Android插件换肤 一.实现原理

学习缺的不是时间，而是耐心

目的

1.搞懂系统获取资源文件到在加载布局的整个流程是自己实现换肤功能的理论基础

2.提高分析源码、追踪源码的能力

要点

1.XmlResourceParser （通过这个类可以将xml中的每一个控件取出来）

2.Factory2（创建兼容的view，为null的话，通过反射的方式创建非兼容的view）

源码阅读

版本：android-29

跟踪setContentView探索view的实例化

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

跟踪setContentView()

 //将我们给他的这个布局加载到root布局中
 public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
        final Resources res = getContext().getResources();
     	//检查是否有预编译，如果有直接使用
        View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
        if (view != null) {
            return view;
        }
        //接下来是重点
        XmlResourceParser parser = res.getLayout(resource);//xml解析器
        try {
            //这个inflate方法中将会真正的拿到xml中的控件添加到root中
            return inflate(parser, root, attachToRoot);
        } finally {
            //关闭此解析器。在此调用之后，接口上的调用不再有效。
            parser.close();
        }
    }

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 {
                advanceToRootNode(parser);
                final String name = parser.getName();

                if (TAG_MERGE.equals(name)) {
                  ...
                } else {
                    // temp是在xml中找到的根视图
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);

                    ViewGroup.LayoutParams params = null;

                    if (root != null) {
                        // 设置temp参数
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {
                            temp.setLayoutParams(params);
                        }
                    }

                    // 解析temp下的子控件
                    rInflateChildren(parser, temp, attrs, true);

                   	//将temp添加到root布局
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }

                     //返回temp
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }

            } catch (Exception e) {
               ...
            } finally {
                // Don't retain static reference on context.
             ...
            }

            return result;
        }
    }

    /**
     * 递归方法，用于降低xml层次结构并实例化
     * 视图，实例化其子级，然后调用onFinishInflate（）
     */
    void rInflate(XmlPullParser parser, View parent, Context context,
            AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {

        final int depth = parser.getDepth();
        int type;
        boolean pendingRequestFocus = false;

        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 (...) {
            ...
            } else {
                //创建view  参数name是LinearLayout、TextView、ImageView...
                final View view = createViewFromTag(parent, name, context, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                //递归调用
                rInflateChildren(parser, view, attrs, true);
                viewGroup.addView(view, params);
            }
        }

        if (pendingRequestFocus) {
            parent.restoreDefaultFocus();
        }

        if (finishInflate) {
            parent.onFinishInflate();
        }
    }

    /**
     * 通过tag获取一个viwe
     */
    @UnsupportedAppUsage
    View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
            boolean ignoreThemeAttr) {
	   ...

        try {
            //通过Factory2创建view
            View view = tryCreateView(parent, name, context, attrs);

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

            return view;
        } catch (Exception e) {
            ...
        }
    }

  /**
     * 通过Factory2创建view
     */
    public final View tryCreateView(@Nullable View parent, @NonNull String name,
        @NonNull Context context,@NonNull AttributeSet attrs) {
  		...

        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);
        }

        return view;
    }

如果 mFactory2不为null 最终会走到这个方法去创建view（创建view的方法二 换肤框架会用到）

    final View createView(View parent, final String name, @NonNull Context context,
            @NonNull AttributeSet attrs, boolean inheritContext,
            boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
        final Context originalContext = context;

        // We can emulate Lollipop's android:theme attribute propagating down the view hierarchy
        // by using the parent's context
        if (inheritContext && parent != null) {
            context = parent.getContext();
        }
        if (readAndroidTheme || readAppTheme) {
            // We then apply the theme on the context, if specified
            context = themifyContext(context, attrs, readAndroidTheme, readAppTheme);
        }
        if (wrapContext) {
            context = TintContextWrapper.wrap(context);
        }

        View view = null;

