一、首先我們先看下Activity是如何顯示View
平常我們在Activity的onCreate會調(diào)用setContentView(R.layout.xxx)，Activity啟動過程與window的源碼流程可參考
Activity Window WMS的源碼關(guān)系流程介紹
總結(jié)幾個步驟：
1、創(chuàng)建Activity：ActivityThread的performLaunchActivity函數(shù)中創(chuàng)建Activity后，調(diào)用Activity.attach函數(shù)
2、創(chuàng)建PhoneWindow：Activity.attach函數(shù)中創(chuàng)建與之關(guān)聯(lián)的PhoneWindow,PhoneWindow會創(chuàng)建DectorView。
3、添加視圖：setContentView添加到PhoneWindow的DectorView中
4、關(guān)聯(lián)PhoneWindow到WMS中：ActivityThread實(shí)行完performLaunchActivity，會handleResumeActivity，走到Activity的onResume，然后設(shè)置Activity的PhoneWindow的type類型：

final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
            .....
            if (r.window == null && !a.mFinished && willBeVisible) {
                r.window = r.activity.getWindow();
                View decor = r.window.getDecorView();
                decor.setVisibility(View.INVISIBLE);
                ViewManager wm = a.getWindowManager();
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
                l.softInputMode |= forwardBit;
              ...
            } 
}

PhoneWindow會創(chuàng)建DectorView，通過WindowManagerImpl-->WindowManagerGlobal-->ViewRootImpl-->(binder)WMS中的Session的addToDisplay函數(shù)，這樣視圖就會顯示出來。
我們看下l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;

WindowManager.java:

public interface WindowManager extends ViewManager {
/**
  * Window type: an application window that serves as the "base" window
  * of the overall application; all other application windows will
  * appear on top of it.
  * In multiuser systems shows only on the owning user's window.
*/
  public static final int TYPE_BASE_APPLICATION   = 1;
}

從英文翻譯來看，這個type是應(yīng)用程序的window類型。

二、Dialog顯示View：

AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
        builder.setMessage("Message部分");
        builder.setTitle("Title部分");
        builder.setView(R.layout.dialog_main);
        builder.setPositiveButton("確定", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                alertDialog.dismiss();
            }
        });
        builder.setNegativeButton("取消", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                alertDialog.dismiss();
            }
        });
        alertDialog = builder.create();
        alertDialog.show();

我們順著alertDialog.show()：

public AlertDialog create() {
            // Context has already been wrapped with the appropriate theme.
            final AlertDialog dialog = new AlertDialog(P.mContext, 0, false);
            P.apply(dialog.mAlert);
            dialog.setCancelable(P.mCancelable);
            if (P.mCancelable) {
                dialog.setCanceledOnTouchOutside(true);
            }
            dialog.setOnCancelListener(P.mOnCancelListener);
            dialog.setOnDismissListener(P.mOnDismissListener);
            if (P.mOnKeyListener != null) {
                dialog.setOnKeyListener(P.mOnKeyListener);
            }
            return dialog;
        }

其中final AlertDialog dialog = new AlertDialog(P.mContext, 0, false);

protected AlertDialog(Context context, boolean cancelable, OnCancelListener cancelListener) {
        this(context, 0);
        setCancelable(cancelable);
        setOnCancelListener(cancelListener);
    }
    AlertDialog(Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {
        super(context, createContextThemeWrapper ? resolveDialogTheme(context, themeResId) : 0,
                createContextThemeWrapper);
        mWindow.alwaysReadCloseOnTouchAttr();
        mAlert = new AlertController(getContext(), this, getWindow());
}

super是Dialog，接著看Dialog的構(gòu)造函數(shù)：

Dialog(@NonNull Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {
        ...
        mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        final Window w = new PhoneWindow(mContext);
        mWindow = w;
        w.setCallback(this);
        w.setOnWindowDismissedCallback(this);
        w.setWindowManager(mWindowManager, null, null);
        w.setGravity(Gravity.CENTER);
        mListenersHandler = new ListenersHandler(this);
    }

這邊會創(chuàng)建Dialog的PhoneWindow，我們看PhoneWindow的構(gòu)造函數(shù)

public PhoneWindow(Context context) {
        super(context);
        mLayoutInflater = LayoutInflater.from(context);
    }

父類Window的

public abstract class Window {
// The current window attributes.
    private final WindowManager.LayoutParams mWindowAttributes =
        new WindowManager.LayoutParams();
//默認(rèn)的PhoneWindow類型
public Window(Context context) {
        mContext = context;
        mFeatures = mLocalFeatures = getDefaultFeatures(context);
    }
}

看WindowManager.LayoutParams:

public LayoutParams() {
            super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
            type = TYPE_APPLICATION;
            format = PixelFormat.OPAQUE;
        }

這個type的默認(rèn)window類型是TYPE_APPLICATION:

      /**
         * Window type: a normal application window.  The {@link #token} must be
         * an Activity token identifying who the window belongs to.
         * In multiuser systems shows only on the owning user's window.
         */
        public static final int TYPE_APPLICATION        = 2;

這個Dialog的window就需要依賴Activity來顯示了。
三、直接使用系統(tǒng)級的Window類型，來添加View：

WindowManager.LayoutParams layoutParams = new WindowManager.LayoutParams();
layoutParams.type = WindowManager.LayoutParams.TYPE_SYSTEM_ERROR;
layoutParams.flags = WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN | WindowManager.LayoutParams.FLAG_FULLSCREEN;
layoutParams.screenOrientation = ActivityInfo.SCREEN_ORIENTATION_PORTRAIT;
layoutParams.gravity = Gravity.CENTER;
layoutParams.x = 0;
layoutParams.y = 0;
layoutParams.width = WindowManager.LayoutParams.MATCH_PARENT;
layoutParams.height = WindowManager.LayoutParams.MATCH_PARENT;
layoutParams.format = PixelFormat.TRANSPARENT;
LayoutInflater layoutInflater = (LayoutInflater) LayoutInflater.from(context);
View view = (View) layoutInflater.inflate(R.layout.window_test_main, null);
WindowManager windowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
windowManager.addView(view, layoutParams);

這個context上下文可以是service、application的。
其中l(wèi)ayoutParams.type = WindowManager.LayoutParams.TYPE_SYSTEM_ERROR;

/**
         * Window type: internal system error windows, appear on top of
         * everything they can.
         * In multiuser systems shows only on the owning user's window.
         */
        public static final int TYPE_SYSTEM_ERROR       = FIRST_SYSTEM_WINDOW+10;

這是系統(tǒng)級的window