一、前言

Android 多屏互联的时代，必然会出现多屏连接的问题，通常意义上的多屏连接包括HDMI/USB、WifiDisplay，除此之外Android 还有OverlayDisplay和VirtualDisplay，其中VirtualDisplay相比不少人录屏的时候都会用到，在Android中他们都是Display，除了物理屏幕，你在OverlayDisplay和VirtualDisplay同样也可以展示弹窗或者展示Activity，所有的Dislplay的差异化通过DisplayManagerService进行了兼容，同样自己的密度和大小以及displayId。

需求

本篇主要解决副屏可插拔之后 Dialog 组建展示问题。存在副屏时，让 Dialog 展示在副屏上，如果不存在，就需要让它自动展示在主屏上。

为什么会有这种需求呢？默认情况下，实现双屏异显的时候， 通常不是使用Presentation就是Activity，然而，Dialog只能展示在主屏上，而Presentation只能展示的副屏上。想象一下这种双屏场景，在切换视频的时候，Loading展示应该是在主屏还是副屏呢 ？毫无疑问，答案当然是副屏。

问题

我们要解决的问题当然是随着场景的切换，Dialog展示在不同的屏幕上。同样，内容也可以这样展示，当存在副屏的时候在副屏上展示内容，当只有主屏的时候在主屏上展示内容。

二、方案

我们这里梳理一下两种方案。

方案：自定义Presentation

作为Presentation的核心点有两个，其中一个是displayId，另一个是WindowType，第一个是通常意义上指定Display Id，第二个是窗口类型，displayId是必须的参数，且不能和DefaultDisplay的id一样。但是WindowType是一个需要重点关注的事情。

早期的 TYPE_PRESENTATION 存在指纹信息 “被借用” 而造成用户资产损失的风险，即便外部无法获取，但是早期的Android 8.0版本利用 （TYPE_PRESENTATION=TYPE_APPLICATION_OVERLAY-1）可以实现屏幕外弹框，在之后的版本做了修复，同时对 TYPE_PRESENTATION 展示必须有 Token 等校验，但是在这种过程中，Presentation的WindowType 变了又变，因此，我们如何获取到兼容每个版本的WindowType呢？

自定义

方法当然是有的，我们不继承Presentation，而是继承Dialog因此自行实现可以参考 Presentation 中的代码，当然难点是 WindowManagerImpl 和WindowType类获取，前者 @hide 标注的，而后者不固定。

解决方式一：

早期我们可以利用 compileOnly layoutlib.jar 的方式倒入 WindowManagerImpl，但是新版本中 layoutlib.jar 中的类已经几乎被删，另外如果要使用 layoutlib.jar，那么你的项目中的 kotlin 版本就会和 layoutlib.jar 产生冲突，虽然可以删除相关的类，但是这种维护方式非常繁琐。

WindowType问题解决

我们知道，创建Presentation的时候，framework源码是设置了WindowType的，我们完全在我们自己的Dialog创建Presentation对象，读取出来设置上即可。

不过，我们先要对Display进行隔离，避免主屏走这段逻辑

WindowManager wm = (WindowManager) outerContext.getSystemService(WINDOW_SERVICE); 
if(display==null || wm.getDefaultDisplay().getDisplayId()==display.getDisplayId()){  
return; 

}

//注意，这里需要借助Presentation的一些属性，否则无法正常弹出弹框，要么有权限问题、要么有token问题

Presentation presentation = new Presentation(outerContext, display, theme);  

WindowManager.LayoutParams standardAttributes =presentation.getWindow().getAttributes();  
final Window w = getWindow(); 
final WindowManager.LayoutParams attr = w.getAttributes(); 

attr.token = standardAttributes.token; w.setAttributes(attr);
//type 源码中是TYPE_PRESENTATION，事实上每个版本是不一样的，因此这里动态获取 w.setGravity(Gravity.FILL);

w.setType(standardAttributes.type); 

WindowManagerImpl 问题

其实我们知道，Presentation的WindowManagerImpl并不是给自己用的，而是给Dialog上的其他组件（如Menu、PopWindow等），将其他组件加到Dialog的 Window上，当然你也可以通过另类方式实现Dialog，抛开通用性不谈的话。那么，其实如果我们没有Menu或者PopWindow，这里实际上是可以不处理的，但是作为一个完整的类，我们这里使用反射处理一下。

