LayoutInflater解析

我们想到的必然是Android内部帮我们解析xml文件，LayoutInflater就是帮我们做了这个工作。
首先LayoutInflater是一个系统服务，这个我们可以从from方法看出来

    /**     * Obtains the LayoutInflater from the given context.     */    public static LayoutInflater from(Context context) {        LayoutInflater LayoutInflater =                (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);        if (LayoutInflater == null) {            throw new AssertionError("LayoutInflater not found.");        }        return LayoutInflater;    }1
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通常我们拿到LayoutInflater对象之后就会调用其inflate方法进行加载布局，inflate是一个重载方法

public View inflate(int resource, ViewGroup root) {        return inflate(resource, root, root != null);    }1
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可以看到，我们调用2个参数的方法时候其默认是添加到父布局中的（父布局一般不为空）

public View inflate(int resource, 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();        }    }1
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这个方法中，其实是使用Resources将资源ID还原为XMlResoourceParser对象，然后调用inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)方法，解析布局的具体步骤都是在这个方法中实现

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {        synchronized (mConstructorArgs) {            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");            final AttributeSet attrs = Xml.asAttributeSet(parser);            Context lastContext = (Context)mConstructorArgs[0];            mConstructorArgs[0] = mContext;            View result = root;            try {                // Look for the root node.                //1.循环寻找根节点，其实就是节点指针遍历的过程                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!");                }                    //2.得到节点的名字，用于判断该节点                final String name = parser.getName();                if (DEBUG) {                    System.out.println("**************************");                    System.out.println("Creating root view: "                            + name);                    System.out.println("**************************");                }                    //3.对节点名字进行判断，然后是merge就将其添加到父布局中（依据Merge的特性必须添加到父布局中）                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, attrs, false, false);                } else {                //4.创建根据节点创建View                    // Temp is the root view that was found in the xml                    final View temp = createViewFromTag(root, name, attrs, false);                    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                        //5.根据attrs生成布局参数                        params = root.generateLayoutParams(attrs);                        if (!attachToRoot) {                            // Set the layout params for temp if we are not                            // attaching. (If we are, we use addView, below)                            //6.如果View不添加到父布局中，那就给其本身设置布局参数                            temp.setLayoutParams(params);                        }                    }                    if (DEBUG) {                        System.out.println("-----> start inflating children");                    }                    // Inflate all children under temp                    // 7.将该节点下的子View全部加载                    rInflate(parser, temp, attrs, true, 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.                    //8.如果添加到父布局中，直接addView                    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.                    //9.如果不添加到父布局，那么将自己返回                    if (root == null || !attachToRoot) {                        result = temp;                    }                }            } catch (XmlPullParserException e) {                InflateException ex = new InflateException(e.getMessage());                ex.initCause(e);                throw ex;            } catch (IOException e) {                InflateException ex = new InflateException(                        parser.getPositionDescription()                        + ": " + e.getMessage());                ex.initCause(e);                throw ex;            } finally {                // Don't retain static reference on context.                mConstructorArgs[0] = lastContext;                mConstructorArgs[1] = null;            }            Trace.traceEnd(Trace.TRACE_TAG_VIEW);            return result;        }    }1
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重点的步骤我已经加上注释了，核心
1.找到根布局标签
2.创建根节点对应的View
3.创建其子View

我们从这里面可以看出来，子View的解析其实都是rInflate方法，如果xml中有根布局，那么就调用createViewFromTag创建布局中的根View。我们也可以明白merge的原来，因为它直接调用rInflate添加到父View中，看到rInflate(parser, root, attrs, false, false)和rInflate(parser, temp, attrs, true, true)第二个参数区别我们就明白了。

接下来我们看下rInflate如何创建多个布局

void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,            boolean finishInflate, boolean inheritContext) throws XmlPullParserException,            IOException {        //获取当前解析器指针所在节点处于布局层次        final int depth = parser.getDepth();        int type;        //进行树的深度优先遍历（如果一个节点有子节点将会再次进入rInflate，否则继续循环）        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();            //如果其中有request_focus标签，那就给这个节点View设置焦点            if (TAG_REQUEST_FOCUS.equals(name)) {                parseRequestFocus(parser, parent);            //如果其中有tag标签，那就给这个节点View设置tag(key,value)            } else if (TAG_TAG.equals(name)) {                parseViewTag(parser, parent, attrs);            } else if (TAG_INCLUDE.equals(name)) {            //如果其中是include标签，如果include标签                if (parser.getDepth() == 0) {                    throw new InflateException("<include /> cannot be the root element");                }                parseInclude(parser, parent, attrs, inheritContext);            } else if (TAG_MERGE.equals(name)) {                throw new InflateException("<merge /> must be the root element");            } else {                    //创建该节点代表的View并添加到父view中，此外遍历子节点                final View view = createViewFromTag(parent, name, attrs, inheritContext);                final ViewGroup viewGroup = (ViewGroup) parent;                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);                rInflate(parser, view, attrs, true, true);                viewGroup.addView(view, params);            }        }            //代表着一个节点含其子节点遍历结束        if (finishInflate) parent.onFinishInflate();    }1
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从上面可以看到，所以创建View都将会交给createViewFromTag(View parent, String name, AttributeSet attrs, boolean inheritContext)中，我们可以看下该方法如何创建View

