Java tutorial
/* * Copyright (C) 2013 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package android.print; import android.annotation.UnsupportedAppUsage; import android.os.Bundle; import android.os.CancellationSignal; import android.os.ParcelFileDescriptor; /** * Base class that provides the content of a document to be printed. * * <h3>Lifecycle</h3> * <p> * <ul> * <li> * Initially, you will receive a call to {@link #onStart()}. This callback * can be used to allocate resources. * </li> * <li> * Next, you will get one or more calls to {@link #onLayout(PrintAttributes, * PrintAttributes, CancellationSignal, LayoutResultCallback, Bundle)} to * inform you that the print attributes (page size, density, etc) changed * giving you an opportunity to layout the content to match the new constraints. * </li> * <li> * After every call to {@link #onLayout(PrintAttributes, PrintAttributes, * CancellationSignal, LayoutResultCallback, Bundle)}, you <strong>may</strong> get * a call to {@link #onWrite(PageRange[], ParcelFileDescriptor, CancellationSignal, * WriteResultCallback)} asking you to write a PDF file with the content for * specific pages. * </li> * <li> * Finally, you will receive a call to {@link #onFinish()}. You can use this * callback to release resources allocated in {@link #onStart()}. * </li> * </ul> * <p> * The {@link #onStart()} callback is always the first call you will receive and * is useful for doing one time setup or resource allocation before printing. You * will not receive a subsequent call here. * </p> * <p> * The {@link #onLayout(PrintAttributes, PrintAttributes, CancellationSignal, * LayoutResultCallback, Bundle)} callback requires that you layout the content * based on the current {@link PrintAttributes}. The execution of this method is * not considered completed until you invoke one of the methods on the passed in * callback instance. Hence, you will not receive a subsequent call to any other * method of this class until the execution of this method is complete by invoking * one of the callback methods. * </p> * <p> * The {@link #onWrite(PageRange[], ParcelFileDescriptor, CancellationSignal, * WriteResultCallback)} requires that you render and write the content of some * pages to the provided destination. The execution of this method is not * considered complete until you invoke one of the methods on the passed in * callback instance. Hence, you will not receive a subsequent call to any other * method of this class until the execution of this method is complete by invoking * one of the callback methods. You will never receive a sequence of one or more * calls to this method without a previous call to {@link #onLayout(PrintAttributes, * PrintAttributes, CancellationSignal, LayoutResultCallback, Bundle)}. * </p> * <p> * The {@link #onFinish()} callback is always the last call you will receive and * is useful for doing one time cleanup or resource deallocation after printing. * You will not receive a subsequent call here. * </p> * </p> * <h3>Implementation</h3> * <p> * The APIs defined in this class are designed to enable doing part or all * of the work on an arbitrary thread. For example, if the printed content * does not depend on the UI state, i.e. on what is shown on the screen, then * you can offload the entire work on a dedicated thread, thus making your * application interactive while the print work is being performed. Note that * while your activity is covered by the system print UI and a user cannot * interact with it, doing the printing work on the main application thread * may affect the performance of your other application components as they * are also executed on that thread. * </p> * <p> * You can also do work on different threads, for example if you print UI * content, you can handle {@link #onStart()} and {@link #onLayout(PrintAttributes, * PrintAttributes, CancellationSignal, LayoutResultCallback, Bundle)} on * the UI thread (assuming onStart initializes resources needed for layout). * This will ensure that the UI does not change while you are laying out the * printed content. Then you can handle {@link #onWrite(PageRange[], ParcelFileDescriptor, * CancellationSignal, WriteResultCallback)} and {@link #onFinish()} on another * thread. This will ensure that the main thread is busy for a minimal amount of * time. Also this assumes that you will generate the printed content in * {@link #onLayout(PrintAttributes, PrintAttributes, CancellationSignal, * LayoutResultCallback, Bundle)} which is not mandatory. If you use multiple * threads, you are responsible for proper synchronization. * </p> */ public abstract class PrintDocumentAdapter { /** * Extra: mapped to a boolean value that is <code>true</code> if * the current layout is for a print preview, <code>false</code> otherwise. * This extra is provided in the {@link Bundle} argument of the {@link * #onLayout(PrintAttributes, PrintAttributes, CancellationSignal, * LayoutResultCallback, Bundle)} callback. * * @see #onLayout(PrintAttributes, PrintAttributes, CancellationSignal, * LayoutResultCallback, Bundle) */ public static final String EXTRA_PRINT_PREVIEW = "EXTRA_PRINT_PREVIEW"; /** * Called when printing starts. You can use this callback to allocate * resources. This method is invoked on the main thread. */ public void onStart() { /* do nothing - stub */ } /** * Called when the print attributes (page size, density, etc) changed * giving you a chance to layout the content such that it matches the * new constraints. This method is invoked on the main thread. * <p> * After you are done laying out, you <strong>must</strong> invoke: {@link * LayoutResultCallback#onLayoutFinished(PrintDocumentInfo, boolean)} with * the last argument <code>true</code> or <code>false</code> depending on * whether the layout changed the content or not, respectively; or {@link * LayoutResultCallback#onLayoutFailed(CharSequence)}, if an error occurred; * or {@link LayoutResultCallback#onLayoutCancelled()} if layout was * cancelled in a response to a cancellation request via the passed in * {@link CancellationSignal}. Note that you <strong>must</strong> call one of * the methods of the given callback for this method to be considered complete * which is you will not receive any calls to this adapter until the current * layout operation is complete by invoking a method on the callback instance. * The callback methods can be invoked from an arbitrary thread. * </p> * <p> * One of the arguments passed to this method is a {@link CancellationSignal} * which is used to propagate requests from the system to your application