怎么处理呢？

我们知道，异显屏的Context是通过createDisplayContext创建的，但是我们这里并不是Hook这个方法，知识在创建这个Context之后，再通过ContextThemeWrapper，设置进去即可。

private static Context createPresentationContext(

      Context outerContext, Display display, int theme) {
if (outerContext == null) {
throw new IllegalArgumentException("outerContext must not be null");

   }
WindowManager outerWindowManager = (WindowManager) outerContext.getSystemService(WINDOW_SERVICE);
if (display == null || display.getDisplayId()==outerWindowManager.getDefaultDisplay().getDisplayId()) {
return outerContext;

   }
Context displayContext = outerContext.createDisplayContext(display);
if (theme == 0) {
TypedValue outValue = new TypedValue();

      displayContext.getTheme().resolveAttribute(

            android.R.attr.presentationTheme, outValue, true);

      theme = outValue.resourceId;

   }

// Derive the display's window manager from the outer window manager.
// We do this because the outer window manager have some extra information
// such as the parent window, which is important if the presentation uses
// an application window type.
//  final WindowManager outerWindowManager =
//        (WindowManager) outerContext.getSystemService(WINDOW_SERVICE);
//   final WindowManagerImpl displayWindowManager =
//         outerWindowManager.createPresentationWindowManager(displayContext);

WindowManager displayWindowManager = null;
try {
ClassLoader classLoader = ComplexPresentationV1.class.getClassLoader();

      Class<?> loadClass = classLoader.loadClass("android.view.WindowManagerImpl");
Method createPresentationWindowManager = loadClass.getDeclaredMethod("createPresentationWindowManager", Context.class);

      displayWindowManager = (WindowManager) loadClass.cast(createPresentationWindowManager.invoke(outerWindowManager,displayContext));

   } catch (ClassNotFoundException | NoSuchMethodException e) {

      e.printStackTrace();

   } catch (IllegalAccessException e) {

      e.printStackTrace();

   } catch (InvocationTargetException e) {

      e.printStackTrace();

   }
final WindowManager windowManager = displayWindowManager;
return new ContextThemeWrapper(displayContext, theme) {
@Override
public Object getSystemService(String name) {
if (WINDOW_SERVICE.equals(name)) {
return windowManager;

         }
return super.getSystemService(name);

      }

   };

}

全部源码

public class ComplexPresentationV1 extends Dialog  {

private static final String TAG = "ComplexPresentationV1";
private static final int MSG_CANCEL = 1;

private  Display mPresentationDisplay;
private  DisplayManager mDisplayManager;
/**

     * Creates a new presentation that is attached to the specified display

     * using the default theme.

     *

     * @param outerContext The context of the application that is showing the presentation.

     * The presentation will create its own context (see {@link #getContext()}) based

     * on this context and information about the associated display.

     * @param display The display to which the presentation should be attached.

     */
public ComplexPresentationV1(Context outerContext, Display display) {
this(outerContext, display, 0);

    }

/**

     * Creates a new presentation that is attached to the specified display

     * using the optionally specified theme.

     *

     * @param outerContext The context of the application that is showing the presentation.

     * The presentation will create its own context (see {@link #getContext()}) based

     * on this context and information about the associated display.

     * @param display The display to which the presentation should be attached.

     * @param theme A style resource describing the theme to use for the window.

     * See <a href="{@docRoot}guide/topics/resources/available-resources.html#stylesandthemes">

     * Style and Theme Resources</a> for more information about defining and using

     * styles.  This theme is applied on top of the current theme in

     * <var>outerContext</var>.  If 0, the default presentation theme will be used.

