:
public class MyAsyncTask extends AsyncTask<String, Integer, Integer> {
#Override
protected Integer doInBackground(String... params) {
int total = params.length;
int successfulPings = 0;
for (int i = 0; i < total; i++) {
if (isReachable(params[i])) {
successfulPings++;
}
publishProgress(i, total);
}
return successfulPings;
}
#Override
protected void onProgressUpdate(Integer... progress) {
int testsSoFar = progress[0];
int totalTests = progress[1];
progressBar.setMax(totalTests);
progressBar.setProgress(testSoFar);
}
#Override
protected void onPostExecute(Integer result) {
Toast.makeTest(context, "Reached " + result + " sites.", Toast.LENGTH_SHORT).show();
}
}
I would initiate this as follows:
String[] urls = ...
MyAsyncTask task = new MyAsyncTask();
task.execute(urls);
The argument passed into execute will be passed into doInBackground. Whatever you do in doInBackground, you need to return something that gets passed in as the argument to onPostExecute. While in doInBackground, you can call publishProgress, where you can do something like I did (but you don't have to).
My basic suggestion is to research more about AsyncTask but I'll as well try and Answer your questions:
1- In the angle brackets, I have Void, Void, Void. What exactly do those represent and how do I know what's correct to place in there?
They are the input argument types for the overridden methods: doInBackground() onPreExecute(), onProgressUpdate() and ... respectively.
2- For each method within the class, I have the each method called as void. When should they be different than void (like boolean, String,
long, etc.)?
I'll give you an example: onPostExecute() is guaranteed to be called after doInBackground(), so if onPostExecute(int someCounter) has an integer in its input, then doInBackground() should return an integer rather than void. So it will be declared like this:
int doInBackground(){
//bla bla bla
int someIntegerValue;
return someIntegerValue;
}
3- For the doInBackground() method, I have Void... voids in the parenthesis. What exactly should I be putting in there? What do they
represent?
They are var-args, basically speaking, they represent Variable number of Arguments.
Read more about them here.
Once again I suggest you delve into threading in Android and get a sense of why AsyncTask is needed.
Good Luck,
The three types used by an asynchronous task are the following:
Params, the type of the parameters sent to the task upon execution. i.e if you want to send some variable/array to your async task background task. You use that information using that variable.
Progress, the type of the progress units published during the background computation. i.e to show the progress of your background progress. ( such as showing how much a video/image is downloaded)
Result, the type of the result of the background computation. i.e the result that you calculated in background process used for passing result to onPostExecute method.
String[] username;
username[0]="user1";
username[1]="user2";
new asynctask().execute(username);
private class asynctask extends AsyncTask <USER, Integer, String>
{
#Override
protected void onPreExecute()
{
// anything you want to do prior starting the async task.
}
#Override
protected String doInBackground(USER... users)
{
int count = users.length;
for (int i = 0; i < count; i++)
retriveinformation(users[i]);
return "Hey";
}
protected void onProgressUpdate(Integer... progress) {
setProgressPercent(progress[0]);
}
protected void onPostExecute(String result) {
// the value result is HEY that is returned by doInBackground.
}
}
Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type Void.
information taken from https://developer.android.com/reference/android/os/AsyncTask.html
Let me try my hand at this. All the info I am parsing is from the link shared by Karakuri in the comments to the question.
The angle brackets' contents, in order, are called "parameters", "progress", and "result". Parameters is the type of parameters (using varargs) that will be fed to "doInBackground". They will be receive by that method in an array. You will use these parameters as your data to work on in the background thread.
"Progress" is the types of values fed into the onProgressUpdate. When you override this method, you will take in data of this type, format it, and use it in the progress display. From my reading, it is up to you to publish your progress in doInBackground to be consumed by onProgressUpdate.
"Result" is the data type of the result of the operation. This will be the return type of doInBackground, as well as the parameter type of onPostExecute.
