如何卸载 Image 对象上显示的图像以释放内存？

问题描述

我正在开发一个 Xamarin.Forms Android 项目，只需按一下按钮即可显示一个接一个的图像。我当前的测试设备是 HTC Nexus 9，显然，由于规格有限，我在尝试加载第 17 张图片时出现以下错误：

java.lang.OutOfMemoryError: Failed to allocate a 11059212 byte allocation with 8286168 free bytes and 7MB until OOM
    at dalvik.system.vmruntime.newNonMovableArray(Native Method)
    at android.graphics.BitmapFactory.nativeDecodeAsset(Native Method)
    at android.graphics.BitmapFactory.decodeStream(BitmapFactory.java:620)
    at android.graphics.BitmapFactory.decodeResourceStream(BitmapFactory.java:455)
    at android.graphics.drawable.Drawable.createFromresourceStream(Drawable.java:1155)
    at android.content.res.ResourcesImpl.loadDrawableForCookie(ResourcesImpl.java:720)
    at android.content.res.ResourcesImpl.loadDrawable(ResourcesImpl.java:571)
    at android.content.res.Resources.getDrawable(Resources.java:771)
    at android.content.Context.getDrawable(Context.java:525)
    at androidx.core.content.ContextCompat.getDrawable(ContextCompat.java:455)
    at androidx.appcompat.widget.ResourceManagerInternal.getDrawable(ResourceManagerInternal.java:144)
    at androidx.appcompat.widget.ResourceManagerInternal.getDrawable(ResourceManagerInternal.java:132)
    at androidx.appcompat.content.res.AppCompatResources.getDrawable(AppCompatResources.java:104)
    at crc64ee486da937c010f4.ButtonRenderer.n_onClick(Native Method)
    at crc64ee486da937c010f4.ButtonRenderer.onClick(ButtonRenderer.java:104)
    at android.view.View.performClick(View.java:5637)
    at android.view.View$PerformClick.run(View.java:22433)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6229)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:891)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:781)

我尝试在 Java Max Heap Size 下将 1G 设置为 .Android > Properties > Android Options > Advanced，但没有任何改变。

我找到了一个 potential solution here，但显然，它适用于基于 Java 的 Android 应用程序，我不知道在 Xamarin.Forms 的 Android 项目上哪里可以找到类似的清单文件。

以下是我使用最简单的 MVVM 实现的代码片段：

.xaml

<Image Source="{Binding ImgSource}"
       WidthRequest="960"
       HeightRequest="720" />
       

<!--#region Page1 Controls-->
<Button Command="{Binding Page1DhcpIconCommand}"
        IsVisible="{Binding Page1ControlVisible}"
        BackgroundColor="{Binding Page1ControlColor}"
        Opacity="0.25"
        RelativeLayout.XConstraint="57"
        RelativeLayout.YConstraint="679"
        RelativeLayout.WidthConstraint="46"
        RelativeLayout.HeightConstraint="37" />
<!--#endregion-->

<!--#region Page2 Controls-->
<Button Command="{Binding Page2RolesCommand}"
        IsVisible="{Binding Page2ControlVisible}"
        BackgroundColor="{Binding Page2ControlColor}"
        Opacity="0.25"
        RelativeLayout.XConstraint="24"
        RelativeLayout.YConstraint="85"
        RelativeLayout.WidthConstraint="46"
        RelativeLayout.HeightConstraint="15" />
<!--#endregion-->

<!--#region Page3 Controls-->
<Button Command="{Binding Page3AddRolesCommand}"
        IsVisible="{Binding Page3ControlVisible}"
        BackgroundColor="{Binding Page3ControlColor}"
        Opacity="0.25"
        RelativeLayout.XConstraint="720"
        RelativeLayout.YConstraint="199"
        RelativeLayout.WidthConstraint="71"
        RelativeLayout.HeightConstraint="21" />
<!--#endregion-->

viewmodel.cs

#region backing Fields
string imgSource;

bool page1ControlVisible;
string page1ControlColor;

bool page2ControlVisible;
string page2ControlColor;

bool page3ControlVisible;
string page3ControlColor;
#endregion
#region Private Variables
private readonly bool _transparentOverride;
#endregion
#region Public Properties
public string ImgSource
{
    get => imgSource;
    set => SetProperty(ref imgSource,value);
}

