在Activity或Fragment页面动态添加View，有其应用场景，比如配合运营在首页动态插入H5活动页（如下图手淘的雪花例示[1]）,在页面头部插入通知View等。本文结合ActivityLifecycleCallbacks[2]及DecorView使用，为类似需求提供一种解决方案。

方案概述

本文方案监听Activity生命周期，在拿到指定Activity后，获取PhoneWindow.mContentParent，并在mContentParent中addView。

监听Activity生命周期

在android API 14+ ,Application.registerActivityLifecycleCallbacks(ActivityLifecycleCallbacks callback)提供了一套回调方法，用于对Activity的生命周期事件进行集中处理。我们可用ActivityLifecycleCallbacks玩出很多花样，比如页面埋点，router预处理，限制Activity实例个数，swipeback等等。在本文它keep当前activity引用。

public interface ActivityLifecycleCallbacks {    void onActivityCreated(Activity activity, Bundle savedInstanceState);    void onActivityStarted(Activity activity);    void onActivityResumed(Activity activity);    void onActivityPaused(Activity activity);    void onActivityStopped(Activity activity);    void onActivitySaveInstanceState(Activity activity, Bundle outState);    void onActivityDestroyed(Activity activity);}

PhoneWindow.mContentParent获取

Android的页面组成[3]如下图（注:在sdk 14+或在19+AppCompat，mContentParent不包含Actionbar，既标题栏[4]），我们关心如何获取mContentParent。

在此简单例出mContentParent的相关代码

/**     *Activity.setContentView，此处的getWindow返回PhoneWindow,在     *activity.attach方法中实例化     **/    public void setContentView(@LayoutRes int layoutResID) {                    getWindow().setContentView(layoutResID);                    initWindowDecorActionBar();    }        /**     *PhoneWindow.setContentView     **/        public void setContentView(int layoutResID) {               ...               if (mContentParent == null) {                mContentParent = generateLayout(mDecor);                }                ...     }        /**     * mContentParent 从DecorView中ID为ID_ANDROID_CONTENT中赋值；ID_ANDROID_CONTENT在PhoneWindow中是一个int常量     * public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;     **/       protected ViewGroup generateLayout(DecorView decor) {     ...       ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);            if (contentParent == null) {                throw new RuntimeException("Window couldn't find content container view");            }      ...      return contentParent;    }  /** *Activity.findViewById **/  public View findViewById(@IdRes int id) {        return getWindow().findViewById(id);    }/** *PhoneWindow.findViewById**/public View findViewById(@IdRes int id) {        return getDecorView().findViewById(id);    }

从上述代码我们可以发现，mContentParent可以通过activity.findViewById(android.R.id.content)获取。

方案实现

此方案两个关键：keep Activity实例和获取id为android.R.id.content的View。下面列出核心代码。

public class Background implements Application.ActivityLifecycleCallbacks {    /**     * 用于检测当前APP是否运行于前台     */    private int appCount = 0;    private WeakReference
    
      mActivity;    @Override    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {    }    @Override    public void onActivityStarted(Activity activity) {        appCount++;    }    @Override    public void onActivityResumed(Activity activity) {        mActivity = new WeakReference<>(activity);    }    @Override    public void onActivityPaused(Activity activity) {    }    @Override    public void onActivityStopped(Activity activity) {        appCount--;    }    @Override    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {    }    @Override    public void onActivityDestroyed(Activity activity) {    }    /**     * 判断当前APP是否在后台     *     * @param context     * @return     */    public boolean inBackRunning(final Context context) {        PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);        boolean isScreenOn = true;        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT_WATCH) {            isScreenOn = pm.isInteractive();        } else {            isScreenOn = pm.isScreenOn();        }        boolean isOnForeground = appCount > 0;        return !isScreenOn || !isOnForeground;    }    /**     * 获取当前页面的Fragment     *     * @return     */    public Fragment getCurFragment() {        if (mActivity == null ||mActivity.get()==null|| !(mActivity.get() instanceof FragmentActivity)) {            return null;        }        FragmentManager fragManager = ((FragmentActivity) mActivity.get()).getSupportFragmentManager();        if (fragManager.getFragments() != null) {            List
     
