自定义View算是Android开发中的重中之重了，很多小伙伴可能或多或少都玩过自定义View，对View的绘制流程也有一定的理解。那么现在我想通过几篇博客来详细介绍View的绘制流程，以便使我们更加深刻的理解自定义View。

如果小伙伴们还没用过自定义View或者用的不多的话，那么建议通过以下几篇文章先来热个身：

OK，View的世界浩如烟海，不过凡事只要抓住纲就好解决，所谓提纲挈领嘛。那我这里就打算从LayoutInflater这个布局管理器开始我们的View绘制详解之旅。so，开始吧！

使用LayoutInflater加载一个布局文件，一般情况下，我们通过下面的方式来获取一个LayoutInflater实例：

LayoutInflater inflater = LayoutInflater.from(this);

点到这个方法里边，我们发现这里实际上是调用了系统服务，如下：

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;    }

看到这里小伙伴们应该明白了，获取LayoutInflater我们还有另外一种方式：

LayoutInflater layoutInflater = (LayoutInflater) getSystemService(LAYOUT_INFLATER_SERVICE);

两种方式都可以获取一个LayoutInflater实例，拿到实例后，接下来就可以加载布局了，加载布局时我们使用inflate方法，该方法有两个重载的方法，一个是两个参数，一个是三个参数，关于这两个方法的差异小伙伴们如果还不懂可以移步这里，不论是两个参数还是三个参数，inflate方法最后都会到达这里：

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;                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("
     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                    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.                    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) {                        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;        }    }

小伙伴们请看，上面这个方法虽然很长，但是逻辑却很简单，前面几行代码都很简单，就是XML的解析，我们看到Google中布局XML的解析使用了PULL解析的方式，在第24行，首先获取了XML文件根节点的名称，第33行，如果根节点是merge的话，那么要求根节点必须添加到某一个容器中，否则会抛异常（如果小伙伴们对merge这个节点尚不了解，请移步这里 ）。如果根节点不是merge，则在第42行通过createViewFromTag方法创建一个View（该方法稍后详述），这个View实际上就是我们要加载布局的根节点，加载到根View之后，加载到根View之后，如果root不为null，则在第52行根据root生成一个LayoutParams，第53行，如果我们设置了不将加载进来的布局加载到root中，即我们传入的attachToRoot为false的话，则将刚刚加载进来的params设置给temp（temp是我们要加载布局的根节点），然后在65行通过rInflateChildren方法开始去读取这个根节点下的所有子控件（该方法稍后详述），第73行，如果我们设置了root不为null，并且要将添加进来的布局加入到root中，则会执行root.addView方法。第79行，如果root为null，则attachToRoot不管是什么都会执行第80行，将temp赋值给result。小伙伴们请注意这个result本身的值就是root。最后将result返回。OK如此看来整个inflate方法思路还是非常清晰的，并没有什么难以理解的地方。接下来我们再来看一看系统是怎么样创建根布局的，以及是怎样创建根布局中的子元素的。首先我们来看看createViewFromTag这个方法。

createViewFromTag这个方法最终会到达这里：

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 {                    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;        }    }

小伙伴们注意，在这个方法的第40行，首先判断了要实例化的节点的名字中是否包含一个点，包含的话说明该View不是由android.jar提供的，这种情况对应一种解析方式，不包含说明该View是由系统提供的，对应一种解析方式，如果name中不包含点，则最终会调用下面的方法：

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

小伙伴们请看，这里系统会自动为View添加上包名，我们再来看看这个createView方法（小伙伴们注意，如果我们的View是自定义View的话，最终也会来到这个方法中）：

public final View createView(String name, String prefix, AttributeSet attrs)            throws ClassNotFoundException, InflateException {        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) {                // Class not found in the cache, see if it's real, and try to add it                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);                    }                }                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 = 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;            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]));            }            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 ? "
    
     " : clazz.getName()), e);            ie.setStackTrace(EMPTY_STACK_TRACE);            throw ie;        } finally {            Trace.traceEnd(Trace.TRACE_TAG_VIEW);        }    }
    

这个方法虽然有点长，但是核心目的却很清晰，就是根据View的名称，通过Java中的反射机制来获取一个View实例并返回。OK，以上就是对创建根布局方法createViewFromTag的讲解，其实严格来说，这个方法是创建一个View，我们一会在创建容器中的子View的时候，还会再用到这个方法。OK，现在我们再回到inflate方法的第65行，这里又一个方法叫做rInflateChildren，我们来看看该方法：

void rInflate(XmlPullParser parser, View parent, Context context,            AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {        final int depth = parser.getDepth();        int type;        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)) {                parseRequestFocus(parser, parent);            } else if (TAG_TAG.equals(name)) {                parseViewTag(parser, parent, attrs);            } else if (TAG_INCLUDE.equals(name)) {                if (parser.getDepth() == 0) {                    throw new InflateException("
     cannot be the root element");                }                parseInclude(parser, context, parent, attrs);            } else if (TAG_MERGE.equals(name)) {                throw new InflateException("
     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 (finishInflate) {            parent.onFinishInflate();        }    }

这个方法倒是很简单，并不长，核心代码就是28行到32行，这里还是调用了上文所说的createViewFromTag方法来获取一个View的实例，然后第31行通过一个递归方法来遍历容器中的所有控件，并将获取到的控件添加到对应的viewGroup中。

OK，以上就是对inflate方法的一个简单解读，整体来说貌似没有什么难点，有问题欢迎留言讨论。

以上。