[原創]Android從xml加載到View對象過程解析

咱們從Activity的setContentView()入手，開始源碼解析，

    //Activity.setContentView
    public void setContentView(int layoutResID) {
        getWindow().setContentView(layoutResID);
        initActionBar();
    }


    //PhoneWindow.setContentView
    public void setContentView(int layoutResID) {
        if (mContentParent == null) {
            installDecor();
        } else {
            mContentParent.removeAllViews();
        }
        mLayoutInflater.inflate(layoutResID, mContentParent); final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

發現是使用mLayoutInflater建立View的，因此咱們去LayoutInflater.inflate()裏面看下，

    public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
        if (DEBUG) System.out.println("INFLATING from resource: " + resource);
        XmlResourceParser parser = getContext().getResources().getLayout(resource);
        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }

先根據resource id 獲取到XmlResourceParseer，意如其名，就是xml的解析器，繼續往下，進入到inflate的核心方法，有些長，咱們只分析關鍵部分：

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
        ......

                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);
                } else {
                    // Temp is the root view that was found in the xml
                    View temp;
                    if (TAG_1995.equals(name)) {
                        temp = new BlinkLayout(mContext, attrs);
                    } else {
                        temp = createViewFromTag(root, name, attrs);
                    }
        ......
            } 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;
            }

            return result;
        }
    }

若是tag的名字不是TAG_1995（名字是個梗），就調用函數createViewFromTag()建立View，進去看看，

    View createViewFromTag(View parent, String name, AttributeSet attrs) {
        if (name.equals("view")) {
            name = attrs.getAttributeValue(null, "class");
        }

        ......

            View view;
            if (mFactory2 != null) view = mFactory2.onCreateView(parent, name, mContext, attrs);
            else if (mFactory != null) view = mFactory.onCreateView(name, mContext, attrs);
            else view = null;

            if (view == null && mPrivateFactory != null) {
                view = mPrivateFactory.onCreateView(parent, name, mContext, attrs);
            }
            
            if (view == null) {
                if (-1 == name.indexOf('.')) {
                    view = onCreateView(parent, name, attrs);
                } else {
                    view = createView(name, null, attrs);
                }
            }

            if (DEBUG) System.out.println("Created view is: " + view);
            return view;
    ......
    }

首先嚐試用3個Fractory建立View，若是成功就直接返回了。注意，咱們能夠利用這個機制，建立本身的Factory來控制View的建立過程。

若是沒有Factory或建立失敗，那麼走默認邏輯。

先判斷name中是否有'.'字符，若是沒有，則認爲使用android本身的View，此時會在name的前面加上包名"android.view."；若是有這個'.'，則認爲使用的自定義View，這時無需添加任何前綴，認爲name已經包含全包名了。

最終，使用這個全包名的name來建立實例，

    private static final HashMap<String, Constructor<? extends View>> sConstructorMap =
            new HashMap<String, Constructor<? extends View>>();

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

    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;
        　　 ......

            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;
            return constructor.newInstance(args);
            ......
    }

從源碼中看到，在建立實例前，會先從一個靜態Map中獲取緩存，

Constructor<? extends View> constructor = sConstructorMap.get(name);

緩存的是Constructor對象，目的是用於建立實例，這裏要注意sConstructorMap是靜態的，而且經過Constructor建立的實例，是使用和Constructor對象同一個ClassLoader來建立的，換句話說，在同一個進程中，同一個自定義View對象，是沒法用不一樣ClassLoader加載的，若是想解決這個問題，就不要讓系統使用createView()接口建立View，作法就是自定義Factory或Factory2來自行建立View。

繼續往下看，若是緩存裏沒有，則建立View的Class對象clazz，並緩存到sConstructorMap中，

            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);
            }

而後就是newInstance了，至此這個View便從xml中變成了java對象，咱們繼續返回到inflate函數中，看看這個View返回以後作了什麼，

                ......
                    // Temp is the root view that was found in the xml
                    View temp;
                    if (TAG_1995.equals(name)) {
                        temp = new BlinkLayout(mContext, attrs);
                    } else {
                        temp = createViewFromTag(root, name, attrs);
                    }

                    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);

                    // 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;
                    }
            ......
            return result;

從createViewFromTag返回後，會調用個rInflate()，其中parent參數就是剛纔建立出的View，應該能猜到裏面作了什麼，

    void rInflate(XmlPullParser parser, View parent, final 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_INCLUDE.equals(name)) {
                if (parser.getDepth() == 0) {
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {
                throw new InflateException("<merge /> must be the root element");
            } else if (TAG_1995.equals(name)) {
                final View view = new BlinkLayout(mContext, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflate(parser, view, attrs, true);
                viewGroup.addView(view, params);                
            } else {
                final View view = createViewFromTag(parent, name, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflate(parser, view, attrs, true);
                viewGroup.addView(view, params);
            }
        }

        if (finishInflate) parent.onFinishInflate();
    }

沒錯，就是遞歸的使用createViewFromTag()建立子View，並經過ViewGroup.addView添加到parent view中。

以後，這個View樹上的全部View都建立完畢。而後會根據inflate()的參數（root和attachToRoot）判斷是否將新建立的View添加到root view中，

            if (root != null && attachToRoot) {
                root.addView(temp, params);
            }

而後，inflate()就將View返回了。

整個分析到此結束。