package com.tomgibara.android.util;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.Callable;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.PriorityBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import android.graphics.Bitmap;
import android.net.Uri;
import android.os.Handler;
import android.os.Process;
import android.view.View;
import android.widget.Adapter;
import android.widget.AdapterView;
import android.widget.GridView;
import android.widget.ImageView;
/**
* <p>
* This excellent class was borrowed from Tom Gibara at:
* http://blog.tomgibara.com/post/7665158012/android-adapter-view-rendering
* Modified only to add this attribution. Thanks!
* </p>
*
* <p>
* Concrete extensions of this class are capable of rendering a multiplicity of
* Views on background threads. Interrupting the rendering of a View to start a
* new rendering is handled by this class. So too is rendering a view in
* multiple passes (initial passes are automatically prioritized over subsequent
* passes). Support for caching renderings is also available.
* <p>
*
* <p>
* This class is mainly intended to be used within
* {@link Adapter#getView(int, View, android.view.ViewGroup)} to handle all
* aspects of the asynchronous rendering of the views within an associated
* {@link AdapterView}.
* </p>
*
* <p>
* Example: A simple scenario would be a {@link GridView} containing
* {@link ImageView}s which are rendered by an instance of this class where
* <code>Param<code> is {@link Uri} and <code>Render</code> is {@link Bitmap}
* and the implementation loads bitmaps over HTTP for display in the view. The
* {@link GridView} would invoke the {@link ViewRenderer} via an {@link Adapter}
* that called the {@link #renderView(View, Object)} method on each call to its
* {@link Adapter#getView(int, View, android.view.ViewGroup)} method. Such an
* implementation might support two pass rendering, loading a low-res image on
* the first pass before loading a high-res image later.
* </p>
*
* <p>
* <strong>Note that this class requires a well defined equals method on the
* class that satisfies Param.</strong>
* </p>
*
* @author Tom Gibara
*
* @param <Param>
* the type of parameters that define renders
* @param <Render>
* the type of renders that are applied to views
*/
public abstract class ViewRenderer<Param, Render> {
// statics
// convenience method for testing equality
private static final boolean equal(Object a, Object b) {
if (a == b) return true;
if (a == null) return false;
if (b == null) return false;
return a.equals(b);
}
// shared default executor, should be good enough in the common case
// and avoids thread proliferation in casual use
private static Executor sDefaultExecutor = null;
// ThreadPoolExecutor is broken for priority queues (see 6539720) so we need to patch it.
// Also newTaskFor() was only introduced in API level 9, so we can't use that either
// instead we patch the submit method directly
private static class Executor extends ThreadPoolExecutor {
public Executor(int threadCount) {
// ThreadPoolExecutor can't be configured to grow the thread count without risking queue rejections
// so we take the easy way out and clamp it with a fixed number of threads.
// TODO introduce our own executor (or equivalent) that doesn't share this weakness
// TODO our own blocking queue which stitched-together lists of the same priority would be much more efficient
super(threadCount, threadCount, 0L, TimeUnit.MILLISECONDS, new PriorityBlockingQueue<Runnable>());
}
@Override
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
ComparableFuture<T> future = new ComparableFuture<T>(task);
execute(future);
return future;
}
@SuppressWarnings("unchecked")
private static class ComparableFuture<T> extends FutureTask<T> implements Comparable<ComparableFuture<T>> {
private final Comparable mComparable;
public ComparableFuture(Callable<T> callable) {
super(callable);
mComparable = (Comparable) callable;
}
@Override
public int compareTo(ComparableFuture<T> that) {
return this.mComparable.compareTo(that.mComparable);
}
}
}
// wraps access to the default executor to allow for lazy creation
private static synchronized Executor getDefaultExecutor() {
return sDefaultExecutor == null ? sDefaultExecutor = new Executor(1) : sDefaultExecutor;
}
// fields
private final Executor mExecutor;
private final boolean mMayInterruptIfRunning;
private final Handler mHandler;
private final int mInheritedThreadPriority;
private final int mPasses;
private volatile long mNextOrder = 0L;
private volatile boolean mStopping = false;
private final Cache mCache;
// constructors
/**
* Constructs a new {@link ViewRenderer} object that coordinates concurrent
* background rendering for display to views. It is primarily designed for
* use with {@link AdapterView} based views. Supplying zero for the thread
* count will cause the {@link ViewRenderer} to use a single thread that
* can be shared among other instances.
*
* @param maxThreadCount
* the number of threads that will be created to support
* background rendering, may be zero.
