LayoutInflater源码解析

android.view.LayoutInflater是布局解析的主要类，包括setContentView也是走的LayoutInflater。LayoutInflater解析布局主要入口就是inflate()方法，我们就从inflate(int resource, ViewGroup root, boolean attachToRoot)方法开始分析。

inflate(int resource, ViewGroup root, boolean attachToRoot)包含三个参数：
resource是布局文件id
root是被解析布局所要附着的父布局
attachToRoot是否需要添加到root布局

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();
    }
}

其中XmlResourceParser继承于XmlPullParser，采用的是pull方式解析xml布局。 接着继续走inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)方法。

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.
            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 (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 {
                // 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) {
                    // 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
                rInflate(parser, temp, attrs, true, true);

                // 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) {
            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;
    }
}

先遍历获取到布局根标签，通过标签名称来判断是否是merge标签，如果是merge标签则取父View，不是的话就通过createViewFromTag创建View，然后继续走rInflate，通过递归方式解析子标签。
我们先分析createViewFromTag方法。

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);
    }

    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;
            }
        }

        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;
    }
}

通过-1 == name.indexOf(‘.’)判断标签名是否是全类名，如果是走createView方法，不是走onCreateView方法。跟踪onCreateView方法可以看到，最终也是走了createView方法，不过传入了prefix参数

public final View createView(String name, String prefix, AttributeSet attrs)
        throws ClassNotFoundException, InflateException {
    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
            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);
            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);
        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) {
        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);
    }
}

通过反射创建View实例。

void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
        boolean finishInflate, boolean inheritContext) 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("<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 {
            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);
        }
    }

    if (finishInflate) parent.onFinishInflate();
}

递归方式解析子标签，并添加到父容器中。