LayoutInflater簡介

LayoutInflater的作用是將layout目錄下的xml佈局檔案載入到程式碼中並例項化為View類物件，它在android原始碼中的目錄為frameworks\base\core\java\android\view\LayoutInflater.java。在編碼過程中有兩種方法獲取它的例項：

1.LayoutInflater inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
2. LayoutInflater inflater = LayoutInflater.from(context);

其實我們看下原始碼就知道方法2就是對方法1的一個簡單封裝，使開發者更容易使用。

    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;
    }

LayoutInflater通過方法inflate載入xml資原始檔，並利用Pull解析器對Xml檔案進行解析，然後返回View物件。
inflate共有4個過載方法
public View inflate(int resource, ViewGroup root)
public View inflate(int resource, ViewGroup root, boolean attachToRoot)
public View inflate(XmlPullParser parser, ViewGroup root)
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)

引數說明如下：
resource :Xml檔案對應的資源ID
root ： 這個xml佈局檔案所屬於的根佈局
1、如果root不為null時，返回值其實就是這個root，這個方法就是把xml解析成view之後載入到這個root下。
2、如果root為null，返回值就是xml佈局裡面的根節點。
3. 如果root不為null，attachToRoot設為true，則會給載入的佈局檔案的指定一個父佈局，即root。
4. 如果root不為null，attachToRoot設為false，與2一樣。

inflate方法實現：

 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();

                 /*merge必須放在佈局檔案的根節點上，因為它不是一個ViewGroup，
                  * 也不是一個View。它相當於聲明瞭一些檢視，等待被新增。所以在通過
                  * LayoutInflate.inflate方法載入xml檔案的時候， 第二個引數必 須指定
                  *指定一個父親節點。
                  */
                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, inflaterContext, attrs, false);
                } else {

                     //建立根節點View物件
                    // 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.
                    //通過遞迴呼叫的方式遍歷xml，建立所有子控制元件的例項
                    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.
                     //如果root不為空，將跟節點例項加入到root中
                    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.
                    //如果root為空則直接返回根結點例項
                    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;
        }
    }

rInflate方法實現：

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();
            //如果控制元件帶有<requestFocus/>標誌(控制元件處於獲得焦點的狀態)，則會
            //呼叫view.requestFocus()；設定該控制元件為焦點狀態
            if (TAG_REQUEST_FOCUS.equals(name)) {
                parseRequestFocus(parser, parent);
            //如果控制元件帶有android:tag標誌，則會呼叫view.setTag(key, value);
            //將值儲存在view中，在使用時我們又可以通過view.getTag（）的方式取出來
            } else if (TAG_TAG.equals(name)) {
                parseViewTag(parser, parent, attrs);
            //如果控制元件帶有include標誌,則會去解析include所包含的xml檔案，解析過程
            //與inflate一樣
            } else if (TAG_INCLUDE.equals(name)) {
                if (parser.getDepth() == 0) {
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, context, parent, attrs);
            //merge必須放在佈局檔案的根節點上,否則會報錯
            } else if (TAG_MERGE.equals(name)) {
                throw new InflateException("<merge /> must be the root element");
            } else {
                //根據獲得的屬性值，建立子控制元件View並加入到它的父控制元件內，然後繼續解析view的子控制元件
                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();
        }
    }

createViewFromTag方法實現

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);
        }

        //建立View,我們可以繼承Factory介面並實現函式onCreateView然後
        //使用函式setFactory(Factory factory)和setFactory2(Factory2 factory)
        //來設定mFactory和mFactory2,自己實現建立的過程，也可以使用該類預設的實現函式
        //onCreateView建立View
        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;
        }
    }

createView方法實現

    //主要就是通過ClassLoder來建立View
    public final View createView(String name, String prefix, AttributeSet attrs)
            throws ClassNotFoundException, InflateException {
        Constructor<? extends View> constructor = sConstructorMap.get(name);
        if (constructor != null && !verifyClassLoader(constructor)) {
            constructor = null;
            sConstructorMap.remove(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);
                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 ? "<unknown>" : clazz.getName()), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }

rInflateChildren方法實現

    //很簡單，就是遞迴呼叫rInflate（）
    final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
            boolean finishInflate) throws XmlPullParserException, IOException {
        rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
    }