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Welcome back, everyone.

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Our report's coming along.

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In this video, we'll implement sorting.

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When the user taps one of the column headings,

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the data will be sorted by that column.

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Examine the DurationsViewModel code carefully

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to decide how you're going to achieve that.

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Yes, it's challenge time.

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Remember that the durations report activity

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will respond to the user tapping the headings,

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and the ViewModel will take care of loading the data.

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Add an onClick listener to the headings in the DurationsReport

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class to allow the data to be sorted by the various columns.

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Test your solution to make sure it works.

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Pause the video now and come back when you're ready to see my solution.

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So how did you get on?

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The only changes we need to make are in the DurationsReport class.

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And DurationsViewModel already deals with re-querying the data

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when its sort order property changes.

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For my solution,

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I'm going to get the DurationsReport class

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to implement the view.OnClickListener interface,

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that's probably the easiest way

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to use the same onClick listener for all the headings.

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Although, you could have declared an onClick listener object instead.

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I'll get android studio to generate the function after oncreate,

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and then we'll add the code.

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We've got an error on line 38 because we should be using

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the safe call operator.

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Using safe call is one solution,

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but this is an implementation of the views

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on ClickListener interface.

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What are the chances of this function being called by something that's null.

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Remember that google haven't finished annotating

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all the java methods and parameters with non-null.

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The view parameter can't be null.

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And another solution is to remove the nullable type marker

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from the declaration.

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All right. When one of the column headings is tapped,

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this function will receive the corresponding text view as an argument.

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We can use the ID of the text view to decide which one was tapped.

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A simple when statement says the view model sort order property.

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We've already got the ViewModel to reload the data

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when sort order changes,

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so that's all we need to do.

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In oncreate, we set the OnClick listener

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for each of the heading widgets.

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Remember, the description column won't exist in portrait.

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Using the save call operator will make sure we don't get a null pointer exception

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when attempting to set its OnClick listener,

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I'll run the app to test it.

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I'll start by tapping the date column.

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That sorts the data by date, which is good.

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The duration column also works fine.

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That scrambled the task names

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letting us test the task name column.

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That also seems to be working.

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If you get strange results when sorting by date, don't worry,

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there's a bug in this program

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but you won't notice it unless you're testing on a different time zone

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to GMT.

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The more hours away from GMT you are,

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the more chance there is you'll experience a bug.

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I'll be talking about this later.

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First, we're going to implement all the functionality

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then we'll look at spotting and fixing this bug.

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All right. We should also test sorting in landscape

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because we've got an extra column when displaying the report in landscape.

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This highlights a problem with our data

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as far as testing the reports goes.

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Our descriptions are a bit artificial,

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that was useful to validate the data in the database earlier,

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but it doesn't let us test sorting very easily.

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Tapping the description column doesn't do a lot

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because the task names and the task description sort

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in almost the same order.

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For a thorough test,

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you should change all the task descriptions to something more realistic.

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I'm not going to do that on the video, it does work though.

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A quick test is to sort on duration,

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and then tap on description

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to sort the column that way.

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That seems to be working as well.

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Part of the reason this was so simple

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was because of the clear separation between the

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DurationReports class and the DurationsViewModel.

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One question we often get asked by students is,

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how can i learn to design my code well?

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That comes with experience.

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Be prepared to create classes that are far from perfect.

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The more times you get it wrong, the sooner you'll get it right.

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It's not always easy to spot good design until you come to make a change.

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If the change is fairly easy and involves a few classes,

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then the design is probably good.

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If you really struggle and have to make changes all over the place,

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that's an indication that the design could be improved.

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All right. I'll stop this video here.

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In the next video, we'll make a start on creating the menu

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for the reports activity. See you then.