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Alright so moving on now, now that we've got the data we want, we're going to create a new photo object passing

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the values we've just extracted from the json data in the constructor.

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So the photo object can then be added to the list that we want, that we'll be returning, so I'd better declare

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an array list at the start of the function as well. This is the doInBackground,

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so let's do that, and I'll declare that under the log so we'll do a val photo list is equal to array list,

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diamond operator and it's going to be photo, so it's an array list of photo objects.

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And let's go ahead now and create an object, a photo object from the json data, so down here under

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the val link, we're going to put val photo object is equal to photo, parentheses title author author Id

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link tags and photoUrl. And because we've now got an array list we can do a photo list dot add

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photo object. Let's also do a log here, so Log.d

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parentheses tag comma dot doInBackground dollar photo object. So that's basically added logging there,

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at the end of the function to print out the data, just so we can check that everything's working as it should be.

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So the code will keep looping until it's processed

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all the items in the array, and we'll then end up with a list containing the details for each of the photos

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in the feed. So that's json passing.

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Now we haven't had to write a lot of code, and all the complexity of parsing json data is dealt with

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by the json objects in the org dot json package, which is pretty cool.

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So let's clear this error down here now, which relates to adding a catch block. So we're going to add a catch parentheses

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e colon JSONException, then a code block, and we'll start by doing a e.printStackTrace, then we'll do a Log dot

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e parentheses tag comma dot doInBackground, colon Error processing

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Json data dot, then we'll do a dollar, left and right curly braces e.dot message, which'll return the message passed by the

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json exception. And then we'll do, basically I'm going to cancel this one, so just a note here to cancel,

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true, because that's essentially what's going to happen, we're cancelling out of it.

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So do a listener dot onError parentheses e, parsing the actual error.

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So the catch block prints a stack trace with the code on line 47, then prints the message contained in an

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exception by calling its get message function.

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Now because our code's thrown an exception, we call the onError function of our listener

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to parse the exception back to the calling code.

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Now as I said earlier, this code's running on a separate thread.

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So that means that the calling code can't trap the exception using a try catch block,

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so we'll use this mechanism to let the caller know about any errors that may happen.

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Now I'm going to come back to this cancel true line that I've included as a comment a bit later.

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It'll makes sense when we've got the code running so I'll describe it then.

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Now the only other thing left to do is to return a list of photos that we've created.

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So let's go ahead and do that, and I'll get rid of that last error. First we'll do a log. so Log.d parentheses

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TAG comma,

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double quotes dot doInBackground ends, and then we'll do a return photo list. So that now clears the

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final error, because our function's now returning something.

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OK so when doInBackground finishes, the onPostExecute function will be called.

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So I'm going to use that to send the list of photos back to the calling code.

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So let's have a look at that, so onPostExecute.

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At the moment we've got the log and the super, so leave that in. On the next line we'll put Listener.onData

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Available, and in parentheses, result. Now we've got an error there, or I have anyway, you may not have.

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This function is declared to have a nullable parameter, and we've specified a not-nullable argument to

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onDataAvailable. Now we've already made sure that doInBackground can't return null,

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so it's OK to change the onPostExecute parameter to be non-nullable.

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So let's go ahead and do that. And then lastly we'll add some logging, so

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Log.d parentheses tag comma dot onPostExecute ends.

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Alright so that's how json parser finished. Now if everything worked,

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the calling class will have the photo list in it's onDataAvailable function, and can display

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the photos or do whatever else it wants with them.

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We can now get main activity to use this class to parse the data

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after get raw data's finished downloading it.

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So go back to main activity, so the place to perform the parsing's in the onDownloadComplete

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function, down here, and specifically if the status we got back is OK.

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So at the moment we're just adding some logging there. So now we're going to do

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val getFlickrJSONData is equal to GetFlickrJsonData parenthesis this, that's the listener

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again, then on the next line getFlickerJsonData.execute, and we're passing data.

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Now we've got this error because main activity isn't implementing the on data available interface yet.

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So let's do that, back up here to the class definition. So we've got AppCompactActivity comma get raw data dot on

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download complete. We also want to add, so comma onDataAvailable.

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OK so we've now got another error but we expected that. We're not yet implementing the required functions for

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this interface.

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So let's go down to the bottom and we'll get Android Studio to generate the stubs for us.

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Now the ones we want are relating to the onDataAvailable here, onDataAvailable and onError.

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So I'm going to select both of those, click OK, and by doing that we've removed that error.

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Now onDataAvailable will receive a list of photo objects, and we'll be using that list to display the

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details on the screen.

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For now though, we're just going to print the list so that we can see that everything's working.

