自定義view加載xml 自定義VIEW中的XML文件解析過程－IT工程師數位筆記本

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自定義VIEW中的XML文件解析過程：面向對象的語言一切都是對象，這個當然有爭議，有的人說JAVA不是完全的面向對象的語言，這里不作討論。事實是XML也會被解析成對象，這就是為什么，在布局文件中，經過一系列的過程，VIEWGROUP和VIEW就會實例化了。

然后，在ACTIVITY中通過FINDVIEWBYID就找出來了，這里面當然是有文章的。這里離不開一個重要的類INFLATER，所有的XML文件，都是通過這個類來解析的，最終會實例化對象。
有的人說SETCONTENTVIEW之后，不就可以了嗎？但這個方法中也使用了INFLATER類，自己可以去看源碼。
首先是獲取INFLATER類，其實也只有一種方式，看源碼：

    /**     * 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;    }

注意，LayoutInflater是個抽象類，這個靜態方法中返回的一個實例是它的子類，具體是什么，可以看systerm_server中怎么做的，這是系統服務的知識點，這里不展開分析。這個context是什么類型的也沒什么要求，隨便什么context拿來用就是。然后就是調用inflate這個函數，這個函數有幾個重載，看看最終要調用的這個。

    /**     * Inflate a new view hierarchy from the specified xml resource. Throws     * {@link InflateException} if there is an error.     *      * @param resource ID for an XML layout resource to load (e.g.,     *        R.layout.main_page)     * @param root Optional view to be the parent of the generated hierarchy (if     *        attachToRoot is true), or else simply an object that     *        provides a set of LayoutParams values for root of the returned     *        hierarchy (if attachToRoot is false.)     * @param attachToRoot Whether the inflated hierarchy should be attached to     *        the root parameter? If false, root is only used to create the     *        correct subclass of LayoutParams for the root view in the XML.     * @return The root View of the inflated hierarchy. If root was supplied and     *         attachToRoot is true, this is root; otherwise it is the root of     *         the inflated XML file.     */    public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {        final Resources res = getContext().getResources();        if (DEBUG) {            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("                    + Integer.toHexString(resource) + ")");        }//這里開始獲得解析流,parser        final XmlResourceParser parser = res.getLayout(resource);        try {            return inflate(parser, root, attachToRoot);        } finally {            parser.close();        }    }

獲得parser之后，調用inflate(parser, root, attachToRoot)，這個函數開始干解析的工作。看源碼。

 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 (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) {                        // 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 against its context.                    rInflateChildren(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;                    }                }            } catch (XmlPullParserException e) {                InflateException ex = new InflateException(e.getMessage());                ex.initCause(e);                throw ex;            } catch (Exception 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;        }    }

這段代碼比較長，所以參數說明和debug都刪掉了，以便看清邏輯。解析過程是這樣的，如果有TAG_MERG，則調用 rInflate(parser, root, inflaterContext, attrs, false)，如果沒有則調用createViewFromTag(root, name, inflaterContext, attrs)，從名字可以看得出，通過反射獲得實例。我們來看看第二個方法。

  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);            ie.initCause(e);            throw ie;        } catch (Exception e) {            final InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class " + name);            ie.initCause(e);            throw ie;        }    }

這個方法干的事就是實例化VIEW，核心方法可以看得出來，就是factory中的oncreateview和LayoutInflate中的createview這兩個方法。這些factory是什么呢？

/**     * Create a new LayoutInflater instance that is a copy of an existing     * LayoutInflater, optionally with its Context changed.  For use in     * implementing {@link #cloneInContext}.     *      * @param original The original LayoutInflater to copy.     * @param newContext The new Context to use.     */    protected LayoutInflater(LayoutInflater original, Context newContext) {        mContext = newContext;        mFactory = original.mFactory;        mFactory2 = original.mFactory2;        mPrivateFactory = original.mPrivateFactory;        setFilter(original.mFilter);    }

從注釋可以看得出來，這些factory可以從original復制而來，但是original中的factory從何而來？

 public interface Factory {        public View onCreateView(String name, Context context, AttributeSet attrs);    }

 public interface Factory2 extends Factory {        public View onCreateView(View parent, String name, Context context, AttributeSet attrs);    }

 private static class FactoryMerger implements Factory2 {        private final Factory mF1, mF2;        private final Factory2 mF12, mF22;        FactoryMerger(Factory f1, Factory2 f12, Factory f2, Factory2 f22) {            mF1 = f1;            mF2 = f2;            mF12 = f12;            mF22 = f22;        }        public View onCreateView(String name, Context context, AttributeSet attrs) {            View v = mF1.onCreateView(name, context, attrs);            if (v != null) return v;            return mF2.onCreateView(name, context, attrs);        }        public View onCreateView(View parent, String name, Context context, AttributeSet attrs) {            View v = mF12 != null ? mF12.onCreateView(parent, name, context, attrs)                    : mF1.onCreateView(name, context, attrs);            if (v != null) return v;            return mF22 != null ? mF22.onCreateView(parent, name, context, attrs)                    : mF2.onCreateView(name, context, attrs);        }    }

 public void setFactory(Factory factory) {        if (mFactorySet) {            throw new IllegalStateException("A factory has already been set on this LayoutInflater");        }        if (factory == null) {            throw new NullPointerException("Given factory can not be null");        }        mFactorySet = true;        if (mFactory == null) {            mFactory = factory;        } else {            mFactory = new FactoryMerger(factory, null, mFactory, mFactory2);        }    }

 public void setFactory2(Factory2 factory) {        if (mFactorySet) {            throw new IllegalStateException("A factory has already been set on this LayoutInflater");        }        if (factory == null) {            throw new NullPointerException("Given factory can not be null");        }        mFactorySet = true;        if (mFactory == null) {            mFactory = mFactory2 = factory;        } else {            mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);        }    }

 public void setPrivateFactory(Factory2 factory) {        if (mPrivateFactory == null) {            mPrivateFactory = factory;        } else {            mPrivateFactory = new FactoryMerger(factory, factory, mPrivateFactory, mPrivateFactory);        }    }

oncreateview方法在哪里被實現？在framework中，被隱藏起來。所以我們來看看createview這個方法。

  public final View createView(String name, String prefix, AttributeSet attrs)            throws ClassNotFoundException, InflateException {        Constructor constructor = sConstructorMap.get(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) {            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 ? "" : clazz.getName()));            ie.initCause(e);            throw ie;        } finally {            Trace.traceEnd(Trace.TRACE_TAG_VIEW);        }    }

這里很明顯，通過構造函數，來產生VIEW的實例，自定義的使用，系統控件使用（name).asSubclass(View.class)。

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