环境

本文以androidx,API 28代码来进行分析setContentView流程
先看一下AppCompatActivity中的onCreate方法代码，如下：

   @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        final AppCompatDelegate delegate = getDelegate();
        delegate.installViewFactory();
        delegate.onCreate(savedInstanceState);
        if (delegate.applyDayNight() && mThemeId != 0) {
            // If DayNight has been applied, we need to re-apply the theme for
            // the changes to take effect. On API 23+, we should bypass
            // setTheme(), which will no-op if the theme ID is identical to the
            // current theme ID.
            if (Build.VERSION.SDK_INT >= 23) {
                onApplyThemeResource(getTheme(), mThemeId, false);
            } else {
                setTheme(mThemeId);
            }
        }
        super.onCreate(savedInstanceState);
    }

其中getDelegate方法返回的是AppCompatDelegate 的实现类AppCompatDelegateImpl

   @NonNull
    public AppCompatDelegate getDelegate() {
        if (mDelegate == null) {
            mDelegate = AppCompatDelegate.create(this, this);
        }
        return mDelegate;
    }

  public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {
        return new AppCompatDelegateImpl(activity, activity.getWindow(), callback);
    }

activity.getWindow()对应的Window为PhoneWindow，在AppCompatDelegateImpl的构造方法里，主要做的事情是初始化变量，给window设置回调和背景

private static final int[] sWindowBackgroundStyleable = {android.R.attr.windowBackground};

AppCompatDelegateImpl(Context context, Window window, AppCompatCallback callback) {
        mContext = context;
        mWindow = window;
        mAppCompatCallback = callback;

        mOriginalWindowCallback = mWindow.getCallback();
        if (mOriginalWindowCallback instanceof AppCompatWindowCallback) {
            throw new IllegalStateException(
                    "AppCompat has already installed itself into the Window");
        }
        mAppCompatWindowCallback = new AppCompatWindowCallback(mOriginalWindowCallback);
        // Now install the new callback
        mWindow.setCallback(mAppCompatWindowCallback);

        final TintTypedArray a = TintTypedArray.obtainStyledAttributes(
                context, null, sWindowBackgroundStyleable);
        final Drawable winBg = a.getDrawableIfKnown(0);
        if (winBg != null) {
            mWindow.setBackgroundDrawable(winBg);
        }
        a.recycle();
    }

delegate.installViewFactory()是为LayoutInflater设置Factory2，拦截监听View的创建，代码如下：

  public void installViewFactory() {
        LayoutInflater layoutInflater = LayoutInflater.from(mContext);
        if (layoutInflater.getFactory() == null) {
            LayoutInflaterCompat.setFactory2(layoutInflater, this);
        } else {
            if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImpl)) {
                Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"
                        + " so we can not install AppCompat's");
            }
        }
    }

  public static void setFactory2(@NonNull LayoutInflater inflater, @NonNull LayoutInflater.Factory2 factory) {
        inflater.setFactory2(factory);

        if (Build.VERSION.SDK_INT < 21) {
            final LayoutInflater.Factory f = inflater.getFactory();
            if (f instanceof LayoutInflater.Factory2) {
                // The merged factory is now set to getFactory(), but not getFactory2() (pre-v21).
                // We will now try and force set the merged factory to mFactory2
                forceSetFactory2(inflater, (LayoutInflater.Factory2) f);
            } else {
                // Else, we will force set the original wrapped Factory2
                forceSetFactory2(inflater, factory);
            }
        }
    }
    /**
     * Like {@link #setFactory}, but allows you to set a {@link Factory2}
     * interface.
     */
    public void setFactory2(Factory2 factory) {
        if (mFactorySet) {
            throw new IllegalStateException("A factory has already been set on this LayoutInflater");
        }
        if (factory == null) {
            throw new NullPointerException("Given factory can not be null");
        }
        mFactorySet = true;
        if (mFactory == null) {
            mFactory = mFactory2 = factory;
        } else {
            mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
        }
    }

从代码中可以看出setFactory2方法只能设置调用一次，否则会报new IllegalStateException("A factory has already been set on this LayoutInflater")的异常，如果我们需要自定义Factory，需要在super.onCreate之前执行,因为系统在super.onCreate中会判断Factory是否为null，如果为null则会默认设置了一个Factory，我们再重复设置就会报错了。onCreate方法中还有适配夜间模式的代码，此处不再展开分析。