Android AsyncLayoutInflater 限制及改进

1、注意事项

For a layout to be inflated asynchronously it needs to have a parent whose generateLayoutParams(AttributeSet) is thread-safe and all the Views being constructed as part of inflation must not create any Handlers or otherwise call myLooper(). If the layout that is trying to be inflated cannot be constructed asynchronously for whatever reason, AsyncLayoutInflater will automatically fall back to inflating on the UI thread.

NOTE that the inflated View hierarchy is NOT added to the parent. It is equivalent to calling inflate(int, ViewGroup, boolean) with attachToRoot set to false. Callers will likely want to call addView(View) in the AsyncLayoutInflater.OnInflateFinishedListener callback at a minimum.

This inflater does not support setting a LayoutInflater.Factory nor LayoutInflater.Factory2. Similarly it does not support inflating layouts that contain fragments.

以上来自 AsyncLayoutInflater 的说明文档：

1.使用异步 inflate，那么需要这个 layout 的 parent 的 generateLayoutParams 函数是线程安全的；
2.所有构建的 View 中必须不能创建 Handler 或者是调用 Looper.myLooper；（因为是在异步线程中加载的，异步线程默认没有调用 Looper.prepare ）；
3.异步转换出来的 View 并没有被加到 parent view中，AsyncLayoutInflater 是调用了 LayoutInflater.inflate(int, ViewGroup, false)，因此如果需要加到 parent view 中，就需要我们自己手动添加；
4.AsyncLayoutInflater 不支持设置 LayoutInflater.Factory 或者 LayoutInflater.Factory2；
5.不支持加载包含 Fragment 的 layout；
6.如果 AsyncLayoutInflater 失败，那么会自动回退到UI线程来加载布局；

2、注意事项说明

以上注意事项2、3、6两项非常容易明白，下面分析下其余几项；

2.1 使用异步 inflate，那么需要这个 layout 的 parent 的 generateLayoutParams 函数是线程安全的；

我们看下 ViewGroup 中的 generateLayoutParams 方法

 /** * Returns a new set of layout parameters based on the supplied attributes set. * @param attrs the attributes to build the layout parameters from * @return an instance of {@link android.view.ViewGroup.LayoutParams} or one * of its descendants */public LayoutParams generateLayoutParams(AttributeSet attrs) {return new LayoutParams(getContext(), attrs);} 

generateLayoutParams 方法只是直接new了一个对象，因而非线程安全情况下创建多次而使用非同一个对象的情况。

2.2 AsyncLayoutInflater 不支持设置 LayoutInflater.Factory 或者 LayoutInflater.Factory2；

这个很好解释，因为 AsyncLayoutInflater 没有提供类似的Api，但是看过之前文章的小伙伴肯定知道这两个类是非常关键的，如果 AsyncLayoutInflater 不支持设置，那么有些情况下效果肯定是不一样的，使用了异步之后导致效果不一样岂不是很坑，下面我们再具体解决。

2.3 不支持加载包含 Fragment 的 layout；

前面的不支持三个字是不是让你心里一凉，其实这三个字不够准确，应该改为不完全支持。这一条要一篇文章的篇幅才能说明白，我们下篇文章再说哈。

3、可改进点

AsyncLayoutInflater 的代码并不多，而且代码质量也很高，所以其中可以优化的地方寥寥，简单说下我的看法：

1.AsyncLayoutInflater 只能通过回调的方式返回真正 Inflate 出来的View，但是假设一种场景，使用 AsyncLayoutInflater 去异步加载 Layout 和使用不是同一个类；
2.AsyncLayoutInflater 中不能 setFactory，这样通过 AsyncLayoutInflater 加载的布局是无法得到系统的兼容（例如 TextView 变为 AppCompatTextView）;
3.因为有任务排队机制，那么可能出现需要使用时任务仍然没有执行的场景，此时等待任务被执行还不如直接在主线程加载；

那么修改方案也很简单：

1.封装 AsyncLayoutInflater，修改调用方法，屏蔽不同类使用造成的影响；
2.直接在 AsyncLayoutInflater 的 Inflater 中进行相关设置；
3.在获取加载出来 View 的 Api 中做判断，如果当前任务没有被执行，则直接在 UI 线程加载；