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We'll also had the usual logging, so let's go ahead and do that. So for onDataAvailable,

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we're going to start with our log, the Log.d parenthesis TAG comma double quotes, dot onData

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Available called, data is dollar data. Then we'll just do a space there for future function.

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I'll do Log.d parentheses TAG comma double quotes dot onDataAvailable ends, and for onError,

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we're just going to do Log.d parentheses TAG comma on

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Error called with dollar, left and right curly braces, e dot, sorry, exception because exception's the name of the

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parameter exception dot message. Alright so let's look at this slide again.

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So you can see that MainActivity now implements two interface: the onDownloadComplete interface

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that's defined in GetRawData, and the onDataAvailable interface from our new GetFlickrJsonData class.

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Now the download started by creating a GetRawObject, or GetRawData object in onCreate. When the

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download completes, the onDownloadComplete function gets called. That kicks off parsing the data by creating

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a GetFlickrJsonData instance,

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and when the parsing completes, the onDataAvailable function gets called. Now we'll add the code to display

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the data later,

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so we still haven't implemented the left hand side of this diagram, but we have finished the classes on the

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right hand side now though.

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So at this point when we're ready to run the program and see if our parsing works. Alright,

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so let's now run this again and see if it works.

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So we're going to come up and start it, and actually what we should do is open log cat. And while I remember we've still

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got a  filter on over here, so we can either take the filter off, or what we can do now that we've written

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the GetFlickrdjacentData class, we can add that to the filter so I think I might do that.

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So I'm going to add a pipe at the end of that, and do a getflickrjsondata, so we can also get output from

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that as well. And we can see that when I've done that,

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we've actually got an error here, system dot error. So something's actually crashed, and the error we've got back is

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'onError called with no value for m'. So that, I suspect, we know for sure now,

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00:09:41,200 --> 00:09:44,950
it's relating to GetFlickrJsonData, the class doInBackground.

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So let's just go and have a look at that,

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and it's most likely something to do with retrieving the fields I would think.

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And this is a hint here, the fact that json media is never used. That tells me that I've made a typo, so

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I'm going to copy that, and that should have actually been here for the photo url.

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So we should have been using that, so we're retrieving the json object into json media.

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The photo is actually being extracted from json media.

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That's the correct way to do it so that should actually, hopefully, fix that problem.

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So let's run this again, I'll clear log cat.

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OK so we've got some different data this time, much better looking,

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and I think we might have fixed that problem.

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So if we scroll up and have a look first, we can see clearly here now that GetFlickrJsonData class is

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logging the data. So it's logging each photo as it's being parsed, and that's in it's doInBackground method as

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you can see there. It's printing it out, but if we go up further,

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we can see where the doInBackground was actually started.

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But in terms of the raw data itself, if you scroll up and have a look. If you recall we've got the raw

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json file that we downloaded, being logged in onDownloadComplete,

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but also if we scroll up and have a look, onPostExecute and GetRawData as well.

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So now that we can see that things are working, we're going to start commenting out that logging, but if we scroll it

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back down again, and have a look at our GetFlickrJsonData, the call, you can see doInBackground starts

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here. So that's from again,

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the GetFlickrJsonData class, and it's parsing each set of details for the photos. You can see that as

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it's doing that, we're getting each one logged, the information relating to the various fields, title, link and so

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on. So that's working nicely, so

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basically title, author, author Id, and link, and actually we can go right across, we can see tags as well.

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You can see the image there, we can see tags, all working nicely.

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So the parsing seems to be working, and when it completes, if we go right down to the end of the log, we've got the onDataAvailable

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function being called which is correct,

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and then onPostExecute actually ends right on the last line. Right, so at this point now that we've got the

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json data parsing correctly, we don't need to log the raw json data any more, so

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let's tidy up that log cat a little bit.

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So going back to, well we're in actually, MainActivity, so we'll stay in there,

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and what we'll do is we'll remove the data being logged in the onDownloadComplete function.

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So what I'm going to do is just remove this part here,

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so we've still got a message to say it's being called, but we just don't get the actual data.

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And I'll fix that, that's a typo, onDownloadComplete it should have been, not on completed, the name of the function.

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Right, so that's main activity, and the other thing is it's been logged in onPostExecute and get raw data. So lets

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also remove that from this line, this logging line, so that we don't see all that raw json data anymore,

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because we've confirmed that the json parser is working, and it's just basically making the log far too

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big for us to actually try and figure out, to make any sense of, so we're removing it from the log.

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Alright, we're going to run it again, and hopefully now the output will be a lot tidier.