* @param passes
* the number of rendering passes required to complete the
* rendering required for a view
* @param mayInterruptIfRunning
* whether background rendering tasks can be interrupted if they
* become redundant
* @param cacheCapacity
* the maximum number of render
*
*/
public ViewRenderer(int threadCount, int passes, boolean mayInterruptIfRunning, int cacheCapacity) {
if (threadCount < 0) throw new IllegalArgumentException("negative thread count");
if (passes <= 0) throw new IllegalArgumentException("passes not positive");
if (cacheCapacity < 0) throw new IllegalArgumentException("negative cache capacity");
mExecutor = threadCount == 0 ? getDefaultExecutor() : new Executor(threadCount);
mPasses = passes;
mMayInterruptIfRunning = mayInterruptIfRunning;
mCache = cacheCapacity == 0 ? null : new Cache(cacheCapacity);
mHandler = new Handler();
mInheritedThreadPriority = Process.getThreadPriority(Process.myTid());
}
// methods
/**
* Causes a view to be rendered using the supplied parameters. The nature of
* the rendering performed is determined by the concrete subclass on which
* this method is called, but in all cases, the view is initialized (by a
* call to {@link #prepare(View, Object, int)} and a Render object created
* in {@link #render(Object, int)} (on a background thread) before being
* displayed (in a call to {@link #update(View, Object, int)}.
*
* @param view
* the view that will display the rendering
* @param param
* the parameters that define the rendering
*/
public void renderView(View view, Param param) {
if (mStopping) throw new IllegalStateException("stopped");
//check the parameters
if (view == null) throw new IllegalArgumentException("null view");
// see if there's a tag which will tell us about past rendering on this view
Task task = getTask(view);
if (task != null) {
if (equal(param, task.mParam)) {
// a task has already done this (or will)
// so there's nothing else to do
return;
} else {
// try to cancel the existing task
// this may save lots of work
task.removeFromView(view);
}
}
// try to obtain a task from the cache
if (mCache == null) {
task = null;
} else {
synchronized (mCache) {
task = mCache.get(param);
}
}
// create a task that will render and later update the view
if (task == null) task = new Task(param);
// associate the task with the view (and schedule it for rendering as necessary)
task.assignToView(view);
}
/**
* Instructs this renderer that any cached renders should be purged. This
* method may be useful for dealing with low memory conditions or situations
* where parameter equality is no longer valid.
*/
public void clearCache() {
mCache.clear();
}
/**
* Stops this renderer and makes it unusable for further rendering. A zero
* value for the timeout blocks indefinitely. A negative timeout value will
* cause the method to return immediately without waiting for background
* rendering operations to complete. Note that even if no timeout is set, or
* the timeout is exceeded, the renderer will not call
* {@link #update(View, Object, int)} or {@link #prepare(View, Object, int)}
* or otherwise modify a view at any time after the this method has been
* called.
*
* @param timeout
* the number of milliseconds for which to wait for rendering
* operations to terminate
*/
public void stop(long timeout) throws InterruptedException {
if (mStopping) return;
mStopping = true;
if (mExecutor != sDefaultExecutor) {
mExecutor.shutdown();
if (timeout == 0) {
timeout = Long.MAX_VALUE;
}
if (timeout >= 0) {
mExecutor.awaitTermination(timeout, TimeUnit.MILLISECONDS);
}
}
}
/**
* Stop the renderer without waiting for calls to
* {@link #render(Object, int)} to complete. This is a convenient way of
* calling {@link #stop(long)} without specifying a timeout.
*/
public void stop() {
try {
stop(-1L);
} catch (InterruptedException e) {
throw new IllegalStateException("Impossible: interrupted without waiting");
}
}
/**
* The thread priority assigned to the specified rendering pass. The default
* implementation returns {@link Process#THREAD_PRIORITY_BACKGROUND} for
* every pass.
*
* @param pass the rendering pass for which the priority is being requested
* @return a thread priority
* @see Process
*/
protected int getThreadPriority(int pass) {
return Process.THREAD_PRIORITY_BACKGROUND;
}
/**
* <p>
* Prepares a view for display in advance of being updated with its rendered
* content. A non-negative immediatePassHint indicates the update method
* will be called immediately after this call to prepare (this may enable
* some optimizations within the prepare method), otherwise the view will
* displayed to the visitor until the background rendering completes. This
* method should return quickly with all time-consuming operations being
* perfomed in one-or-more rendering passes.
* </p>
*
* <p>
* Implementations will typically prepare the supplied view to display a
* blank placeholder, or a "loading" indicator. Note that a view may be
* prepared several times without receiving a render if it is part of an
* {@link AdapterView} that is recycling its views.
* </p>
*
* @param view
* a view will be displayed to the user presently
* @param param
* defines the content that the view will display
* @param immediatePassHint
* the pass to which rendering has already progressed
*/
protected abstract void prepare(View view, Param param, int immediatePassHint);
/**
* Converts a Param into a Render for subsequent display in a view. The
* supplied pass parameter may be used to generate progressively more
* complete Renders. The first pass is zero. The number of passes is
* specified at construction time.
*
* @param param
* defines the rendering that needs to be produced
* @param pass
* which rendering pass is being performed
* @return a rendering of the supplied parameters
*/
//TODO consider supplying the previous Render for multi-pass rendering
protected abstract Render render(Param param, int pass);
/**
* Applies previously rendered content to a view.
