深入理解 LayoutInflater -- 通过 setContentView 入手

Activity

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

public Window getWindow() {
    return mWindow;
}


final void attach(...) {
    attachBaseContext(context);
    ...
    mWindow = new PhoneWindow(this, window, activityConfigCallback);
    ...
}

PhoneWindow

@Override
public void setContentView(int layoutResID) {
    if (mContentParent == null) {
        // 加载  Decor
        installDecor();
    } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        // 移除所有
        mContentParent.removeAllViews();
    }

    if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                getContext());
        transitionTo(newScene);
    } else {
        // 布局添加到 mContentParent
        mLayoutInflater.inflate(layoutResID, mContentParent);
    }
     ...
}

public PhoneWindow(Context context) {
    super(context);
    mLayoutInflater = LayoutInflater.from(context);
}

LayoutInflater.class

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) {
   ...
    try {
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
        ...
        // 存储父视图
        View result = root;

        try {
            // Look for the root node.
            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();

            // 处理 merge
            if (TAG_MERGE.equals(name)) {
                if (root == null || !attachToRoot) {
                    throw new InflateException(" can be used only with a valid "
                            + "ViewGroup root and attachToRoot=true");
                }
                // 所有子View添加到根标签中
                rInflate(parser, root, inflaterContext, attrs, false);
            } else {
                // 通过 xml 的 tag 来解析 layout 根视图
                // name是要解析视图的类名
                final View temp = createViewFromTag(root, name, inflaterContext, attrs);

                ViewGroup.LayoutParams params = null;

                if (root != null) {
                    
                    // 如果父布局不为空，获取参数
                    params = root.generateLayoutParams(attrs);
                    if (!attachToRoot) {
                        // 如果 attachToRoot 为 false，将 temp 设置布局参数
                        temp.setLayoutParams(params);
                    }
                }

                // 解析所有子视图
                rInflateChildren(parser, temp, attrs, true);

                // 如果 Root 不为空，且 attachToRoot 为 true，将 temp 添加到父视图中
                if (root != null && attachToRoot) {
                    // 添加到 root
                    root.addView(temp, params);
                }

                // 否则返回 temp
                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);
        }
       // 返回处理好的视图
        return result;
    }
}

LayoutInflate 解析单个视图

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;
        // 可以通过设置 Factory 自行解析 View，默认为kong，可忽略这段
        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);
        }

        // 没有 Factory 情况下创建 View
        if (view == null) {
            final Object lastContext = mConstructorArgs[0];
            mConstructorArgs[0] = context;
            try {
                // 内置 View（名称中无 . ）
                if (-1 == name.indexOf('.')) {
                    view = onCreateView(parent, name, attrs);
                } else {
                     // 自定义控件
                    view = createView(name, null, attrs);
                }
            } finally {
                mConstructorArgs[0] = lastContext;
            }
        }
        return view;
        ...
}

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

// 调用的同一个方法，只不过系统内部添加了前缀
protected View onCreateView(String name, AttributeSet attrs)
        throws ClassNotFoundException {
    return createView(name, "android.view.", attrs);
}

// 通过完整类名反射机制构造 View 对象，解析单个 View 的方法
public final View createView(String name, String prefix, AttributeSet attrs)
        throws ClassNotFoundException, InflateException {
    // 缓存中获取
    Constructor constructor = sConstructorMap.get(name);
    if (constructor != null && !verifyClassLoader(constructor)) {
        constructor = null;
        sConstructorMap.remove(name);
    }
    Class clazz = null;

    try {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);

        if (constructor == null) {
            // 前缀不为空，拼接完整 View 路径。
            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);
                }
            }
            // 获取构造函数
            constructor = clazz.getConstructor(mConstructorSignature);
            // 放入缓存
            constructor.setAccessible(true);
            sConstructorMap.put(name, constructor);
        } else {
            ...
        }

        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;

        // 通过反射构造View
        final View view = constructor.newInstance(args);
        if (view instanceof ViewStub) {
            // Use the same context when inflating ViewStub later.
            final ViewStub viewStub = (ViewStub) view;
            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;

    } 
    // 省略各种 catch、finaly 代码
}

LayoutInflate 解析视图树的方法

void rInflate(XmlPullParser parser, View parent, Context context,
        AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
    // 获取视图树的深度
    final int depth = parser.getDepth();
    int type;
    boolean pendingRequestFocus = false;
    // 遍历解析 View
    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();
        // requestFocus ：控件获取焦点
        if (TAG_REQUEST_FOCUS.equals(name)) {
            pendingRequestFocus = true;
            consumeChildElements(parser);
        //tag ：控件 tag，level 27
        } else if (TAG_TAG.equals(name)) {
            parseViewTag(parser, parent, attrs);
        // include ：包含
        } else if (TAG_INCLUDE.equals(name)) {
            if (parser.getDepth() == 0) {
                throw new InflateException(" cannot be the root element");
            }
            parseInclude(parser, context, parent, attrs);
        // merge（必须作为根视图，抛出异常）
        } else if (TAG_MERGE.equals(name)) {
            throw new InflateException(" must be the root element");
        } else {
            // 解析单个元素
            final View view = createViewFromTag(parent, name, context, attrs);
            final ViewGroup viewGroup = (ViewGroup) parent;
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
            // 其实就是递归调用，深度优先遍历
            rInflateChildren(parser, view, attrs, true);
            // 添加到 viewGroup
            viewGroup.addView(view, params);
        }
    }

    if (pendingRequestFocus) {
        parent.restoreDefaultFocus();
    }

    if (finishInflate) {
        parent.onFinishInflate();
    }
}

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

参考《Android 源码设计解析与实战》，结合 Andorid P 源码进行梳理。