For parameters that include the "..." (called varargs), you can put in as many arguments as you want. They must be separated by commas, and must all be of the type specified before the "...". The items will be passed to the function in an array. So, to answer your questions:
You'll change the first parameter in the angle brackets from void when your background task needs to receive data to work on. The type will be the type of data you'd like to pass in. You'll change the second parameter when you wish to keep track of the progress. The type will be whatever type you'd like to use for keeping track, and you will need to do all the tracking yourself in "doInBackground". You'll change the third parameter when your background task needs to return data. The type will be the type of data you wish to return. The data will be supplied to "onPostExecute". This is important because AsyncTask.execute() has no return value, so you must use any necessary data in that method to get it out to the class that called AsyncTask.execute().
onPreExecute appears to always be type void. doInBackground will be of the type of result, the 3rd argument in your angle brackets. onProgressUpdate appears to always be void. onPostExecute appears to be always void.
The parameter of doInBackground will be a type label followed by the three dot and a name. The type for the label must be the same as the first parameter in the angle brackets. The name is anything you want. That name will then be assigned to an array of the type named in the type label, and passed in for you to use in the body of doInBackground.
Some thoughts: if you are a beginner, leave "progress" void for now, and don't override onProgressUpdate. It is very important in production code, but a lot of extra hard work for beginner learning projects. onPreExecute is also likely more trouble than it is worth at your level. Just remember they exist, and when you need them, you will likely know. Also, good work on not avoiding the subject of multithreading just because it is difficult.
Related
I have a method inside AsyncTask class which I want to reuse. But it uses the object of type String... url which is accessed as url[0],url[1]
To reuse this function I need create a object of Type String... or
convert String... to String[]
I did not find any documentation for String...
#Override
protected String doInBackground(String... url) {
Thread.currentThread().setName("MetaDataDownloader");
String jsonStr=metadataDownloader(url);
//this uses param as url[0] and so on
}
That is because AsyncTask by design could receive more than one parameter (hence the varargs...).
Change your method like this:
#Override
protected String doInBackground(String... urls) {
Thread.currentThread().setName("MetaDataDownloader");
// if you want to handle all URLs
if (urls != null) {
for (String url : urls) {
String jsonStr=metadataDownloader(url);
}
}
}
You can create constructor in async task and pass what argument you would like to . Do not use parameters in doInBackground.
You can look at this Java Document. It's arbitrary number of arguments.
ou can use a construct called varargs to pass an arbitrary number of values to a method. You use varargs when you don't know how many of a particular type of argument will be passed to the method. It's a shortcut to creating an array manually (the previous method could have used varargs rather than an array).
To use varargs, you follow the type of the last parameter by an ellipsis (three dots, ...), then a space, and the parameter name. The method can then be called with any number of that parameter, including none.
--> to use your doInBackground(String... url), just treat it as array of parameters and cast it to whatever you like.
I am calling an API through POST method and its constantly returning NULL. What could be the possible reason? I have posted the whole code. protected void onPostExecute(Void result) is actually getting NULL.
The structure of what you're doing is not right. You're using an AsyncTask with a return type of Void. The only possible value of type Void is null, so that's the only thing you can possibly get back in your onPostExecute() method. That's what happens when you return null from your doInBackground() method.
Void is appropriate for when generics force you to use a type, but you don't really need one. So you'd use it only when you don't actually need anything to be returned. In your case, where you're wanting to retrieve something that results from the call, you need a different type.
If all you want is to know whether it's succeeded or not, you could use Boolean, and have it return true when it succeeds (where you currently have return null), and false when it fails (in your catch blocks).
From the javadoc
The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void.
so result is always null.
You must change your asyncTask to return something different than Void
Something like :
private class MyTask extends AsyncTask<String, Void, String> {
#Override
protected String doInBackground(String... params) {
//Do request
return "response"; //return request's response as String
}
#Override
protected void onPostExecute(String result) {
if(result.equals("response")){
Toast.makeText(getApplicationContext(), "HTTP POST is working...", Toast.LENGTH_LONG).show();
} else{
Toast.makeText(getApplicationContext(), "Invalid POST req...", Toast.LENGTH_LONG).show();
}
}
}
I am creating an app in Android and I have a large number of string data, lets way 10,000 or more words (items), I want to display it in a list view. My question is where should I put my (source) data
String Array in Xml file
In DB (then i have to place an external DB)
Reading data from a simple text file, CSV etc
Here my only concern is speed, which way is faster and why.