*
* @param view
* a view that needs to update with rendered content
* @param render
* the content that is to be applied to the view
* @param pass
* the rendering pass that generated the rendered content
*/
protected abstract void update(View view, Render render, int pass);
/**
* Retrieves the last object that was associated with a view by the
* renderer. If no object has yet been associated with the view, null is
* returned.
*
* @see ViewRenderer.setTag
* @param view
* a rendered view
* @return the associated object or null
*/
protected Object getTag(View view) { return view.getTag(); }
/**
* Associates an object with the view. The default implementation simply
* uses the {@link View.setTag(Object)} method. This may interfere with some
* layouts, so using the {@link View.setTag(int,Object)} method is
* preferable (API level 4 and above). A conservative implementation may be
* to use a {@link WeakHashMap}.
*
* @param view
* a rendered view
* @param tag
* the object with which the view is to be tagged, may be null
*/
protected void setTag(View view, Object tag) { view.setTag(tag); }
// private utility methods
@SuppressWarnings("unchecked")
private Task getTask(View view) {
Object obj = getTag(view);
return obj == null || (obj instanceof ViewRenderer.Task) ? (Task) obj : null;
}
private void setTask(View view, Task task) {
setTag(view, task);
}
// inner classes
private class Task implements Runnable, Callable<Void>, Comparable<Task> {
// immutable fields for task
private final long mOrder = mNextOrder++;
private final Param mParam;
// only set/mutated on UI thread
// TODO look at strategy's for defraying cost of a hashset on each task
// note that this set also incurs the cost of repeated iterator creation
private final HashSet<View> mViews = new HashSet<View>();
private Future<Void> mFuture;
// only set on render thread
private int mPass = 0; // from the UI thread, this is actually 'the next pass'
private Render mRender = null;
private boolean mFailed = false;
public Task(Param param) {
mParam = param;
}
void assignToView(View view) {
// mPass is actually 'the next pass' at this point
int pass = mPass - 1;
// prepare the view for display while rendering
prepare(view, mParam, pass);
// update the view with whatever work we have already done
if (pass >= 0) update(view, mRender, pass);
//check if we have work left to do
if (mPass < mPasses) {
// add the view to our set
mViews.add(view);
// tag the view so that we can cancel the task later if we need to
setTask(view, this);
// all set, queue us up
enqueue();
}
}
void removeFromView(View view) {
// mViews can't be null because keep tags and set in sync
Task task = getTask(view);
if (task == this) setTask(view, null);
mViews.remove(view);
if (mViews.isEmpty()) {
cancel();
}
}
void removeFromViews() {
for (View view : mViews) {
Task task = getTask(view);
if (task == this) setTask(view, null);
}
mViews.clear();
cancel();
}
// called to apply render to view
@Override
public void run() {
if (mStopping) return;
if (mFailed) {
// render didn't complete (probably got cancelled)
// remove from the view (but we may still get cached)
removeFromViews();
}
// the views may be null/empty if we were removed from it while queued by the handler
if (mViews != null && !mViews.isEmpty()) {
try {
for (View view : mViews) {
// we are safe to update this view,
// if another task had been started on the view
// the view would have already have been removed from our set
update(view, mRender, mPass - 1);
}
} finally {
if (mPass < mPasses) {
// reschedule again if we need to perform more passes
enqueue();
} else {
// we're done, remove us from all our views
// we lose the ability to avoid doing the same rendering for the same view (very rare)
// but this is worth it, so that we can re-use the render from the cache (much more likely)
removeFromViews();
}
}
}
// try putting us into the cache
encache();
}
// called to produce render
@Override
public Void call() throws Exception {
if (mStopping) return null;
Process.setThreadPriority(getThreadPriority(mPass));
try {
Render render = render(mParam, mPass);
if (render == null) {
mFailed = true;
} else {
// record the render, we will apply to the view it cache it later
mRender = render;
// increment the pass now - since later calls aren't guaranteed to occur
mPass++;
}
mHandler.post(this);
return null;
} finally {
Process.setThreadPriority(mInheritedThreadPriority);
}
}
public int compareTo(Task that) {
if (this == that) return 0;
if (this.mPass != that.mPass) return this.mPass < that.mPass ? -1 : 1;
if (this.mOrder != that.mOrder) return this.mOrder < that.mOrder ? -1 : 1;
return 0;
}
private void enqueue() {
mFailed = false;
mFuture = mExecutor.submit((Callable<Void>) this);
}
private void encache() {
if (mPass > 0 && mCache != null) {
synchronized (mCache) {
Task that = mCache.get(mParam);
if (that == null || this.mPass > that.mPass) {
mCache.put(mParam, this);
}
}
}
}
// may only be called when all views have been removed
private void cancel() {
mFuture.cancel(mMayInterruptIfRunning);
}
}
private class Cache extends LinkedHashMap<Param, Task> {
// serialization boilerplate
private static final long serialVersionUID = -5867267874566891476L;
private final int mCapacity;
public Cache(int capacity) {
super(capacity, 0.75f, true);
mCapacity = capacity;
}
@Override
protected boolean removeEldestEntry(Map.Entry<Param, Task> eldest) {
return size() >= mCapacity;
}
}
}