     */
public ComplexPresentationV1(Context outerContext, Display display, int theme) {
super(createPresentationContext(outerContext, display, theme), theme);
WindowManager wm = (WindowManager) outerContext.getSystemService(WINDOW_SERVICE);
if(display==null || wm.getDefaultDisplay().getDisplayId()==display.getDisplayId()){
return;

        }

        mPresentationDisplay = display;

        mDisplayManager = (DisplayManager)getContext().getSystemService(DISPLAY_SERVICE);

//注意，这里需要借助Presentation的一些属性，否则无法正常弹出弹框，要么有权限问题、要么有token问题
Presentation presentation = new Presentation(outerContext, display, theme);

        WindowManager.LayoutParams standardAttributes = presentation.getWindow().getAttributes();

final Window w = getWindow();
final WindowManager.LayoutParams attr = w.getAttributes();

        attr.token = standardAttributes.token;

        w.setAttributes(attr);

        w.setType(standardAttributes.type); 
//type 源码中是TYPE_PRESENTATION，事实上每个版本是不一样的，因此这里动态获取

        w.setGravity(Gravity.FILL);

        setCanceledOnTouchOutside(false);

    }

/**

     * Gets the {@link Display} that this presentation appears on.

     *

     * @return The display.

     */
public Display getDisplay() {
return mPresentationDisplay;

    }

/**

     * Gets the {@link Resources} that should be used to inflate the layout of this presentation.

     * This resources object has been configured according to the metrics of the

     * display that the presentation appears on.

     *

     * @return The presentation resources object.

     */
public Resources getResources() {
return getContext().getResources();

    }

@Override
protected void onStart() {
super.onStart();

if(mPresentationDisplay ==null){
return;

        }

        mDisplayManager.registerDisplayListener(mDisplayListener, mHandler);

// Since we were not watching for display changes until just now, there is a
// chance that the display metrics have changed.  If so, we will need to
// dismiss the presentation immediately.  This case is expected
// to be rare but surprising, so we'll write a log message about it.
if (!isConfigurationStillValid()) {

            Log.i(TAG, "Presentation is being dismissed because the "

                    + "display metrics have changed since it was created.");

            mHandler.sendEmptyMessage(MSG_CANCEL);

        }

    }

@Override
protected void onStop() {
if(mPresentationDisplay ==null){
return;

        }

        mDisplayManager.unregisterDisplayListener(mDisplayListener);
super.onStop();

    }

/**

     * Inherited from {@link Dialog#show}. Will throw

     * {@link android.view.WindowManager.InvalidDisplayException} if the specified secondary

     * {@link Display} can't be found.

     */
@Override
public void show() {
super.show();

    }

/**

     * Called by the system when the {@link Display} to which the presentation

     * is attached has been removed.

     *

     * The system automatically calls {@link #cancel} to dismiss the presentation

     * after sending this event.

     *

     * @see #getDisplay

     */
public void onDisplayRemoved() {

    }

/**

     * Called by the system when the properties of the {@link Display} to which

     * the presentation is attached have changed.

     *

     * If the display metrics have changed (for example, if the display has been

     * resized or rotated), then the system automatically calls

     * {@link #cancel} to dismiss the presentation.

     *

     * @see #getDisplay

     */
public void onDisplayChanged() {

    }

private void handleDisplayRemoved() {

        onDisplayRemoved();

        cancel();

    }

private void handleDisplayChanged() {

        onDisplayChanged();

// We currently do not support configuration changes for presentations
// (although we could add that feature with a bit more work).
// If the display metrics have changed in any way then the current configuration
// is invalid and the application must recreate the presentation to get
// a new context.
if (!isConfigurationStillValid()) {

            Log.i(TAG, "Presentation is being dismissed because the "

                    + "display metrics have changed since it was created.");

            cancel();

        }

    }

private boolean isConfigurationStillValid() {
if(mPresentationDisplay ==null){
return true;

        }
DisplayMetrics dm = new DisplayMetrics();

        mPresentationDisplay.getMetrics(dm);
try {
Method equalsPhysical = DisplayMetrics.class.getDeclaredMethod("equalsPhysical", DisplayMetrics.class);
return (boolean) equalsPhysical.invoke(dm,getResources().getDisplayMetrics());

        } catch (NoSuchMethodException e) {

            e.printStackTrace();

        } catch (IllegalAccessException e) {

            e.printStackTrace();

        } catch (InvocationTargetException e) {

            e.printStackTrace();

        }
return false;

    }

private static Context createPresentationContext(

            Context outerContext, Display display, int theme) {
if (outerContext == null) {
throw new IllegalArgumentException("outerContext must not be null");

        }
WindowManager outerWindowManager = (WindowManager) outerContext.getSystemService(WINDOW_SERVICE);
if (display == null || display.getDisplayId()==outerWindowManager.getDefaultDisplay().getDisplayId()) {
return outerContext;

