Android源碼分析-setContentView加載佈局流程

上篇文章追溯了Android源碼中Activity的啓動流程，那麼Activity啓動之後，是如何加載佈局的呢？這篇文章我們繼續來追溯這一塊的Android源碼。

Activity->setContentView

public void setContentView(@LayoutRes int layoutResID) {
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
}

這裏調用了Window的setContentView方法，而Window是一個抽象類，PhoneWindow是其唯一派生子類，所以這裏調用的是PhoneWindow中的setContentView方法。

PhoneWindow->setContentView

@Override
public void setContentView(int layoutResID) {
    // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
    // decor, when theme attributes and the like are crystalized. Do not check the feature
    // before this happens.
    if (mContentParent == null) {////////////////////////////////////////////////// TAG1
        //創建DecorView，並添加到mContentParent上
        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 {
        //將佈局加載到mContentParent上
        mLayoutInflater.inflate(layoutResID, mContentParent);/////////////////////// TAG2
    }
    mContentParent.requestApplyInsets();
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
        //回調通知界面加載完畢
        cb.onContentChanged();
    }
    mContentParentExplicitlySet = true;
}

首先看標識TAG1處，這裏判斷mContentParent是否爲空，後面我們可以知道這個mContentParent表示的是DecorView中展示內容的佈局（除去標題欄）。到這一步，我們還沒有創建過DecorView，也就更不可能生成mContentParent了，所以這裏的判斷是爲true的。也就是會執行installDecor()方法。

Activity->installDecor

private void installDecor() {
    mForceDecorInstall = false;
    ///////////////////////////////////////////////////////////////////////// TAG3
    if (mDecor == null) {
        //如果mDecor爲空，創建一個DecorView賦給mDecor
        mDecor = generateDecor(-1);
        mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
        mDecor.setIsRootNamespace(true);
        if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
            mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
        }
    } else {
        mDecor.setWindow(this);
    }

    ///////////////////////////////////////////////////////////////////////// TAG4
    if (mContentParent == null) {
        //根據主題以及設置的FEATURE爲mDecor添加默認佈局
        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);
        //添加其他資源
        
        ......
        
        //設置轉場動畫
        if (hasFeature(FEATURE_ACTIVITY_TRANSITIONS)) {
            ......
        }
    }
}

TAG3處沒什麼好說的，就是創建一個DecorView賦給mDecor。
這裏我們重點看TAG4處新建mContentParent的過程，調用的是generateLayout(mDecor)方法。

PhoneWindow->generateLayout(DecorView decor)

protected ViewGroup generateLayout(DecorView decor) {
    // Apply data from current theme.
    //獲取當前設置的窗口主題
    //這也就是爲什麼我們要在setContentView之前調用requesetFeature的原因
    TypedArray a = getWindowStyle();
    
    ......//根據設置加載默認主題

    // Inflate the window decor.
    int layoutResource;
    int features = getLocalFeatures();
    // System.out.println("Features: 0x" + Integer.toHexString(features));
    // 根據用戶設置的Feature來設置DecorView的佈局資源
    if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
        layoutResource = R.layout.screen_swipe_dismiss;
        setCloseOnSwipeEnabled(true);
    } else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
        if (mIsFloating) {
            TypedValue res = new TypedValue();
            getContext().getTheme().resolveAttribute(
                    R.attr.dialogTitleIconsDecorLayout, res, true);
            layoutResource = res.resourceId;
        } else {
            layoutResource = R.layout.screen_title_icons;
        }
        // XXX Remove this once action bar supports these features.
        removeFeature(FEATURE_ACTION_BAR);
        // System.out.println("Title Icons!");
    } else if ......一系列的判斷

    mDecor.startChanging();
    //將符合配置的佈局創建並添加到DecorView中
    mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
    ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
    if (contentParent == null) {
        throw new RuntimeException("Window couldn't find content container view");
    }
    if ((features & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
        ProgressBar progress = getCircularProgressBar(false);
        if (progress != null) {
            progress.setIndeterminate(true);
        }
    }
    //右劃退出
    if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
        registerSwipeCallbacks(contentParent);
    }
    // Remaining setup -- of background and title -- that only applies
    // to top-level windows.
    // 設置背景和標題
    if (getContainer() == null) {
        ......
    }
    mDecor.finishChanging();
    return contentParent;
}

