1. AppCompatDelegate 的 setContentView()

分析 Android 中的 View，我们先从进入应用的看到的的一个 View 入手，第一个 View 就是 通过 setContentView() 这个方法进行加载的。我们来看 setContentView() 的源码：

public void setContentView(@LayoutRes int layoutResID) {
    this.getDelegate().setContentView(layoutResID);
}

AppCompatActivity 中的 setContentView() 又调用了 AppCompatDelegate 中的 setContentView() 方法，那 AppCompatDelegate 是做什么的呢？

AppCompat 出现在 v7 包，它的作用是让 API 等级在 7 之上的设备也能使用 ActionBar，在 v7:21 之后，AppCompat 可以让 API 在 7 之上的设备使用 MD、ToolBar 等效果。之前的 ActionBarActivity 也被取代为 AppCompatActivity。但 AppCompatActivity 的内部回调是由 AppCompatDelegate 来实现的。

AppCompatDelegate 是一个抽象类，它的实现类是 AppCompatDelegateImpl，现在看 AppCompatDelegateImpl 中的 setContentView() 方法：

public void setContentView(int resId) {
    // 创建 DecorView，DecorView 是视图中的顶级 View
    this.ensureSubDecor();
    // 获取 DecorView 中的 content 部分
    ViewGroup contentParent = (ViewGroup)this.mSubDecor.findViewById(16908290);
    contentParent.removeAllViews();
    // 将我们编写的界面填充到 content 中
    LayoutInflater.from(this.mContext).inflate(resId, contentParent);
    this.mOriginalWindowCallback.onContentChanged();
}

2. DecorView

在 AppCompatDelegateImpl 的 setContentView() 中，通过 ensureSubDecor() 方法为视图创建 DecorView，

private void ensureSubDecor() {
    if (!this.mSubDecorInstalled) {
        // DecorView 不存在，调用 createSubDecor() 创建 DecorView
        this.mSubDecor = this.createSubDecor();
        CharSequence title = this.getTitle();
        if (!TextUtils.isEmpty(title)) {
            if (this.mDecorContentParent != null) {
                this.mDecorContentParent.setWindowTitle(title);
            } else if (this.peekSupportActionBar() != null) {
                this.peekSupportActionBar().setWindowTitle(title);
            } else if (this.mTitleView != null) {
                this.mTitleView.setText(title);
            }
        }

        this.applyFixedSizeWindow();
        this.onSubDecorInstalled(this.mSubDecor);
        this.mSubDecorInstalled = true;
        AppCompatDelegateImpl.PanelFeatureState st = this.getPanelState(0, false);
        if (!this.mIsDestroyed && (st == null || st.menu == null)) {
            this.invalidatePanelMenu(108);
        }
    }
}

private ViewGroup createSubDecor() {
    // 获取设置的主题属性
    TypedArray a = this.mContext.obtainStyledAttributes(styleable.AppCompatTheme);
    // 如果使用的主题不是 Theme.AppCompat，或者没又继承自 Theme.AppCompat，抛出异常。
    if (!a.hasValue(styleable.AppCompatTheme_windowActionBar)) {
        a.recycle();
        throw new IllegalStateException("You need to use a Theme.AppCompat theme (or descendant) with this activity.");
    } else {
        // 根据主题的属性进行设置
        if (a.getBoolean(styleable.AppCompatTheme_windowNoTitle, false)) {
            // 在 requestWindowFeature() 方法中
            // 设置 this.mWindowNoTitle = true
            this.requestWindowFeature(1);
        } else if (a.getBoolean(styleable.AppCompatTheme_windowActionBar, false)) {
            this.requestWindowFeature(108);
        }

        if (a.getBoolean(styleable.AppCompatTheme_windowActionBarOverlay, false)) {
            this.requestWindowFeature(109);
        }

        if (a.getBoolean(styleable.AppCompatTheme_windowActionModeOverlay, false)) {
            this.requestWindowFeature(10);
        }
    
        // 记录是否为浮动的主题
        this.mIsFloating = a.getBoolean(styleable.AppCompatTheme_android_windowIsFloating, false);
        a.recycle();
        this.mWindow.getDecorView();
        LayoutInflater inflater = LayoutInflater.from(this.mContext);
        ViewGroup subDecor = null;
        