        }
Context displayContext = outerContext.createDisplayContext(display);
if (theme == 0) {
TypedValue outValue = new TypedValue();

            displayContext.getTheme().resolveAttribute(

                    android.R.attr.presentationTheme, outValue, true);

            theme = outValue.resourceId;

        }

// Derive the display's window manager from the outer window manager.
// We do this because the outer window manager have some extra information
// such as the parent window, which is important if the presentation uses
// an application window type.
//  final WindowManager outerWindowManager =
//        (WindowManager) outerContext.getSystemService(WINDOW_SERVICE);
//   final WindowManagerImpl displayWindowManager =
//         outerWindowManager.createPresentationWindowManager(displayContext);

WindowManager displayWindowManager = null;
try {
ClassLoader classLoader = ComplexPresentationV1.class.getClassLoader();

            Class<?> loadClass = classLoader.loadClass("android.view.WindowManagerImpl");
Method createPresentationWindowManager = loadClass.getDeclaredMethod("createPresentationWindowManager", Context.class);

            displayWindowManager = (WindowManager) loadClass.cast(createPresentationWindowManager.invoke(outerWindowManager,displayContext));

        } catch (ClassNotFoundException | NoSuchMethodException e) {

            e.printStackTrace();

        } catch (IllegalAccessException e) {

            e.printStackTrace();

        } catch (InvocationTargetException e) {

            e.printStackTrace();

        }
final WindowManager windowManager = displayWindowManager;
return new ContextThemeWrapper(displayContext, theme) {
@Override
public Object getSystemService(String name) {
if (WINDOW_SERVICE.equals(name)) {
return windowManager;

                }
return super.getSystemService(name);

            }

        };

    }

private final DisplayManager.DisplayListener mDisplayListener = new DisplayManager.DisplayListener() {
@Override
public void onDisplayAdded(int displayId) {

        }

@Override
public void onDisplayRemoved(int displayId) {
if (displayId == mPresentationDisplay.getDisplayId()) {

                handleDisplayRemoved();

            }

        }

@Override
public void onDisplayChanged(int displayId) {
if (displayId == mPresentationDisplay.getDisplayId()) {

                handleDisplayChanged();

            }

        }

    };

private final Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_CANCEL:

                    cancel();
break;

            }

        }

    };

}

Delagate方式：

反射，利用反射本身就是一种方式，当然 android 9 开始，很多 @hide 反射不被允许，但是办法也是很多的，比如 freeflection 开源项目。

此外还有一个需要注意的是 Presentation 继承的是 Dialog 构造方法是无法被包外的子类使用，但是影响不大，我们在和Presentation的包名下创建我们的自己的Dialog依然可以解决。

这种方式借壳 Dialog，这种事只是套用 Dialog 一层，以动态代理方式实现，不过相比前一种方案来说，这种方案也有很多缺陷，比如他的onCreate\onShow\onStop\onAttachToWindow\onDetatchFromWindow等方法并没有完全和Dialog同步，需要做下兼容。

兼容

onAttachToWindow\onDetatchFromWindow

WindowManager wm = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
if (display != null && display.getDisplayId() != wm.getDefaultDisplay().getDisplayId()) {

    dialog = new Presentation(context, display, themeResId);

} else {

    dialog = new Dialog(context, themeResId);

}
//下面兼容attach和detatch问题

mDecorView = dialog.getWindow().getDecorView();

mDecorView.addOnAttachStateChangeListener(this);

onShow和\onStop

@Override

public void show() {

    if (!isCreate) {

        onCreate(null);
isCreate = true;

    }

    dialog.show();

    if (!isStart) {

        onStart();
isStart = true;

    }

}



@Override

public void dismiss() {

    dialog.dismiss();

    if (isStart) {

        onStop();
isStart = false;

    }

}

从兼容代码上来看，显然没有做到Dialog那种同步，因此只适合在单一线程中使用。

总结

本篇总结了2种异显屏弹窗，总体上来说，都有一定的瑕疵，但是第一种方案显然要好的多，主要是View更新上和可扩展上，当然第二种对于非多线程且不关注严格回调的需求，也是足以应付，在实际情况中，合适的才是最重要的。