MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

Sequential Async Operations: Chaining Promises
In the previous code example, you might have noticed how we throw exceptions whenever we don’t get
a good result back from any given async operation. Furthermore, we have only a single error handler at
the end, and there’s this odd construct of returning the result (a promise) from each subsequent async
operation instead of just processing the promise then and there.
Though it may look odd at first, this is actually the most common pattern for dealing with sequential
async operations because it works better than the more obvious approach of nesting. Nesting means to
call the next async API within the completed handler of the previous one, fulfilling each with done.
Here’s how the async calls in previous code would be placed with this approach (extraneous code
removed for simplicity):
captureUI.captureFileAsync(Windows.Media.Capture.CameraCaptureUIMode.photo)
.done(function (capturedFileTemp) {
//...
local.createFolderAsync("HereMyAm", ...)
.done(function (myFolder) {
//...
capturedFile.copyAsync(myFolder, newName)
.done(function (newFile) {
})
})
});
The one advantage to this approach is that each completed handler will have access to all the
variables declared before it. Yet the disadvantages begin to pile up. For one, there is usually enough
intervening code between the async calls that the overall structure becomes visually messy. More
significantly, error handling becomes significantly more difficult. When promises are nested, error
handling must be done at each level; if you throw an exception at the innermost level, for instance, it
won’t be picked up by any of the outer error handlers. Each promise thus needs its own error handler,
making real spaghetti of the basic code structure:
captureUI.captureFileAsync(Windows.Media.Capture.CameraCaptureUIMode.photo)
.done(function (capturedFileTemp) {
//...
local.createFolderAsync("HereMyAm", ...)
.done(function (myFolder) {
//...
capturedFile.copyAsync(myFolder, newName)
.done(function (newFile) {
},
function (error) {
})
},
function (error) {
});
},
function (error) {
});
94