        // 根据不同的设置，给 subDecor 填充内容
        if (!this.mWindowNoTitle) {
            if (this.mIsFloating) {
                // Dialog 的主题
                subDecor = (ViewGroup)inflater.inflate(layout.abc_dialog_title_material, (ViewGroup)null);
                this.mHasActionBar = this.mOverlayActionBar = false;
            } else if (this.mHasActionBar) {
                // 添加 ActionBar
                TypedValue outValue = new TypedValue();
                this.mContext.getTheme().resolveAttribute(attr.actionBarTheme, outValue, true);
                Object themedContext;
                if (outValue.resourceId != 0) {
                    themedContext = new ContextThemeWrapper(this.mContext, outValue.resourceId);
                } else {
                    themedContext = this.mContext;
                }

                subDecor = (ViewGroup)LayoutInflater.from((Context)themedContext).inflate(layout.abc_screen_toolbar, (ViewGroup)null);
                this.mDecorContentParent = (DecorContentParent)subDecor.findViewById(id.decor_content_parent);
                this.mDecorContentParent.setWindowCallback(this.getWindowCallback());
                if (this.mOverlayActionBar) {
                    this.mDecorContentParent.initFeature(109);
                }

                if (this.mFeatureProgress) {
                    this.mDecorContentParent.initFeature(2);
                }

                if (this.mFeatureIndeterminateProgress) {
                    this.mDecorContentParent.initFeature(5);
                }
            }
        } else {
            if (this.mOverlayActionMode) {
                subDecor = (ViewGroup)inflater.inflate(layout.abc_screen_simple_overlay_action_mode, (ViewGroup)null);
            } else {
                subDecor = (ViewGroup)inflater.inflate(layout.abc_screen_simple, (ViewGroup)null);
            }

            if (VERSION.SDK_INT >= 21) {
                ViewCompat.setOnApplyWindowInsetsListener(subDecor, new OnApplyWindowInsetsListener() {
                    public WindowInsetsCompat onApplyWindowInsets(View v, WindowInsetsCompat insets) {
                        int top = insets.getSystemWindowInsetTop();
                        int newTop = AppCompatDelegateImpl.this.updateStatusGuard(top);
                        if (top != newTop) {
                            insets = insets.replaceSystemWindowInsets(insets.getSystemWindowInsetLeft(), newTop, insets.getSystemWindowInsetRight(), insets.getSystemWindowInsetBottom());
                        }

                        return ViewCompat.onApplyWindowInsets(v, insets);
                    }
                });
            } else {
                ((FitWindowsViewGroup)subDecor).setOnFitSystemWindowsListener(new OnFitSystemWindowsListener() {
                    public void onFitSystemWindows(Rect insets) {
                        insets.top = AppCompatDelegateImpl.this.updateStatusGuard(insets.top);
                    }
                });
            }
        }

        // 把 DecorView 添加到 Window 上 并且返回 DecorView
        if (subDecor == null) {
            throw new IllegalArgumentException("AppCompat does not support the current theme features: { windowActionBar: " + this.mHasActionBar + ", windowActionBarOverlay: " + this.mOverlayActionBar + ", android:windowIsFloating: " + this.mIsFloating + ", windowActionModeOverlay: " + this.mOverlayActionMode + ", windowNoTitle: " + this.mWindowNoTitle + " }");
        } else {
            if (this.mDecorContentParent == null) {
                this.mTitleView = (TextView)subDecor.findViewById(id.title);
            }

            ViewUtils.makeOptionalFitsSystemWindows(subDecor);
            ContentFrameLayout contentView = (ContentFrameLayout)subDecor.findViewById(id.action_bar_activity_content);
            ViewGroup windowContentView = (ViewGroup)this.mWindow.findViewById(16908290);
            if (windowContentView != null) {
                while(windowContentView.getChildCount() > 0) {
                    View child = windowContentView.getChildAt(0);
                    windowContentView.removeViewAt(0);
                    contentView.addView(child);
                }

                windowContentView.setId(-1);
                contentView.setId(16908290);
                if (windowContentView instanceof FrameLayout) {
                    ((FrameLayout)windowContentView).setForeground((Drawable)null);
                }
            }
            
