Tasks and Progress Monitoring in Java

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Tasks and Progress Monitoring
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Three of the classes that you have seen in this chapter HttpRequest, RssTask, and AtomTask perform asynchronous operations in a background thread while reporting their status and delivering results in the main application threadThese classes all derive from javafxasyncTask, which is intended to be the basis for all asynchronous operations on the JavaFX platformAs you ll see in the next section, this class can be used to implement your own background operations In this section, we look at the features of the Task class that allow you to monitor and control the state of an asynchronous operation
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Tasks and Progress Monitoring
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Task State Variables
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The common state of an asynchronous operation is held in the variables of the Task class, which are listed in Table 27-10 Subclasses may add additional state, as appropriate For example, the HttpRequest class adds variables that indicate whether data is currently being read or written
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Table 27-10 Variable
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onStarted onDone
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Variables of the Task Class Type
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function() :Void function() :Void Boolean Boolean
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Access
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Default
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null null
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Description
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Function called when the task is about to start Function called when the task has completed (successfully or otherwise) Indicates whether the task has been started
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true when the task has been stopped by calling the stop()
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started stopped
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function
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succeeded failed done causeOfFailure Boolean Boolean Boolean Object
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null
Indicates that the task ran to a successful completion
true if the task failed to com-
plete normally Indicates that the task is no longer running A value that indicates the reason why the task failed, if this is known This will typically be a Throwable The progress made by the task so far, or -1 if the task is not yet started or the progress made is not known The value of progress that would indicate that the task is complete, or -1 if the task is not yet started or the progress made is not known
progress
Number
progressMax
Number
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(Continued) Variable
percentDone
Type
Number
Access
Default
Description
The progress made so far as a proportion of the maximum The value will be in the range 0 to 1, or -1 if the operation is not started or if the progress made or maximum progress value is not known
The variables started, stopped, succeeded, failed, and done describe the current state of the operation Initially, all these variables are falseWhen the start() function is called, the started variables is set to true and remains in that state, even when the operation has completedYou cannot, therefore use the started variable on its own to determine whether the operation is in progressWhen the operation ends, the done variable is always set to true, so the correct test for whether the operation is in progress is as follows:
(started and not done)
An operation can terminate for any of three reasons:
The stop() function has been called In this case, the stopped, failed, and done variables are all set to true The operation fails In this case, a message giving the reason for the failure may be stored in the causeOfFailure variable, and the failed and done variables will be set to true The operation succeeds, in which case both the succeeded and done variables will be set to true
Note that because of the way these variables are managed, it is possible for an operation to be simultaneously started and stopped or started and done The values of these variables are typically used to enable or disable user interface controls whose state should depend on that of the operation with which they are connected You ll see an example of this in Listing 27-13 later in this section Some state changes are also reported by a callback function the onStarted function is called just before the operation is initiated, and the onDone function is invoked just after it terminatesYou have already seen examples in this chapter that use the onDone function to do things such as trigger parsing of XML that has been read by an HttpRequest