正确使用AsyncTasks并处理View和Activity所需的对象

Question

So I am currently writing an App and have a few questions about how to deal with AsyncTasks. I describe my problem as detailed as I can. I have the following classes:

1. MainActivity - This Activity only executes setContentView(R.layout.myLayout) and creates an Object of my CustomLocationListener . The Layout contains a custom view and a ProgressBar .
2. CustomLocationListener - This is basically just a Class which implements LocationListener and has the default LocationListener methods. In addition to that I wrote a method to check which providers are enabled to prior use the NETWORK_PROVIDER .
3. Display - This is just an assisstant class. I use it only to get display metrics to calculate some things.
4. MyCustomView - This class extends the View class. It also has an Attribute of the type Display (my own class).

Now you should know everything important to my code. Well, my question now is, how to deal with AsyncTasks, since this are the steps the App should go through:

1. Get the current location of the users device
2. Execute some queries to a database server (mysql) and save them non persistent (the data consists of city names)
3. Calculate the distance between the users location and the cities in the database
4. Do some other calculations to figure out the positions where to draw on the screen
5. Draw everything

On top of that I would like to implement a progress bar, which should be active untill all calculations are done and the drawing phase begins. So, I'm kinda confused where to code what. I mean, both calculations (step 3. and 4.) have to know about each other. The calculation of step 4 needs the results of the calculation of step 3. And my Activity class should also know about the state of the calculations, since I have to make the ProgressBar invisible when I start to draw.

I hope I was detailed enough and I appreciate your help!

Answer 1

Well, from the AsyncTask docs you can gather this:

When an asynchronous task is executed, the task goes through 4 steps:

1. onPreExecute(), invoked on the UI thread before the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface.

2. doInBackground(Params...), invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step.

3. onProgressUpdate(Progress...), invoked on the UI thread after a call to publishProgress(Progress...). The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.

4. onPostExecute(Result), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

So what I would suggest for you is to use two AsyncTasks to do your geo and server (one for each) work in the doInBackground(Params...) method and return the value to your MainActivity in the onPostExecute(Result) method. Also, keep your MainActivity aware of the progress by using the onProgressUpdate(Progress..) method.

When they are done, you can choose to use another AsyncTask to do the calculations if they stall the UI thread noticeably. Use the same method of execution for it.

When this is all done you are ready to draw a view... (I think is what you're saying)

Have a default view load before you start the calculations that says "Loading..." or whatever. Have another view that is not visible ready for when you are ready to display your calculated things and just set it visible when it is ready to be drawn on.