记UWP开发——多线程操作/并发操作中的坑

一切都要从新版风车动漫UWP的图片缓存功能说起。

起因便是风车动漫官网的番剧更新都很慢，所以图片更新也非常慢。在开发新版的过程中，我很简单就想到了图片多次重复下载导致的资源浪费问题。

所以我给app加了一个缓存机制：

创建一个用户控件CoverView，将首页GridView.ItemTemplate里的Image全部换成CoverView

CoverView一旦接到ImageUrl的修改，就会自动向后台的PictureHelper申请指定Url的图片

PictureHelper会先判断本地是否有这个Url的图片，没有的话从风车动漫官网下载一份，保存到本地，然后返回给CoverView

关键就是PictureHelper的GetImageAsync方法

本地缓存图片的代码片段：

    //缓存文件名以MD5的形式保存在本地
    string name = StringHelper.MD5Encrypt16(Url);


    if (imageFolder == null)
        imageFolder = await cacheFolder.CreateFolderAsync("imagecache", CreationCollisionOption.OpenIfExists);
    StorageFile file;
    IRandomAccessStream stream = null;
    if (File.Exists(imageFolder.Path + "\" + name))
    {
        file = await imageFolder.GetFileAsync(name);
        stream = await file.OpenReadAsync();
    }

    //文件不存在or文件为空，通过http下载
    if (stream == null || stream.Size == 0)
    {
        file = await imageFolder.CreateFileAsync(name, CreationCollisionOption.ReplaceExisting);
        stream = await file.OpenAsync(FileAccessMode.ReadWrite);
        IBuffer buffer = await HttpHelper.GetBufferAsync(Url);
        await stream.WriteAsync(buffer);
    }
    
    //...

嗯...一切都看似很美好....

但是运行之后，发现了一个很严重的偶发Exception

查阅google良久后，得知了发生这个问题的原因：

主页GridView一次性加载了几十个Item后，几十个Item中的CoverView同时调用了PictureHelper的GetImageAsync方法

几十个PictureHelper的GetImageAsync方法又同时访问缓存文件夹，导致了非常严重的IO锁死问题，进而引发了大量的UnauthorizedAccessException

有=又查阅了许久之后，终于找到了解决方法：

SemaphoreSlim异步锁

使用方法如下：

        private static SemaphoreSlim asyncLock = new SemaphoreSlim(1);//1：信号容量，即最多几个异步线程一起执行，保守起见设为1

        public async static Task<WriteableBitmap> GetImageAsync(string Url)
        {
            if (Url == null)
                return null;
            try
            {
                await asyncLock.WaitAsync();

                //缓存文件名以MD5的形式保存在本地
                string name = StringHelper.MD5Encrypt16(Url);


                if (imageFolder == null)
                    imageFolder = await cacheFolder.CreateFolderAsync("imagecache", CreationCollisionOption.OpenIfExists);
                StorageFile file;
                IRandomAccessStream stream = null;
                if (File.Exists(imageFolder.Path + "\" + name))
                {
                    file = await imageFolder.GetFileAsync(name);
                    stream = await file.OpenReadAsync();
                }

                //文件不存在or文件为空，通过http下载
                if (stream == null || stream.Size == 0)
                {
                    file = await imageFolder.CreateFileAsync(name, CreationCollisionOption.ReplaceExisting);
                    stream = await file.OpenAsync(FileAccessMode.ReadWrite);
                    IBuffer buffer = await HttpHelper.GetBufferAsync(Url);
                    await stream.WriteAsync(buffer);
                }

                //...

            }
            catch(Exception error)
            {
                Debug.WriteLine("Cache image error:" + error.Message);
                return null;
            }
            finally
            {
                asyncLock.Release();
            }
        }
    

成功解决了并发访问IO的问题

但是在接下来的Stream转WriteableBitmap的过程中，问题又来了....

这个问题比较好解决

                BitmapDecoder bitmapDecoder = await BitmapDecoder.CreateAsync(stream);
                WriteableBitmap bitmap = null;
                await Window.Current.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, async delegate
                {
                    bitmap = new WriteableBitmap((int)bitmapDecoder.PixelWidth, (int)bitmapDecoder.PixelHeight);
                    stream.Seek(0);
                    await bitmap.SetSourceAsync(stream);
                });
                stream.Dispose();
                return bitmap;

使用UI线程来跑就ok了

然后！问题又来了

WriteableBitmap到被return为止，都很正常

但是到接下来，我在CoverView里做其他一些bitmap的操作时，出现了下面这个问题

又找了好久，最后回到bitmap的PixelBuffer一看，擦，全是空的？

虽然bitmap成功的new了出来，PixelHeight/Width啥的都有了，当时UI线程中的SetSourceAsync压根没执行完，所以出现了内存保护的神奇问题

明明await了啊？

最后使用这样一个奇技淫巧，最终成功完成

                BitmapDecoder bitmapDecoder = await BitmapDecoder.CreateAsync(stream);
                WriteableBitmap bitmap = null;
                TaskCompletionSource<bool> task = new TaskCompletionSource<bool>();
                await Window.Current.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, async delegate
                {
                    bitmap = new WriteableBitmap((int)bitmapDecoder.PixelWidth, (int)bitmapDecoder.PixelHeight);
                    stream.Seek(0);
                    await bitmap.SetSourceAsync(stream);
                    task.SetResult(true);
                });
                await task.Task;

关于TaskCompletionSource，请参阅

https://www.cnblogs.com/loyieking/p/9209476.html

最后总算是完成了....

        public async static Task<WriteableBitmap> GetImageAsync(string Url)
        {
            if (Url == null)
                return null;
            try
            {
                await asyncLock.WaitAsync();

                //缓存文件名以MD5的形式保存在本地
                string name = StringHelper.MD5Encrypt16(Url);


                if (imageFolder == null)
                    imageFolder = await cacheFolder.CreateFolderAsync("imagecache", CreationCollisionOption.OpenIfExists);
                StorageFile file;
                IRandomAccessStream stream = null;
                if (File.Exists(imageFolder.Path + "\" + name))
                {
                    file = await imageFolder.GetFileAsync(name);
                    stream = await file.OpenReadAsync();
                }

                //文件不存在or文件为空，通过http下载
                if (stream == null || stream.Size == 0)
                {
                    file = await imageFolder.CreateFileAsync(name, CreationCollisionOption.ReplaceExisting);
                    stream = await file.OpenAsync(FileAccessMode.ReadWrite);
                    IBuffer buffer = await HttpHelper.GetBufferAsync(Url);
                    await stream.WriteAsync(buffer);
                }

                BitmapDecoder bitmapDecoder = await BitmapDecoder.CreateAsync(stream);
                WriteableBitmap bitmap = null;
                TaskCompletionSource<bool> task = new TaskCompletionSource<bool>();
                await Window.Current.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, async delegate
                {
                    bitmap = new WriteableBitmap((int)bitmapDecoder.PixelWidth, (int)bitmapDecoder.PixelHeight);
                    stream.Seek(0);
                    await bitmap.SetSourceAsync(stream);
                    task.SetResult(true);
                });
                await task.Task;
                stream.Dispose();
                return bitmap;

            }
            catch(Exception error)
            {
                Debug.WriteLine("Cache image error:" + error.Message);
                return null;
            }
            finally
            {
                asyncLock.Release();
            }
        }