Android LayoutInflater布局文件解析过程

1:View的创建流程

LayoutInflater是通过inflate方法将一个xml布局解析成为一个View。我们都知道inflate方法通常有三个参数，分别是：resource、root、attachToRoot，表示的含义如下：

1:resource：xml布局的id。
2:root：解析成之后的View的父View，此参数只在attachToRoot为true才生效。
3:attachToRoot：决定解析出来的View是否添加到root上。

1.1:inflate方法

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 {
                advanceToRootNode(parser);
                final String name = parser.getName();
                if (TAG_MERGE.equals(name)) {//如果是merge标签则直接解析下面的view
                    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 {//如果不是，那么创建view
                    // Temp is the root view that was found in the xml
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);
                    ViewGroup.LayoutParams params = null;
                    if (root != null) {
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {
                            temp.setLayoutParams(params);
                        }
                    }
                    // Inflate all children under temp against its context.
                    rInflateChildren(parser, temp, attrs, true);//递归解析下面的子view
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }
                    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(
                        getParserStateDescription(inflaterContext, attrs)
                        + ": " + 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;
        }
    }

inflate处理了两件事
1:处理merge标签，解析下面的子view
2:当根标签不是merge的话递归处理下面的子view

1.2:rInflate方法

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)) {--->解析request_focus标签
                pendingRequestFocus = true;
                consumeChildElements(parser);
            } else if (TAG_TAG.equals(name)) {---->解析tag标签
                parseViewTag(parser, parent, attrs);
            } else if (TAG_INCLUDE.equals(name)) {--------->解析include标签
                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 {
                final View view = createViewFromTag(parent, name, context, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflateChildren(parser, view, attrs, true);
                viewGroup.addView(view, params);
            }
        }
        if (pendingRequestFocus) {
            parent.restoreDefaultFocus();
        }
        if (finishInflate) {
            parent.onFinishInflate();
        }
    }

1.3:createViewFromTag

View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
            boolean ignoreThemeAttr) {
        if (name.equals("view")) {--------->如果节点名字为view，则取节点下面的class属性值作为名称
            name = attrs.getAttributeValue(null, "class");
        }
        // Apply a theme wrapper, if allowed and one is specified.
        if (!ignoreThemeAttr) {-------->是否忽略主题资源
            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();
        }
        try {
            View view = tryCreateView(parent, name, context, attrs);---->根据名称和属性值创建一个view,如果创建失败
            if (view == null) {
                final Object lastContext = mConstructorArgs[0];
                mConstructorArgs[0] = context;
                try {
                    if (-1 == name.indexOf('.')) {------------>是否是自定义的view,有.说明是自定义的view,因为自定义的view一般是包名+类名
                        view = onCreateView(context, parent, name, attrs);----->context和属性值传给view的构造方法
                    } else {
                        view = createView(context, name, null, attrs);---->原生的控件像TextView
                    }
                } finally {
                    mConstructorArgs[0] = lastContext;
                }
            }
            return view;
        } catch (InflateException e) {
            throw e;
        } catch (ClassNotFoundException e) {
            final InflateException ie = new InflateException(
                    getParserStateDescription(context, attrs)
                    + ": Error inflating class " + name, e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } catch (Exception e) {
            final InflateException ie = new InflateException(
                    getParserStateDescription(context, attrs)
                    + ": Error inflating class " + name, e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        }
    }

1.4:tryCreateView

 public final View tryCreateView(@Nullable View parent, @NonNull String name,
        @NonNull Context context,
        @NonNull AttributeSet attrs) {
        if (name.equals(TAG_1995)) {--------->处理特殊名称
            // Let's party like it's 1995!
            return new BlinkLayout(context, attrs);
        }
        View view;-------->// mFactory和mFactory2是两个工厂类，可以对视图的创建进行hook，暂时不分析
        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);
        }
        return view;
    }

1.5:onCreateView

public final View createView(@NonNull Context viewContext, @NonNull String name,
            @Nullable String prefix, @Nullable AttributeSet attrs)
            throws ClassNotFoundException, InflateException {
        Objects.requireNonNull(viewContext);
        Objects.requireNonNull(name);
        Constructor<? extends View> constructor = sConstructorMap.get(name);--->缓存了所有解析过得构造方法
        if (constructor != null && !verifyClassLoader(constructor)) {
            constructor = null;
            sConstructorMap.remove(name);
        }
        Class<? extends View> clazz = null;----->待解析view的class
        try {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
            if (constructor == null) {----------->如果没有解析过或是类加载器不一样
                // Class not found in the cache, see if it's real, and try to add it
                clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                        mContext.getClassLoader()).asSubclass(View.class);--->获取class
                if (mFilter != null && clazz != null) {
                    boolean allowed = mFilter.onLoadClass(clazz);
                    if (!allowed) {
                        failNotAllowed(name, prefix, viewContext, attrs);
                    }
                }
                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
                        clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                                mContext.getClassLoader()).asSubclass(View.class);
                        boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                        mFilterMap.put(name, allowed);
                        if (!allowed) {
                            failNotAllowed(name, prefix, viewContext, attrs);
                        }
                    } else if (allowedState.equals(Boolean.FALSE)) {
                        failNotAllowed(name, prefix, viewContext, attrs);
                    }
                }
            }
            Object lastContext = mConstructorArgs[0];
            mConstructorArgs[0] = viewContext;
            Object[] args = mConstructorArgs;
            args[1] = attrs;----->构造方法的第二个参数，所有的属性值attrs
            try {
                final View view = constructor.newInstance(args);---->根据属性值来构造一个view
                if (view instanceof ViewStub) {
                    // Use the same context when inflating ViewStub later.
                    final ViewStub viewStub = (ViewStub) view;
                    viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
                }
                return view;
            } finally {
                mConstructorArgs[0] = lastContext;
            }
        } catch (NoSuchMethodException e) {
            final InflateException ie = new InflateException(
                    getParserStateDescription(viewContext, attrs)
                    + ": 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(
                    getParserStateDescription(viewContext, attrs)
                    + ": 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(
                    getParserStateDescription(viewContext, attrs) + ": Error inflating class "
                            + (clazz == null ? "<unknown>" : clazz.getName()), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

LayoutInflater是通过反射的方式创建View，并将context以及AttributeSet对象作为参数传入。

1.6:解析子节点rInflateChildren

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

调用rInflate方法来解析下面的子节点

3:总结

1:LayoutInflater是android用来解析xml布局文件的一个类
2:LayoutInflater内部使用Pull解析的方式，并对其进行了一定的扩展。
3:LayoutInflater在生成View节点的时候，是通过反射的方式创建View对象，
反射调用的构造方法是带两个参数的那个，所以在定义View的时候必须重写带两个参数的构造方法。
4:LayoutInflater在创建View对象的时候，会将xml节点的解析器AttributeSet传入到View的构造方法中。AttributeSet定义了用来解析xml节点属性的API。View通过AttributeSet生成TypedArray，并从中读取View节点中定义的属性。
最后LayoutInflater将会通过递归的方式创建xml根节点下的所有孩子节点。