#region Page1 Properties & Commands
public bool Page1ControlVisible
{
    get => page1ControlVisible;
    set => SetProperty(ref page1ControlVisible,value);
}
public string Page1ControlColor
{
    get => page1ControlColor;
    set => SetProperty(ref page1ControlColor,value);
}

public Command Page1DhcpIconCommand { get; }
#endregion

#region Page2 Properties & Commands
public bool Page2ControlVisible
{
    get => page2ControlVisible;
    set => SetProperty(ref page2ControlVisible,value);
}
public string Page2ControlColor
{
    get => page2ControlColor;
    set => SetProperty(ref page2ControlColor,value);
}

public Command Page2RolesCommand { get; }
#endregion

#region Page3 Properties & Commands
public bool Page3ControlVisible
{
    get => page3ControlVisible;
    set => SetProperty(ref page3ControlVisible,value);
}
public string Page3ControlColor
{
    get => page3ControlColor;
    set => SetProperty(ref page3ControlColor,value);
}

public Command Page3AddRolesCommand { get; }
#endregion
#endregion


public DHCPConfigurationviewmodel()
{
    _transparentOverride = true;

    ImgSource = "img_dhcp_config_01.jpg";

    #region Page1 Properties & Commands
    Page1ControlVisible = true;
    Page1ControlColor = _transparentOverride ? "Transparent" : "Red";

    Page1DhcpIconCommand = new Command(Page1DhcpIcon);
    #endregion

    #region Page2 Properties & Commands
    Page2ControlVisible = false;
    Page2ControlColor = _transparentOverride ? "Transparent" : "Orange";

    Page2RolesCommand = new Command(Page2Roles);
    #endregion

    #region Page3 Properties & Commands
    Page3ControlVisible = false;
    Page3ControlColor = _transparentOverride ? "Transparent" : "Yellow";

    Page3AddRolesCommand = new Command(Page3AddRoles);
    #endregion
}

private void Page1DhcpIcon()
{
    ImgSource = "img_dhcp_config_02.jpg";

    Page1ControlVisible = false;
    Page2ControlVisible = true;
}

private void Page2Roles()
{
    ImgSource = "img_dhcp_config_03.jpg";

    Page2ControlVisible = false;
    Page3ControlVisible = true;
}

private void Page3AddRoles()
{
    ImgSource = "img_dhcp_config_04.jpg";

    Page3ControlVisible = false;
    Page4ControlVisible = true;
}

解决方法

为此，您可以检查文档 Improve Xamarin.Forms App Performance。

特别部分：Optimize image resources：

显示图像资源可以大大增加应用程序的内存占用。因此，它们应该只在需要时创建并且应该在应用程序不再需要时立即发布他们。例如，如果应用程序通过阅读来显示图像它的数据来自流，确保仅在流时创建流需要，并确保流不再被释放必需的。这可以通过在页面打开时创建流来实现创建，或者当 Page.Appearing 事件触发时，然后处理 Page.Disappearing 事件触发时的流。

使用 ImageSource.FromUri 方法下载要显示的图像时，通过确保 UriImageSource.CachingEnabled 属性设置为 [] 来缓存下载的图像。

您还可以尝试 nuget Xamarin.FFImageLoading.Forms，这是一个 xamarin 库，可在 Xamarin.Forms 上快速轻松地加载图像。更多详情，请查看：https://github.com/luberda-molinet/FFImageLoading .

android android image image xamarin.forms