方法裏先是獲取了我們設置的窗口主題，並加載相應的主題。然後又根據我們設置的Feature來選擇相應的佈局資源，最後調用mDecor.onResourcesLoaded(mLayoutInflater, layoutResource)方法將佈局實例化並添加到DecorView上。

DecorView->onResourcesLoaded

void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
    if (mBackdropFrameRenderer != null) {
        loadBackgroundDrawablesIfNeeded();
        mBackdropFrameRenderer.onResourcesLoaded(
                this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable,
                mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState),
                getCurrentColor(mNavigationColorViewState));
    }
    mDecorCaptionView = createDecorCaptionView(inflater);
    final View root = inflater.inflate(layoutResource, null);
    if (mDecorCaptionView != null) {
        if (mDecorCaptionView.getParent() == null) {
            addView(mDecorCaptionView,
                    new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        }
        mDecorCaptionView.addView(root,
                new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
    } else {
        // Put it below the color views.
        addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    }
    mContentRoot = (ViewGroup) root;
    initializeElevation();
}

這裏通過inflater.inflate方法將佈局創建出來，然後通過addView將佈局添加到DecorView上，關於inflate的源碼後面會進行分析。
到這裏，TAG1處的installDecor方法流程就走完了。它創建了Activity的DecorView，設置了DecorView的主題以及佈局。
現在，回到PhoneWindow的setContentView方法，看TAG2處的代碼。這裏調用了mLayoutInflater.inflate(layoutResID, mContentParent)方法，將我們設置的佈局加載到mContentParent上。

LayoutInflater->inflate

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
    return inflate(resource, root, root != null);
}

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
    final Resources res = getContext().getResources();
    if (DEBUG) {
        Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                + Integer.toHexString(resource) + ")");
    }
    final XmlResourceParser parser = res.getLayout(resource);
    try {
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}

這裏通過res.getLayout(resource)方法對佈局資源進行了XML解析，將佈局資源解析成XmlResourceParser對象，然後調用同名的inflate方法進行下一步操作。

LayoutInflater->inflate

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
        final Context inflaterContext = mContext;
        final AttributeSet attrs = Xml.asAttributeSet(parser);
        Context lastContext = (Context) mConstructorArgs[0];
        mConstructorArgs[0] = inflaterContext;
        View result = root;
        try {
            // Look for the root node.
            int type;
            // 循環查找頂節點
            while ((type = parser.next()) != XmlPullParser.START_TAG &&
                    type != XmlPullParser.END_DOCUMENT) {
                // Empty
            }
            if (type != XmlPullParser.START_TAG) {
                throw new InflateException(parser.getPositionDescription()
                        + ": No start tag found!");
            }
            final String name = parser.getName();
            if (DEBUG) {
                System.out.println("**************************");
                System.out.println("Creating root view: "
                        + name);
                System.out.println("**************************");
            }
            if (TAG_MERGE.equals(name)) {
                if (root == null || !attachToRoot) {
                    throw new InflateException("<merge /> can be used only with a valid "
                            + "ViewGroup root and attachToRoot=true");
                }
                rInflate(parser, root, inflaterContext, attrs, false);
            } else {
                // Temp is the root view that was found in the xml
                // 先生成頂節點佈局
                final View temp = createViewFromTag(root, name, inflaterContext, attrs);
                ViewGroup.LayoutParams params = null;
                if (root != null) {
                    if (DEBUG) {
                        System.out.println("Creating params from root: " +
                                root);
                    }
                    // Create layout params that match root, if supplied
                    params = root.generateLayoutParams(attrs);
                    if (!attachToRoot) {
                        // Set the layout params for temp if we are not
                        // attaching. (If we are, we use addView, below)
                        temp.setLayoutParams(params);
                    }
                }
                if (DEBUG) {
                    System.out.println("-----> start inflating children");
                }
                // Inflate all children under temp against its context.
                // 實例化所有子View，以temp頂節點爲父佈局，遞歸生成View樹
                rInflateChildren(parser, temp, attrs, true);
                if (DEBUG) {
                    System.out.println("-----> done inflating children");
                }
                // We are supposed to attach all the views we found (int temp)
                // to root. Do that now.
                if (root != null && attachToRoot) {
                    // 將生成的View樹添加到root父佈局上，
                    // setContentView時這裏的root是mContentParent
                    root.addView(temp, params);
                }
                // Decide whether to return the root that was passed in or the
                // top view found in xml.
                if (root == null || !attachToRoot) {
                    // 如果root根視圖爲空，則返回在XML中找到的頂視圖
                    result = temp;
                }
            }
        } catch (XmlPullParserException e) {
            final InflateException ie = new InflateException(e.getMessage(), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } catch (Exception e) {
            final InflateException ie = new InflateException(parser.getPositionDescription()
                    + ": " + e.getMessage(), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } finally {
            // Don't retain static reference on context.
            mConstructorArgs[0] = lastContext;
            mConstructorArgs[1] = null;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        return result;
    }
}