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Alright, so you can see now that I've removed that raw data, things are a heck of a lot tidier than what they

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were before now.

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And we're still getting the function calls logged, so you can see the order that everything's happening.

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So the onCreate function in MainActivity creates a get raw data object to perform the downloading.

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Once the download's complete, the onPostExecute function, well that calls onDownloadComplete, and on

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DownloadComplete,

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actually here, you can see it here being called, that actually creates a GetFlickrJsonData object

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to parse the json into our list of photos, and we can see if I scroll down a little bit further, doInBackground starts.

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Then we can see each photo being created as it's going through the json data,

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and parsing it. And keep in mind here that when that's finished, the GetFlickrJsonData's onPostExecute should be called,

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and you can see that down there, it starts onPostExecute.

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And by the way the Android framework takes care of that.

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That's what the execute method of an async task does.

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So it runs the doInBackground function on a separate thread,

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then calls onPostExecute with the result.

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So onPostExecute then calls the onDataAvailable function in MainActivity that sends it

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the list of photos.

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So you can see all the photos in the list there,

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and that's because that's the array list we're printing out now. Instead of an individual list of photos that you just saw

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higher in the log,

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this is the complete list of photos in an array list being printed out. Alright so the last

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thing we want to do is to check what happens if there's an error, while parsing the data.

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Now I did have a typo anyway which actually caused that to actually error, before we actually fixed

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it earlier in this video,

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but let's just go through and show you how to simulate that and just check that

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what happens if we've got some invalid json data or if the json parser fails some reason. So we can

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go back to MainActivity to do that.

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That's the quickest way, and to duplicate the call, the execute, the GetFlickrJsonData.execute, duplicate that with

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command d, control d on a PC, and if you just send some, basically some bogus data in there, I'm just

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going to say bogus data. If we run that, and you can see we've got an error.

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Very similar error, or the actual log's very similar to what happened before I actually fixed the typo

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I made in GetFlickrJsonData. But you can see here it's actually saying what the

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actual error is, 'Error processing Json data. Value bogus of type java dot

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lang dot String cannot be converted to a JSON Object'.

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And again that's correct because it's not valid json data.

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So I'm getting the error log there.

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And also if you look further down, you can see that onError is also called, with the same message there.

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So we're basically seeing the log twice because the error is actually passed rather back to main activity in it's

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onError function.

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So that's why I may have passing rather an exception to the caller from code that's running on a separate thread.

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But there's a slight problem with this as we can see in the log cat.

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So even though we got an error while parsing the data, the onPostExecute method still gets called,

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so that in turn passes an empty list to onDataAvailable, and you can see this down here in this part

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of the log.

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Now we really don't want that to happen if there's an error.

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Now fortunately though the async task class has a way to prevent that.

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So async task will only call onPostExecute when doInBackground finishes.

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It won't do it, won't do that call if the task's cancelled.

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Now async task has a cancel method which lets the main thread cancel a long running task if necessary.

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Now we will be doing that later, when we look at how to handle the activity being destroyed by an

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orientation change. But there's no reason why an async task can't cancel itself though,

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and that seems like a good idea to do, if we get an error.

188
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So going back to GetFlickerJsonData, that's why we've got this cancel commented out code here, in the

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catch for the json exception errors. So I'm going to uncomment that line now, and just format it correctly.

190
00:17:16,319 --> 00:17:22,319
So if we cancel the task when there's an exception, the async task code won't call onPostExecute, which

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00:17:22,319 --> 00:17:23,640
is what we want here.

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00:17:23,640 --> 00:17:31,300
So let's just run the app again and confirm that that is the case, and that happens if there is an error.

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Open the log cat,

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00:17:32,790 --> 00:17:35,880
and you can see that we've got a different result this time. Actually I might just clear the log and run it

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00:17:35,880 --> 00:17:42,510
again, so we can see the fresh output,

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and that looks better as you can see. We get the exception message logged by

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the GetFlickrJsonData dot onPostExecute,

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and again by MainActivity onDownloadComplete, and then MainActivity onError,

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but then the doInBackground actually ends down here as you can see, after showing the actual errors.

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So basically onPostExecute doesn't get called in that scenario, and MainActivity doesn't have to then

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deal with an empty list. Alright so that's our two async tasks

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now working. What I'll do before we end the video is I'll go back to MainActivity.

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I'm going to fix this line here. I'm

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actually just going to delete that line entirely, and uncomment the previous line so we've got some valid

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data when we actually start the next video.

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So I'll stop the video now and in the next one we're going to sort out the url so that we can add the

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tag parameters to it, and that's going to allow us to return data for different searches.

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So I'll see you in the next video.