Exception Handling
Often developers can predict scenarios which might cause issues or errors in their programs and they will attempt to avoid these scenarios as much as possible. However, sometimes they are unavoidable and even very likely to happen but you'd rather not have the entire application crash as a result. Exception handling enables programmers to cater for these scenarios and override the normal program flow during runtime when these exceptions occur. You can find more about Exception Handling in Helium here but in this lesson we're going to stick to a simpler example.
When trying to process each MezBatchItem instance in the function above, we want to catch any issues that might occur as Helium attempts to create a StockUpdate instance using the fromCsvLine() function. Helium provides exactly this functionality in a Try Catch Block which allows the other instances to continue processing without halting the entire application in the middle of processing the CSV file.
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void saveStockUpdate() {
MezBatch stockUpdateBatch = Mez:createBatch(fileUpload._id, fileUpload.data);
// Get the batch items related to the batch being processed
MezBatchItem[] stockUpdateBatchItems = MezBatchItem:relationshipIn(batch, stockUpdateBatch);
// Results from CSV processing will be stored in this collection
StockUpdate[] stockUpdates;
// Iterate over the batch items and process one by one using fromCsvLine
foreach(MezBatchItem stockUpdateBatchItem: stockUpdateBatchItems) {
try{
StockUpdate stockUpdate = StockUpdate:fromCsvLine(stockUpdateBatch.header, stockUpdateBatchItem.value);
stockUpdate.stocktakeDate = selectedDateOfStocktake;
stockUpdate.shop = selectedShop;
stockUpdate.stock = getStockFromName(stockUpdate.stockName);
stockUpdate.save();
stockUpdates.append(stockUpdate);
// Once a batch item has been successfully processed it can be marked as such
stockUpdateBatchItem.processed = true;
}
catch(exception) {
// Handle the exception by logging it and creating a follow up task
handleException(stockUpdateBatch, stockUpdateBatchItem, exception.message);
}
}
Mez:alert("alert.uploaded_data_saved");
fileUpload = FileUpload:new();
} |
This Try Catch Block will attempt to complete everything in the try{} code block from top to bottom, and if it encounters any Helium exceptions, it will jump immediately to the catch{} block to continue with processing until the entire function is resolved. Please note that the handleException() function in the catch{} block above has not yet been declared. Also that the exception caught in the catch{} block has several values that can be used and here we are using the message value.
For our application we would like to display any exceptions that might occur to the Shop Owner so that they can rectify it and upload these corrections in a new csv file. We're going to give them a message explaining just that and show a table indicating exactly what was the exception. First we'll add a non-persistent object to the model that can logically house these exceptions temporarily as we don't want to keep these messages for use outside of their session.
| Code Block | ||||||
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object CsvException {
string header;
string value;
string exception;
} |
Then we can declare the handleException() function implemented in the catch{} block above, also adding a list (unprocessedItems) to the presenter that will house these exceptions and a boolean value (showExceptions) to toggle the table in the view which shows these exceptions to the user.
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unit StockLevelsUpdate;
CsvException[] unprocessedItems;
bool showExceptions;
// Init function to initialize unit variables
void init() {
showExceptions = false;
...
}
void saveStockUpdate() {
unprocessedItems.clear();
...
if (unprocessedItems.length() > 0) {
Mez:alert("alert.uploaded_data_saved_with_exc");
showExceptions = true;
} else {
Mez:alert("alert.uploaded_data_saved");
showExceptions = false;
}
fileUpload = FileUpload:new();
}
...
void handleException(MezBatch updateBatch, MezBatchItem updateItem, string exception){
//Mark that the MezBatchItem has not been processed successfully
updateItem.processed = false;
//Log the exception
Mez:log(exception);
//Add to exception list to display to User
CsvException item = CsvException:new();
item.header = updateBatch.header;
item.value = updateItem.value;
item.exception = exception;
unprocessedItems.append(item);
}
|
The view just gets an added table that displays the unprocessedItems when the showExceptions is true.
| Code Block | ||||||
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<table>
<visible variable="showExceptions"/>
<collectionSource variable="unprocessedItems"/>
<column heading="column_heading.header">
<attributeName>header</attributeName>
</column>
<column heading="column_heading.value">
<attributeName>value</attributeName>
</column>
<column heading="column_heading.exception">
<attributeName>exception</attributeName>
</column>
</table> |
This would be an example of what your application should look like then.
As can be seen in the values, the user attempted to add values for level and price which could not be formatted to the expected column type. Instead of the whole csv processing throwing an error, only two lines failed and have to be corrected before uploading again while the other 8 were successfully persisted. Remember to update the en.lang file with the new translations and see the attached source code for any omitted parts if necessary.
| Info |
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Take note that at present, exceptions resulting from SQLException cannot be handled by the try catch statement. |