       fragments = fragManager.getFragments();            for (Fragment fragment : fragments) {                if (fragment != null && fragment.isVisible())                    return fragment;            }            return null;        }        return null;    }    /**     * 获取当前运行的Activity     *     * @return     */    public Activity getCurActivity() {        return mActivity.get();    }}/** *  页面顶部显示通知View实例 `[5]` **/public class MessageBar extends FrameLayout {            private Runnable mDismissRunnable = new Runnable() {            @Override            public void run() {                dismiss();            }        };                private AppMessage mAppMessage = new AppMessage.Builder().build();            public MessageBar(Context context) {            super(context);        }            public MessageBar(Context context, AttributeSet attrs) {            super(context, attrs);        }            public MessageBar(Context context, AttributeSet attrs, int defStyle) {            super(context, attrs, defStyle);        }            public static MessageBar with(Context context) {            return new MessageBar(context);        }            public MessageBar setAppMessage(AppMessage appMessage) {            if (appMessage != null) {                mAppMessage = appMessage;            }            return this;        }            public void dismiss() {            finish();        }            private void finish() {            clearAnimation();            ViewGroup parent = (ViewGroup) getParent();            if (parent != null) {                parent.removeView(this);            }        }            /**         * Displays the {@link MessageBar} at the bottom of the         * {@link Activity} provided.         *         * @param targetActivity         */        public void show(Activity targetActivity) {            ViewGroup root = (ViewGroup) targetActivity.findViewById(android.R.id.content);            MarginLayoutParams params = init(targetActivity, root);            showInternal(targetActivity, params, root);            MediaUtil.getInstance(targetActivity).playSound(R.raw.im_notification, targetActivity);            VibratorUtil.vibrator(targetActivity);        }            private MarginLayoutParams init(Activity targetActivity, ViewGroup parent) {                FrameLayout layout = (FrameLayout) LayoutInflater.from(targetActivity)                    .inflate(R.layout.mercury_template, this, true);            customUI(layout);            MarginLayoutParams params;            params = createMarginLayoutParams(                    parent, LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);            return params;        }            private void customUI(FrameLayout layout) {             //TODO init custom view         }            private static MarginLayoutParams createMarginLayoutParams(ViewGroup viewGroup, int width, int height) {            if (viewGroup instanceof FrameLayout) {                LayoutParams params = new LayoutParams(width, height);                params.gravity = TOP;                return params;            } else if (viewGroup instanceof RelativeLayout) {                RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(width, height);                params.addRule(RelativeLayout.ALIGN_PARENT_TOP, RelativeLayout.TRUE);                return params;            } else if (viewGroup instanceof LinearLayout) {                LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(width, height);                params.gravity = TOP;                return params;            } else {                throw new IllegalStateException("Requires FrameLayout or RelativeLayout for the parent of Snackbar");            }        }                private void showInternal(Activity targetActivity, MarginLayoutParams params, ViewGroup parent) {                parent.removeView(this);                // We need to make sure the MessageBar elevation is at least as high as            // any other child views, or it will be displayed underneath them            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {                for (int i = 0; i < parent.getChildCount(); i++) {                    View otherChild = parent.getChildAt(i);                    float elvation = otherChild.getElevation();                    if (elvation > getElevation()) {                        setElevation(elvation);                    }                }            }            parent.addView(this, params);                bringToFront();                // As requested in the documentation for bringToFront()            if (Build.VERSION.SDK_INT < Build.VERSION_CODES.KITKAT) {                parent.requestLayout();                parent.invalidate();            }            focusForAccessibility(this);            startTimer();            }            private void startTimer() {            postDelayed(mDismissRunnable, 3000);        }                private void focusForAccessibility(View view) {            final AccessibilityEvent event = AccessibilityEvent.obtain(AccessibilityEvent.TYPE_VIEW_FOCUSED);                AccessibilityEventCompat.asRecord(event).setSource(view);            try {                view.sendAccessibilityEventUnchecked(event);            } catch (IllegalStateException e) {                // accessibility is off.            }        }            }
     
    

参考

[1] 利用动态加载实现手机淘宝的节日特效：