在Activity 生命週期函式執行過程詳解中介紹了ActivityThread、Instrumentation、ActivityManagerService啟動activity的過程，本文主要介紹Activity從載入佈局檔案到顯示的過程。

SetContentView

在activity的onCreate()生命週期函式中，會呼叫setContentView()來設定我們的佈局檔案，先來了解下setContentView()函式

  public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);//呼叫了getWindow()
 

        initWindowDecorActionBar();
    }

間接呼叫了getWindow()的setContentView()函式，來看看getWindow()

 public Window getWindow() {
        return mWindow;
    }

來看看mWindow是什麼時候初始化的

 final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor) {
        attachBaseContext(context);
        ...
 

        mWindow = new PhoneWindow(this);
        ...
    }

在activity的attach()函式中初始化，是一個PhoneWindow，那接著而看看PhoneWindow的setContentView()方法

    @Override
    public void setContentView(int layoutResID) {

        // before this happens.
        if (mContentParent == null) {
            installDecor();
        } else
 
 if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

mContentParent為null時先呼叫 installDecor()，然後在通過mLayoutInflater.inflate(layoutResID, mContentParent);進行設定，先來看看installDecor()

private void installDecor() {
            //mDecor為null，進行初始化
        if (mDecor == null) {
            mDecor = generateDecor();
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        }
        //mContentParent為null，進行初始化
        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);

            // Set up decor part of UI to ignore fitsSystemWindows if appropriate.
            mDecor.makeOptionalFitsSystemWindows();

            final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
                    R.id.decor_content_parent);

            if (decorContentParent != null) {
                mDecorContentParent = decorContentParent;
                mDecorContentParent.setWindowCallback(getCallback());
                if (mDecorContentParent.getTitle() == null) {
                    mDecorContentParent.setWindowTitle(mTitle);
                }

                final int localFeatures = getLocalFeatures();
                for (int i = 0; i < FEATURE_MAX; i++) {
                    if ((localFeatures & (1 << i)) != 0) {
                        mDecorContentParent.initFeature(i);
                    }
                }

                mDecorContentParent.setUiOptions(mUiOptions);

                if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) != 0 ||
                        (mIconRes != 0 && !mDecorContentParent.hasIcon())) {
                    mDecorContentParent.setIcon(mIconRes);
                } else if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) == 0 &&
                        mIconRes == 0 && !mDecorContentParent.hasIcon()) {
                    mDecorContentParent.setIcon(
                            getContext().getPackageManager().getDefaultActivityIcon());
                    mResourcesSetFlags |= FLAG_RESOURCE_SET_ICON_FALLBACK;
                }
                if ((mResourcesSetFlags & FLAG_RESOURCE_SET_LOGO) != 0 ||
                        (mLogoRes != 0 && !mDecorContentParent.hasLogo())) {
                    mDecorContentParent.setLogo(mLogoRes);
                }


                PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
                if (!isDestroyed() && (st == null || st.menu == null) && !mIsStartingWindow) {
                    invalidatePanelMenu(FEATURE_ACTION_BAR);
                }
            } else {
               // mContentParent不為null
                mTitleView = (TextView)findViewById(R.id.title);
                if (mTitleView != null) {
                    mTitleView.setLayoutDirection(mDecor.getLayoutDirection());
                     // 不顯示標題的時候隱藏
                    if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
                        View titleContainer = findViewById(
                                R.id.title_container);
                        if (titleContainer != null) {
                            titleContainer.setVisibility(View.GONE);
                        } else {
                            mTitleView.setVisibility(View.GONE);
                        }
                        if (mContentParent instanceof FrameLayout) {
                            ((FrameLayout)mContentParent).setForeground(null);
                        }
                    } else {
                     // 設定標題
                        mTitleView.setText(mTitle);
                    }
                }
            }
        }
    }

DecorView為null時呼叫generateDecor()進行初始化DecorView，mContentParent為null時呼叫generateLayout(mDecor)初始化mContentParent，然後設定標題，若不顯示標題則隱藏。

先來看看generateDecor()

    protected DecorView generateDecor() {
        return new DecorView(getContext(), -1);
    }

只是進行初始化DecorView，接著看generateLayout(mDecor)

   protected ViewGroup generateLayout(DecorView decor) {
        //...省略程式碼
        //通過主題載入不同的佈局檔案
        View in = mLayoutInflater.inflate(layoutResource, null);
        //設定DecorView
        decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        mContentRoot = (ViewGroup) in;
        //獲取contentParent
        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
        //...省略程式碼
        return contentParent;

   }

我們再回到PhoneWindow的setContentView()函式中

  public void setContentView(int layoutResID) {

        if (mContentParent == null) {
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
            //佈局檔案被設定到ContentParent
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

通過轉載網上的一張圖片來說明

1.activity在onCreate()中間接呼叫PhoneWindow的setContentView()
2.PhoneWindow根據主題進行初始化DecorView以及title和ContentParent
3.將對應的佈局檔案設定到ContentParent中

在Activity 生命週期函式執行過程詳解中我們知道，View的顯示是在handleResumeActivity()中處理的，那麼我們接著看handleResumeActivity()

handleResumeActivity()

直接看handleResumeActivity()程式碼是如何展示view的

    final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume) {

             //省略部分程式碼

            if (r.window == null && !a.mFinished && willBeVisible) {
                //獲取PhoneWindow
                r.window = r.activity.getWindow();
                //獲取PhoneWindow的DecorView
                View decor = r.window.getDecorView();
                //DecorView設定為不顯示INVISIBLE
                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;
                if (a.mVisibleFromClient) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);
                }

                 //省略部分程式碼

                    if (r.activity.mVisibleFromClient) {
                        ViewManager wm = a.getWindowManager();
                        //將decorview加入到加入到ViewManager中
                        wm.updateViewLayout(decor, l);
                    }
                }
                r.activity.mVisibleFromServer = true;
                mNumVisibleActivities++;
                if (r.activity.mVisibleFromClient) {
                    //顯示view
                    r.activity.makeVisible();
                }
            }
    }

1.將DecorView設定為Invisible
2.將DecorView加入到ViewManger中
3.通過activity的makeVisible()進行顯示

來看看makeVisible()函式

   void makeVisible() {
        if (!mWindowAdded) {
            ViewManager wm = getWindowManager();
            wm.addView(mDecor, getWindow().getAttributes());
            mWindowAdded = true;
        }
        mDecor.setVisibility(View.VISIBLE);
    }

設定DecorView為visible，而呼叫setVisibility會間接觸發view的繪製流程，然後更新介面。
至於view的繪製流程以及ViewManager用單獨的文章介紹。