Note: I am currently placing data in Xml as string array and getting it in Array in activity, but it is slow it takes few sec/moments to load data from xml but only for the first time.
Execute the code to parse/load content form json/db in a AsyncTask for more speed. I load 5000 rows with ~ 400 chars per row. It takes without AsyncTask much longer.
private class YourTask extends AsyncTask<String, Void, String> {
#Override
protected String doInBackground(String... s) {
//Here you have to make the loading / parsing tasks
//Don't call any UI actions here. For example a Toast.show() this will couse Exceptions
// UI stuff you have to make in onPostExecute method
}
#Override
protected void onPreExecute() {
// This method will called during doInBackground is in process
// Here you can for example show a ProgressDialog
}
#Override
protected void onPostExecute(Long result) {
// onPostExecute is called when doInBackground finished
// Here you can for example fill your Listview with the content loaded in doInBackground method
}
}
To execute you just have to call:
new YourTask().execute("");
Here you can learn more about AsyncTasks:
AsyncTask developer Guide
I am currently having trouble getting a value from an AsyncTask that gets data from a JSON connection. I have looked at a few examples, but mostly I have only seen posting results from AsyncTask.
First I have an object called Dog that only has a String, Name. I am trying to get the Name of the dog from the server.
Using the following code in my oncreate, I start the DogAsyncTask while assing in an URL called n and a Dog d_in.
Dog d_in = new Dog("DogName");
DogAsyncTask task = new DogAsyncTask(d_in);
String n = "www.dog.com";
task.execute(n);
Log.e("Out", d_in.getName());
My AsyncTask is as follows:
private class DogAsyncTask extends AsyncTask<String, Void, String> {
Dog d = null;
DogAsyncTask(Dog d){
this.d = d;
}
ProgressDialog mProgressDialog;
#Override
protected void onPostExecute(String result) {
d.setName(result);
Log.e("Dog", d.getName());
mProgressDialog.dismiss();
}
#Override
protected void onPreExecute() {
mProgressDialog = ProgressDialog.show(AsyncTestActivity.this, "Loading...", "Data is Loading...");
}
#Override
protected String doInBackground(String... name) {
//Go to url = name and then gets String below.
String outfromjson = "new dog name"; //This will be a function that gets a name from JSON
return outfromjson;
}
}
I tried using something like
Log.e("Out", task.d.getName());
but I keep getting the default name of the dog which is "DogName". How to I carry values out of AsyncTask?
OK, The thing what is happening here is:
You want to pass Dog as reference (pointers as in C / C++ / Objective-C),
and you want to reflect change in original variable using other variable.
but keep in mind that Java doesn't pass object by reference, Java passes objects by value (only value is transferred, another object is created and assigned value).
So what you can do is you can assign that result-holder object again as a value to the original object.
You can do the following,
Declare and implement an interface in your activity and then use it's methods as a callback from the onPostExecute method.
I'm unfamiliar with the parameter syntax in doInBackground(Params... params)
What is this type of thing called, and how do I utilize it?
As devA and VVV have said, that is called "varargs". Effectively, the following two lines of code are equivalent:
public void makeLemonade(String[] args) {
and
public void makeLemonade(String... args) {
the code inside the method would be the same, but when it was called, they would be called differently. The first would need to be called like this:
makeLemonade(new String[]{"lemon1", "lemon2", "lemon3"});
while the second one's method signature could have 0 to (an assumed)infinite number of arguments, but they would all need to be String arguments. All of the following calls would work:
makeLemonade("lemon1");
makeLemonade("lemon4", "lemon7", "lemon11", "lemon12");
makeLemonade();
// ... etc ...
A subtle difference between the two is that you can call makeLemonade() legally here if you're using varargs.
They are called Var Args.. Just like an array.. U can pass multiple items and just access like params[0].. etc..
Params... params represents a vararg. It was added in J2SE5.0.
It means you can pass any number of arguments or you can say array of arguments params[0]
Thanks
Deepak