Implemented half sync half async pattern #109

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/AsyncTask.java

@@ -0,0 +1,44 @@
+package com.iluwatar.halfsynchalfasync;
+
+import java.util.concurrent.Callable;
+
+/**
+ * Represents some computation that is performed asynchronously and its result. 
+ * The computation is typically done is background threads and the result is posted 
+ * back in form of callback. The callback does not implement {@code isComplete}, {@code cancel} 
+ * as it is out of scope of this pattern.
+ * 
+ * @param <O> type of result
+ */
+public interface AsyncTask<O> extends Callable<O> {
+	/**
+	 * Is called in context of caller thread before call to {@link #call()}. Large
+	 * tasks should not be performed in this method as it will block the caller thread.
+	 * Small tasks such as validations can be performed here so that the performance penalty
+	 * of context switching is not incurred in case of invalid requests.
+	 */
+	void onPreCall();
+	
+	/**
+	 * A callback called after the result is successfully computed by {@link #call()}. In our
+	 * implementation this method is called in context of background thread but in some variants,
+	 * such as Android where only UI thread can change the state of UI widgets, this method is called
+	 * in context of UI thread.
+	 */
+	void onPostCall(O result);
+	
+	/**
+	 * A callback called if computing the task resulted in some exception. This method
+	 * is called when either of {@link #call()} or {@link #onPreCall()} throw any exception.
+	 * 
+	 * @param throwable error cause
+	 */
+	void onError(Throwable throwable);
+	
+	/**
+	 * This is where the computation of task should reside. This method is called in context
+	 * of background thread.
+	 */
+	@Override
+	O call() throws Exception;
+}