            // 把 DecorView 添加到 Window 上 
            this.mWindow.setContentView(subDecor);
            contentView.setAttachListener(new OnAttachListener() {
                public void onAttachedFromWindow() {
                }

                public void onDetachedFromWindow() {
                    AppCompatDelegateImpl.this.dismissPopups();
                }
            });
            return subDecor;
        }
    }
}

创建好 DecorView 之后，DecorView 会被添加到 Windows（实现类是 PhoneWindow） 中，然后返回 DecorView。
并且 DecorView 是视图的顶级容器，我们可以通过 Android Studio 的 Layout Inspector 来查看一个界面的 View Tree。

一个 DecorView 包含这一个纵向的 LinearLayout，LinearLayout 内部是一个 FrameLayout，FrameLayout 是一个包含了内容部分和标题部分的容器。

在 AppCompatDelegateImpl 中的 setContentView() 方法中还有一句：

ViewGroup contentParent = (ViewGroup)this.mSubDecor.findViewById(16908290);

这句代码得到的 contentParent 就是刚刚创建的 DecorView 中的 内容根部局（id/content (ContentFrameLayout)）。

ContentFrameLayout 是 FrameLayout 的子类，我们编写的 xml 布局就是被添加到它的内部。

然后查看为内容根布局添加视图的过程。

3. LayoutInflater 的 inflate()

inflate() 的代码如下：

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) + ")");
    }
    
    // 获取解析当前布局 xml 文件的 parser 对象
    final XmlResourceParser parser = res.getLayout(resource);
    try {
        // 调用 inflate() 方法，开始解析 xml 文件，并返回得到界面
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}

在上述方法中，会先获取解析 xml 文件的 parser 对象，然后调用另一个 infalte() 方法进行解析。

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 {
            // 通过 while 循环遍历 xml 中的节点，直到找到 root
            int type;
            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();

            // 如果是 merge 节点，执行 rInflate() 方法，按照层次递归的去实例化 xml 文件的子项
            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
                // 不是 merge 节点，就通过 tag 标签创建一个 view
                final View temp = createViewFromTag(root, name, inflaterContext, attrs);

                ViewGroup.LayoutParams params = null;

                if (root != null) {
                    // 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);
                    }
                }

                // Inflate all children under temp against its context.
                rInflateChildren(parser, temp, attrs, true);

                // We are supposed to attach all the views we found (int temp)
                // to root. Do that now.
                // 将创建的 View 添加到 root 视图中
                if (root != null && attachToRoot) {
                    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) {
                    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;
    }
}

在这个 inflate() 方法中，会先寻找 xml 文件中的起始节点，如果起始节点是 merge，就执行 rInflate() 方法，如果不是 merge，就执行 createViewFromTag() 方法去创建一个新的 View，最后把 View 添加到内容根部局中。

4. createViewFromTag()

现在看 createViewFromTag() 的源码：

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();
    }

    // 如果 name 的值为 blink，返回一个 BlinkLayout
    if (name.equals(TAG_1995)) {
        // Let's party like it's 1995!
        return new BlinkLayout(context, attrs);
    }

    try {
        View view;
        // 依次寻找合适的 Factory 对象去创建 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 {
                if (-1 == name.indexOf('.')) {
                    view = onCreateView(parent, name, attrs);
                } else {
                    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;
    }
}

我们看一看代码中的 mFactory2、mFactory、mPrivateFactory 是什么。

在 LayoutInflater.java 的属性中，有如下几个变量：

private Factory mFactory;
private Factory2 mFactory2;
private Factory2 mPrivateFactory;

Factory 是一个接口，Factory2 是继承了 Factory 的接口，它们都有个一个 onCreateView() 的方法。

5. onCreateView()

我们去看它们在 AppCompatDelegateImpl 中的实现：

public View createView(View parent, String name, @NonNull Context context, @NonNull AttributeSet attrs) {
    if (this.mAppCompatViewInflater == null) {
        TypedArray a = this.mContext.obtainStyledAttributes(styleable.AppCompatTheme);
        String viewInflaterClassName = a.getString(styleable.AppCompatTheme_viewInflaterClass);
        if (viewInflaterClassName != null && !AppCompatViewInflater.class.getName().equals(viewInflaterClassName)) {
            try {
                Class viewInflaterClass = Class.forName(viewInflaterClassName);
                this.mAppCompatViewInflater = (AppCompatViewInflater)viewInflaterClass.getDeclaredConstructor().newInstance();
            } catch (Throwable var8) {
                Log.i("AppCompatDelegate", "Failed to instantiate custom view inflater " + viewInflaterClassName + ". Falling back to default.", var8);
                this.mAppCompatViewInflater = new AppCompatViewInflater();
            }
        } else {
                this.mAppCompatViewInflater = new AppCompatViewInflater();
        }
    }