for * canceling the current layout operation. For example, a cancellation may be * requested if the user changes a print option that may affect layout while * you are performing a layout operation. In such a case the system will make * an attempt to cancel the current layout as another one will have to be performed. * Typically, you should register a cancellation callback in the cancellation * signal. The cancellation callback <strong>will not</strong> be made on the * main thread and can be registered as follows: * </p> * <pre> * cancellationSignal.setOnCancelListener(new OnCancelListener() { * @Override * public void onCancel() { * // Cancel layout * } * }); * </pre> * <p> * <strong>Note:</strong> If the content is large and a layout will be * performed, it is a good practice to schedule the work on a dedicated * thread and register an observer in the provided {@link * CancellationSignal} upon invocation of which you should stop the * layout. * </p> * * @param oldAttributes The old print attributes. * @param newAttributes The new print attributes. * @param cancellationSignal Signal for observing cancel layout requests. * @param callback Callback to inform the system for the layout result. * @param extras Additional information about how to layout the content. * * @see LayoutResultCallback * @see CancellationSignal * @see #EXTRA_PRINT_PREVIEW */ public abstract void onLayout(PrintAttributes oldAttributes, PrintAttributes newAttributes, CancellationSignal cancellationSignal, LayoutResultCallback callback, Bundle extras); /** * Called when specific pages of the content should be written in the * form of a PDF file to the given file descriptor. This method is invoked * on the main thread. *<p> * After you are done writing, you should close the file descriptor and * invoke {@link WriteResultCallback#onWriteFinished(PageRange[])}, if writing * completed successfully; or {@link WriteResultCallback#onWriteFailed( * CharSequence)}, if an error occurred; or {@link WriteResultCallback#onWriteCancelled()}, * if writing was cancelled in a response to a cancellation request via the passed * in {@link CancellationSignal}. Note that you <strong>must</strong> call one of * the methods of the given callback for this method to be considered complete which * is you will not receive any calls to this adapter until the current write * operation is complete by invoking a method on the callback instance. The callback * methods can be invoked from an arbitrary thread. * </p> * <p> * One of the arguments passed to this method is a {@link CancellationSignal} * which is used to propagate requests from the system to your application for * canceling the current write operation. For example, a cancellation may be * requested if the user changes a print option that may affect layout while * you are performing a write operation. In such a case the system will make * an attempt to cancel the current write as a layout will have to be performed * which then may be followed by a write. Typically, you should register a * cancellation callback in the cancellation signal. The cancellation callback * <strong>will not</strong> be made on the main thread and can be registered * as follows: * </p> * <pre> * cancellationSignal.setOnCancelListener(new OnCancelListener() { * @Override * public void onCancel() { * // Cancel write * } * }); * </pre> * <p> * <strong>Note:</strong> If the printed content is large, it is a good * practice to schedule writing it on a dedicated thread and register an * observer in the provided {@link CancellationSignal} upon invocation of * which you should stop writing. * </p> * * @param pages The pages whose content to print - non-overlapping in ascending order. * @param destination The destination file descriptor to which to write. * @param cancellationSignal Signal for observing cancel writing requests. * @param callback Callback to inform the system for the write result. * * @see WriteResultCallback * @see CancellationSignal */ public abstract void onWrite(PageRange[] pages, ParcelFileDescriptor destination, CancellationSignal cancellationSignal, WriteResultCallback callback); /** * Called when printing finishes. You can use this callback to release * resources acquired in {@link #onStart()}. This method is invoked on * the main thread. */ public void onFinish() { /* do nothing - stub */ } /** * Base class for implementing a callback for the result of {@link * PrintDocumentAdapter#onWrite(PageRange[], ParcelFileDescriptor, CancellationSignal, * WriteResultCallback)}. */ public static abstract class WriteResultCallback { /** * @hide */ @UnsupportedAppUsage public WriteResultCallback() { /* do nothing - hide constructor */ } /** * Notifies that all the data was written. * * @param pages The pages that were written. Cannot be <code>null</code> * or empty. <br /> * Returning {@link PageRange#ALL_PAGES} indicates that all pages that were * requested as the {@code pages} parameter in {@link #onWrite} were written. */ public void onWriteFinished(PageRange[] pages) { /* do nothing - stub */ } /** * Notifies that an error occurred while writing the data. * * @param error The <strong>localized</strong> error message. * shown to the user. May be <code>null</code> if error is unknown. */ public void onWriteFailed(CharSequence error) { /* do nothing - stub */ } /** * Notifies that write was cancelled as a result of a cancellation request. */ public void onWriteCancelled() { /* do nothing - stub */ } } /** * Base class for implementing a callback for the result of {@link * PrintDocumentAdapter#onLayout(PrintAttributes, PrintAttributes, * CancellationSignal, LayoutResultCallback, Bundle)}. */ public static abstract class LayoutResultCallback { /** * @hide */ @UnsupportedAppUsage public LayoutResultCallback() { /* do nothing - hide constructor */ } /** * Notifies that the layout finished and whether the content changed. * * @param info An info object describing the document. Cannot be <code>null</code>. * @param changed Whether the layout changed. * * @see PrintDocumentInfo */ public void onLayoutFinished(PrintDocumentInfo info, boolean changed) { /* do nothing - stub */ } /** * Notifies that an error occurred while laying out the document. * * @param error The <strong>localized</strong> error message. * shown to the user. May be <code>null</code> if error is unknown. */ public void onLayoutFailed(CharSequence error) { /* do nothing - stub */ } /** * Notifies that layout was cancelled as a result of a cancellation request. */ public void onLayoutCancelled() { /* do nothing - stub */ } } }