先說下這個方法的整體流程，首先，循環上面解析好的佈局資源XmlResourceParser對象，取出佈局的根節點，調用createViewFromTag方法先生成根節點View。然後再調用rInflateChildren方法遞歸的生成整個佈局View樹。最後將生成的View樹添加到DecorView的root根佈局上。

先來看下createViewFromTag方法是如何生成View對象的。

LayoutInflater->createViewFromTag

private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
    return createViewFromTag(parent, name, context, attrs, false);
}

View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
        boolean ignoreThemeAttr) {
    if (name.equals("view")) {
        name = attrs.getAttributeValue(null, "class");
    }
    // Apply a theme wrapper, if allowed and one is specified.
    if (!ignoreThemeAttr) {
        final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
        final int themeResId = ta.getResourceId(0, 0);
        if (themeResId != 0) {
            context = new ContextThemeWrapper(context, themeResId);
        }
        ta.recycle();
    }
    if (name.equals(TAG_1995)) {
        // Let's party like it's 1995!
        return new BlinkLayout(context, attrs);
    }
    try {
        View view;
        if (mFactory2 != null) {
            view = mFactory2.onCreateView(parent, name, context, attrs);
        } else if (mFactory != null) {
            view = mFactory.onCreateView(name, context, attrs);
        } else {
            view = null;
        }
        if (view == null && mPrivateFactory != null) {
            view = mPrivateFactory.onCreateView(parent, name, context, attrs);
        }
        if (view == null) {
            final Object lastContext = mConstructorArgs[0];
            mConstructorArgs[0] = context;
            try {
                // 生成View對象
                if (-1 == name.indexOf('.')) {
                    // Android系統自帶View
                    view = onCreateView(parent, name, attrs);
                } else {
                    // 自定義View
                    view = createView(name, null, attrs);
                }
            } finally {
                mConstructorArgs[0] = lastContext;
            }
        }
        return view;
    } catch (InflateException e) {
        throw e;
    } catch (ClassNotFoundException e) {
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Error inflating class " + name, e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } catch (Exception e) {
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Error inflating class " + name, e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    }
}

這裏正常情況下，最後會調用onCreateView或createView方法來生成View對象，這裏的onCreateView方法中會爲View的節點名添加"android.view."前綴，然後再調用createView方法生成View對象。做這一步操作是因爲createView方法是通過反射生成的對象，所以需要完整的包名+類名，而Android系統自帶的View都是放在android.view包下。

LayoutInflater->createView

protected View onCreateView(View parent, String name, AttributeSet attrs)
        throws ClassNotFoundException {
    return onCreateView(name, attrs);
}

protected View onCreateView(String name, AttributeSet attrs)
        throws ClassNotFoundException {
    return createView(name, "android.view.", attrs);
}