    boolean inheritContext = false;
    if (IS_PRE_LOLLIPOP) {
        inheritContext = attrs instanceof XmlPullParser ? ((XmlPullParser)attrs).getDepth() > 1 : this.shouldInheritContext((ViewParent)parent);
    }

    return this.mAppCompatViewInflater.createView(parent, name, context, attrs, inheritContext, IS_PRE_LOLLIPOP, true, VectorEnabledTintResources.shouldBeUsed());
}

方法的最后可以看出创建视图的工作交给了 AppCompatViewInflater 的 createView() 去完成

6. createView()

final View createView(View parent, String name, @NonNull Context context, @NonNull AttributeSet attrs, boolean inheritContext, boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
    Context originalContext = context;
    if (inheritContext && parent != null) {
        context = parent.getContext();
    }

    if (readAndroidTheme || readAppTheme) {
        context = themifyContext(context, attrs, readAndroidTheme, readAppTheme);
    }

    if (wrapContext) {
        context = TintContextWrapper.wrap(context);
    }

    View view = null;
    byte var12 = -1;
    switch(name.hashCode()) {
    case -1946472170:
        if (name.equals("RatingBar")) {
            var12 = 11;
        }
        break;
    case -1455429095:
        if (name.equals("CheckedTextView")) {
            var12 = 8;
        }
        break;
    case -1346021293:
        if (name.equals("MultiAutoCompleteTextView")) {
            var12 = 10;
        }
        break;
    case -938935918:
        if (name.equals("TextView")) {
            var12 = 0;
        }
        break;
    case -937446323:
        if (name.equals("ImageButton")) {
            var12 = 5;
        }
        break;
    case -658531749:
        if (name.equals("SeekBar")) {
            var12 = 12;
        }
        break;
    case -339785223:
        if (name.equals("Spinner")) {
            var12 = 4;
        }
        break;
    case 776382189:
        if (name.equals("RadioButton")) {
            var12 = 7;
        }
        break;
    case 1125864064:
        if (name.equals("ImageView")) {
            var12 = 1;
        }
        break;
    case 1413872058:
        if (name.equals("AutoCompleteTextView")) {
            var12 = 9;
        }
        break;
    case 1601505219:
        if (name.equals("CheckBox")) {
            var12 = 6;
        }
        break;
    case 1666676343:
        if (name.equals("EditText")) {
            var12 = 3;
        }
        break;
    case 2001146706:
        if (name.equals("Button")) {
            var12 = 2;
        }
    }

    switch(var12) {
    case 0:
        view = this.createTextView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 1:
        view = this.createImageView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 2:
        view = this.createButton(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 3:
        view = this.createEditText(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 4:
        view = this.createSpinner(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 5:
        view = this.createImageButton(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 6:
        view = this.createCheckBox(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 7:
        view = this.createRadioButton(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 8:
        view = this.createCheckedTextView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 9:
        view = this.createAutoCompleteTextView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 10:
        view = this.createMultiAutoCompleteTextView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 11:
        view = this.createRatingBar(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 12:
        view = this.createSeekBar(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    default:
        view = this.createView(context, name, attrs);
    }

    if (view == null && originalContext != context) {
        view = this.createViewFromTag(context, name, attrs);
    }

    if (view != null) {
        this.checkOnClickListener((View)view, attrs);
    }

    return (View)view;
}

通过 createTextView() 等源码可以发现，creatView() 方法把常用的组件都变成了 AppCompat 的类型，从而达到了兼容的目的。

至此，setContentView() 的流程就走完了。但是添加好布局文件之后，视图并不会显示到界面上，还需要通过 WindowsManagerService 去渲染界面才能使界面显示。这部分到内容在后面会讲到。

零碎的东西很多，为了方便大家记忆，我把上面的内容做成了思维导图，需要的朋友可以保存下来，偶尔看一下，帮助自己记忆。

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