View createViewFromTag(View parent, String name, AttributeSet attrs, boolean inheritContext) {        if (name.equals("view")) {            name = attrs.getAttributeValue(null, "class");        }        Context viewContext;        if (parent != null && inheritContext) {            viewContext = parent.getContext();        } else {            viewContext = mContext;        }        // Apply a theme wrapper, if requested.        final TypedArray ta = viewContext.obtainStyledAttributes(attrs, ATTRS_THEME);        final int themeResId = ta.getResourceId(0, 0);        if (themeResId != 0) {            viewContext = new ContextThemeWrapper(viewContext, themeResId);        }        ta.recycle();        if (name.equals(TAG_1995)) {            // Let's party like it's 1995!            return new BlinkLayout(viewContext, attrs);        }        if (DEBUG) System.out.println("******** Creating view: " + name);        try {            View view;            if (mFactory2 != null) {                view = mFactory2.onCreateView(parent, name, viewContext, attrs);            } else if (mFactory != null) {                view = mFactory.onCreateView(name, viewContext, attrs);            } else {                view = null;            }            if (view == null && mPrivateFactory != null) {                view = mPrivateFactory.onCreateView(parent, name, viewContext, attrs);            }            if (view == null) {                final Object lastContext = mConstructorArgs[0];                mConstructorArgs[0] = viewContext;                try {                    if (-1 == name.indexOf('.')) {                        view = onCreateView(parent, name, attrs);                    } else {                        view = createView(name, null, attrs);                    }                } finally {                    mConstructorArgs[0] = lastContext;                }            }            if (DEBUG) System.out.println("Created view is: " + view);            return view;        } catch (InflateException e) {            throw e;        } catch (ClassNotFoundException e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class " + name);            ie.initCause(e);            throw ie;        } catch (Exception e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class " + name);            ie.initCause(e);            throw ie;        }    }1
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其实很简单，就是4个降级处理
if(factory2!=null){
factory2.onCreateView();
}else if(factory!=null){
factory.onCreateView();
}else if(mPrivateFactory!=null){
mPrivateFactory.onCreateView();
}else{
onCreateView()
}

其他的onCreateView我们不去设置的话为null，我们看下自己的onCreateView(),其实这个方法会调用createView()

public final View createView(String name, String prefix, AttributeSet attrs)            throws ClassNotFoundException, InflateException {            //从构造器Map（缓存）中获取需要的构造器        Constructor<? extends View> constructor = sConstructorMap.get(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                //如果缓存中没有需要的构造器，那就通过ClassLoader加载需要的类                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);                    }                }                //将使用过的构造器缓存                constructor = clazz.getConstructor(mConstructorSignature);                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 = 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[] args = mConstructorArgs;            args[1] = attrs;            constructor.setAccessible(true);            //通过反射获取需要的实例对象            final View view = constructor.newInstance(args);            if (view instanceof ViewStub) {                // Use the same context when inflating ViewStub later.                final ViewStub viewStub = (ViewStub) view;                //ViewStub将创建一个属于自己的LayoutInflater，因为它需要在不同的时机去inflate                viewStub.setLayoutInflater(cloneInContext((Context) args[0]));            }            return view;        } catch (NoSuchMethodException e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class "                    + (prefix != null ? (prefix + name) : name));            ie.initCause(e);            throw ie;        } catch (ClassCastException e) {            // If loaded class is not a View subclass            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Class is not a View "                    + (prefix != null ? (prefix + name) : name));            ie.initCause(e);            throw ie;        } catch (ClassNotFoundException e) {            // If loadClass fails, we should propagate the exception.            throw e;        } catch (Exception e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class "                    + (clazz == null ? "<unknown>" : clazz.getName()));            ie.initCause(e);            throw ie;        } finally {            Trace.traceEnd(Trace.TRACE_TAG_VIEW);        }    }1
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大体步骤就是，
1.从缓存中获取特定View构造器，如果没有，则加载对应的类，并缓存该构造器，
2.利用构造器反射构造对应的View
3.如果是ViewStub则复制一个LayoutInflater对象传递给它