4、封装

因为 AsyncLayoutInflater 是 final 的，因而不能使用继承，我们就将其 Copy 一份直接修改其中代码，修改点就是 针对章节3中可改进的地方。不多说，直接 Show The Code。

 /**
 * 实现异步加载布局的功能，修改点：
 * 
 * 1. super.onCreate之前调用没有了默认的Factory；
 * 2. 排队过多的优化；
 */
public class AsyncLayoutInflaterPlus {private static final String TAG = "AsyncLayoutInflaterPlus";private Handler mHandler;private LayoutInflater mInflater;private InflateRunnable mInflateRunnable;// 真正执行加载任务的线程池private static ExecutorService sExecutor = Executors.newFixedThreadPool(Math.max(2,Runtime.getRuntime().availableProcessors() - 2));// InflateRequest poolprivate static Pools.SynchronizedPool sRequestPool = new Pools.SynchronizedPool<>(10);private Future future;public AsyncLayoutInflaterPlus(@NonNull Context context) {mInflater = new AsyncLayoutInflaterPlus.BasicInflater(context);mHandler = new Handler(mHandlerCallback);}@UiThreadpublic void inflate(@LayoutRes int resid, @Nullable ViewGroup parent, @NonNull CountDownLatch countDownLatch,@NonNull AsyncLayoutInflaterPlus.OnInflateFinishedListener callback) {if (callback == null) {throw new NullPointerException("callback argument may not be null!");}AsyncLayoutInflaterPlus.InflateRequest request = obtainRequest();request.inflater = this;request.resid = resid;request.parent = parent;request.callback = callback;request.countDownLatch = countDownLatch;mInflateRunnable = new InflateRunnable(request);future = sExecutor.submit(mInflateRunnable);}public void cancel() {future.cancel(true);}/** * 判断这个任务是否已经开始执行 * * @return */public boolean isRunning() {return mInflateRunnable.isRunning();}private Handler.Callback mHandlerCallback = new Handler.Callback() {@Overridepublic boolean handleMessage(Message msg) {AsyncLayoutInflaterPlus.InflateRequest request = (AsyncLayoutInflaterPlus.InflateRequest) msg.obj;if (request.view == null) {request.view = mInflater.inflate(request.resid, request.parent, false);}request.callback.onInflateFinished(request.view, request.resid, request.parent);request.countDownLatch.countDown();releaseRequest(request);return true;}};public interface OnInflateFinishedListener {void onInflateFinished(View view, int resid, ViewGroup parent);}private class InflateRunnable implements Runnable {private InflateRequest request;private boolean isRunning;public InflateRunnable(InflateRequest request) {this.request = request;}@Overridepublic void run() {isRunning = true;try {request.view = request.inflater.mInflater.inflate(request.resid, request.parent, false);} catch (RuntimeException ex) {// Probably a Looper failure, retry on the UI threadLog.w(TAG, "Failed to inflate resource in the background! Retrying on the UI"+ " thread", ex);}Message.obtain(request.inflater.mHandler, 0, request).sendToTarget();}public boolean isRunning() {return isRunning;}}private static class InflateRequest {AsyncLayoutInflaterPlus inflater;ViewGroup parent;int resid;View view;AsyncLayoutInflaterPlus.OnInflateFinishedListener callback;CountDownLatch countDownLatch;InflateRequest() {}}private static class BasicInflater extends LayoutInflater {private static final String[] sClassPrefixList = {"android.widget.","android.webkit.","android.app."};BasicInflater(Context context) {super(context);if (context instanceof AppCompatActivity) {// 加上这些可以保证AppCompatActivity的情况下，super.onCreate之前// 使用AsyncLayoutInflater加载的布局也拥有默认的效果AppCompatDelegate appCompatDelegate = ((AppCompatActivity) context).getDelegate();if (appCompatDelegate instanceof LayoutInflater.Factory2) {LayoutInflaterCompat.setFactory2(this, (LayoutInflater.Factory2) appCompatDelegate);}}}@Overridepublic LayoutInflater cloneInContext(Context newContext) {return new AsyncLayoutInflaterPlus.BasicInflater(newContext);}@Overrideprotected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {for (String prefix : sClassPrefixList) {try {View view = createView(name, prefix, attrs);if (view != null) {return view;}} catch (ClassNotFoundException e) {// In this case we want to let the base class take a crack// at it.}}return super.onCreateView(name, attrs);}}public AsyncLayoutInflaterPlus.InflateRequest obtainRequest() {AsyncLayoutInflaterPlus.InflateRequest obj = sRequestPool.acquire();if (obj == null) {obj = new AsyncLayoutInflaterPlus.InflateRequest();}return obj;}public void releaseRequest(AsyncLayoutInflaterPlus.InflateRequest obj) {obj.callback = null;obj.inflater = null;obj.parent = null;obj.resid = 0;obj.view = null;sRequestPool.release(obj);}