public final View createView(String name, String prefix, AttributeSet attrs)
        throws ClassNotFoundException, InflateException {
     // 獲取靜態全局變量中是否有緩存相應的構造方法
    Constructor<? extends View> constructor = sConstructorMap.get(name);
    if (constructor != null && !verifyClassLoader(constructor)) {
        constructor = null;
        sConstructorMap.remove(name);
    }
    Class<? extends View> clazz = null;
    try {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
        if (constructor == null) {
            // Class not found in the cache, see if it's real, and try to add it
            // 通過反射機制獲取View的Class對象
            clazz = mContext.getClassLoader().loadClass(
                    prefix != null ? (prefix + name) : name).asSubclass(View.class);
            if (mFilter != null && clazz != null) {
                boolean allowed = mFilter.onLoadClass(clazz);
                if (!allowed) {
                    failNotAllowed(name, prefix, attrs);
                }
            }
            // 獲取構造方法，用於View的實例化
            constructor = clazz.getConstructor(mConstructorSignature);
            constructor.setAccessible(true);
            sConstructorMap.put(name, constructor);
        } else {
            // If we have a filter, apply it to cached constructor
            if (mFilter != null) {
                // Have we seen this name before?
                Boolean allowedState = mFilterMap.get(name);
                if (allowedState == null) {
                    // New class -- remember whether it is allowed
                    // 通過反射機制獲取View的Class對象
                    clazz = mContext.getClassLoader().loadClass(
                            prefix != null ? (prefix + name) : name).asSubclass(View.class);
                    boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                    mFilterMap.put(name, allowed);
                    if (!allowed) {
                        failNotAllowed(name, prefix, attrs);
                    }
                } else if (allowedState.equals(Boolean.FALSE)) {
                    failNotAllowed(name, prefix, attrs);
                }
            }
        }
        Object lastContext = mConstructorArgs[0];
        if (mConstructorArgs[0] == null) {
            // Fill in the context if not already within inflation.
            mConstructorArgs[0] = mContext;
        }
        Object[] args = mConstructorArgs;
        args[1] = attrs;
        // 實例化View對象
        final View view = constructor.newInstance(args);
        if (view instanceof ViewStub) {
            // Use the same context when inflating ViewStub later.
            final ViewStub viewStub = (ViewStub) view;
            // 如果是ViewStub 就爲其設置LayoutInflater 以便後續inflate
            viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
        }
        mConstructorArgs[0] = lastContext;
        return view;
    } catch (NoSuchMethodException e) {
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } catch (ClassCastException e) {
        // If loaded class is not a View subclass
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } catch (ClassNotFoundException e) {
        // If loadClass fails, we should propagate the exception.
        throw e;
    } catch (Exception e) {
        final InflateException ie = new InflateException(
                attrs.getPositionDescription() + ": Error inflating class "
                        + (clazz == null ? "<unknown>" : clazz.getName()), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

createView中先是通過ClassLoader的反射機制，獲取到對應的Class對象，然後獲取到它的構造方法。最後通過構造方法實例化這個View對象並返回。

這就是createViewFromTag生成View對象的流程。最後我們來看LayoutInflater->inflate中的rInflateChildren方法是如何生成View樹的。

LayoutInflater->rInflateChildren

final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
        boolean finishInflate) throws XmlPullParserException, IOException {
    rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}

void rInflate(XmlPullParser parser, View parent, Context context,
        AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
    final int depth = parser.getDepth();
    int type;
    boolean pendingRequestFocus = false;
    while (((type = parser.next()) != XmlPullParser.END_TAG ||
            parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
        if (type != XmlPullParser.START_TAG) {
            continue;
        }
        final String name = parser.getName();
        if (TAG_REQUEST_FOCUS.equals(name)) {
            pendingRequestFocus = true;
            consumeChildElements(parser);
        } else if (TAG_TAG.equals(name)) {
            parseViewTag(parser, parent, attrs);
        } else if (TAG_INCLUDE.equals(name)) {
            if (parser.getDepth() == 0) {
                throw new InflateException("<include /> cannot be the root element");
            }
            parseInclude(parser, context, parent, attrs);
        } else if (TAG_MERGE.equals(name)) {
            throw new InflateException("<merge /> must be the root element");
        } else {
            // 生成View對象
            final View view = createViewFromTag(parent, name, context, attrs);
            // 父佈局
            final ViewGroup viewGroup = (ViewGroup) parent;
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
            // 遞歸調用rInflateChildren，遞歸生成View樹
            rInflateChildren(parser, view, attrs, true);
            viewGroup.addView(view, params);
        }
    }
    if (pendingRequestFocus) {
        parent.restoreDefaultFocus();
    }
    if (finishInflate) {
        parent.onFinishInflate();
    }
}

可以看到，方法裏循環取出子節點。特殊節點這裏不做分析，我們看最後的else裏。
首先先調用上面已經分析過的createViewFromTag方法生成當前的節點View對象，然後這裏會把當前這個View作爲父節點遞歸調用rInflateChildren方法，如果當前View有子節點，就會遞歸的生成子節點並調用addView方法層層的插入對應的父節點。最終就會生成一個完整的View樹，整體的頁面佈局也就顯現出來了。