} 

/**
 * 调用入口类；同时解决加载和获取View在不同类的场景
 */
public class AsyncLayoutLoader {private int mLayoutId;private View mRealView;private Context mContext;private ViewGroup mRootView;private CountDownLatch mCountDownLatch;private AsyncLayoutInflaterPlus mInflater;private static SparseArrayCompat sArrayCompat = new SparseArrayCompat();public static AsyncLayoutLoader getInstance(Context context) {return new AsyncLayoutLoader(context);}private AsyncLayoutLoader(Context context) {this.mContext = context;mCountDownLatch = new CountDownLatch(1);}@UiThreadpublic void inflate(@LayoutRes int resid, @Nullable ViewGroup parent) {inflate(resid, parent, null);}@UiThreadpublic void inflate(@LayoutRes int resid, @Nullable ViewGroup parent,AsyncLayoutInflaterPlus.OnInflateFinishedListener listener) {mRootView = parent;mLayoutId = resid;sArrayCompat.append(mLayoutId, this);if (listener == null) {listener = new AsyncLayoutInflaterPlus.OnInflateFinishedListener() {@Overridepublic void onInflateFinished(View view, int resid, ViewGroup parent) {mRealView = view;}};}mInflater = new AsyncLayoutInflaterPlus(mContext);mInflater.inflate(resid, parent, mCountDownLatch, listener);}/** * getLayoutLoader 和 getRealView 方法配对出现 * 用于加载和获取View在不同类的场景 * * @param resid * @return */public static AsyncLayoutLoader getLayoutLoader(int resid) {return sArrayCompat.get(resid);}/** * getLayoutLoader 和 getRealView 方法配对出现 * 用于加载和获取View在不同类的场景 * * @param resid * @return */public View getRealView() {if (mRealView == null && !mInflater.isRunning()) {mInflater.cancel();inflateSync();} else if (mRealView == null) {try {mCountDownLatch.await();} catch (InterruptedException e) {e.printStackTrace();}setLayoutParamByParent(mContext, mRootView, mLayoutId, mRealView);} else {setLayoutParamByParent(mContext, mRootView, mLayoutId, mRealView);}return mRealView;}/** * 根据Parent设置异步加载View的LayoutParamsView * * @param context * @param parent * @param layoutResId * @param view */private static void setLayoutParamByParent(Context context, ViewGroup parent, int layoutResId, View view) {if (parent == null) {return;}final XmlResourceParser parser = context.getResources().getLayout(layoutResId);try {final AttributeSet attrs = Xml.asAttributeSet(parser);ViewGroup.LayoutParams params = parent.generateLayoutParams(attrs);view.setLayoutParams(params);} catch (Exception e) {e.printStackTrace();} finally {parser.close();}}private void inflateSync() {mRealView = LayoutInflater.from(mContext).inflate(mLayoutId, mRootView, false);}

} 

5、总结

本文主要是分析 AsyncLayoutInflater 的使用注意事项，并对其中的限制进行了改进，此处不再累述。

下一篇文章我们一起探究下为什么 AsyncLayoutInflater 文档上写不支持包含 Fragment 标签的异步，以及真的不能异步吗？