        // We need to 'inject' our tint aware Views in place of the standard framework versions
        switch (name) {
            case "TextView":
                view = createTextView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "ImageView":
                view = createImageView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "Button":
                view = createButton(context, attrs);
                verifyNotNull(view, name);
                break;
            case "EditText":
                view = createEditText(context, attrs);
                verifyNotNull(view, name);
                break;
            case "Spinner":
                view = createSpinner(context, attrs);
                verifyNotNull(view, name);
                break;
            case "ImageButton":
                view = createImageButton(context, attrs);
                verifyNotNull(view, name);
                break;
            case "CheckBox":
                view = createCheckBox(context, attrs);
                verifyNotNull(view, name);
                break;
            case "RadioButton":
                view = createRadioButton(context, attrs);
                verifyNotNull(view, name);
                break;
            case "CheckedTextView":
                view = createCheckedTextView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "AutoCompleteTextView":
                view = createAutoCompleteTextView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "MultiAutoCompleteTextView":
                view = createMultiAutoCompleteTextView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "RatingBar":
                view = createRatingBar(context, attrs);
                verifyNotNull(view, name);
                break;
            case "SeekBar":
                view = createSeekBar(context, attrs);
                verifyNotNull(view, name);
                break;
            case "ToggleButton":
                view = createToggleButton(context, attrs);
                verifyNotNull(view, name);
                break;
            default:
                // The fallback that allows extending class to take over view inflation
                // for other tags. Note that we don't check that the result is not-null.
                // That allows the custom inflater path to fall back on the default one
                // later in this method.
                view = createView(context, name, attrs);
        }

        if (view == null && originalContext != context) {
            // If the original context does not equal our themed context, then we need to manually
            // inflate it using the name so that android:theme takes effect.
            view = createViewFromTag(context, name, attrs);
        }

        if (view != null) {
            // If we have created a view, check its android:onClick
            checkOnClickListener(view, attrs);
        }

        return view;
    }

如果 mFactory2是null 会走下面的这个方法（创建view的方法二 换肤框架会用到）

    /**
     *  通过反射去创建view
     */
    @Nullable
    public final View createView(@NonNull Context viewContext, @NonNull String name,
            @Nullable String prefix, @Nullable AttributeSet attrs)
            throws ClassNotFoundException, InflateException {
        Objects.requireNonNull(viewContext);
        Objects.requireNonNull(name);
        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 = Class.forName(prefix != null ? (prefix + name) : name, false,
                        mContext.getClassLoader()).asSubclass(View.class);

                if (mFilter != null && clazz != null) {
                    boolean allowed = mFilter.onLoadClass(clazz);
                    if (!allowed) {
                        failNotAllowed(name, prefix, viewContext, 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 = Class.forName(prefix != null ? (prefix + name) : name, false,
                                mContext.getClassLoader()).asSubclass(View.class);

                        boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                        mFilterMap.put(name, allowed);
                        if (!allowed) {
                            failNotAllowed(name, prefix, viewContext, attrs);
                        }
                    } else if (allowedState.equals(Boolean.FALSE)) {
                        failNotAllowed(name, prefix, viewContext, attrs);
                    }
                }
            }

            Object lastContext = mConstructorArgs[0];
            mConstructorArgs[0] = viewContext;
            Object[] args = mConstructorArgs;
            args[1] = attrs;

            try {
                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;
            } finally {
                mConstructorArgs[0] = lastContext;
            }
        } catch (Exception e) {
            ...
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

​ 到这里 setContentView(R.layout.activity_main)就走到头了，但是中间有一个重要的变量 mFactory2的创建并不在setContView的流程中。那 mFactory2必然在setContentView之前。接下来就去跟踪一下 super.onCreate(savedInstanceState)吧

mFactory2实例化的过程

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        final AppCompatDelegate delegate = getDelegate();
        //从名字可以看出这个方法初始化了factory
        delegate.installViewFactory();
        delegate.onCreate(savedInstanceState);
        super.onCreate(savedInstanceState);
    }

    @Override
    public void installViewFactory() {
        LayoutInflater layoutInflater = LayoutInflater.from(mContext);
        //通过这个判断可以得知Factory2只可以创建一次
        if (layoutInflater.getFactory() == null) {
            LayoutInflaterCompat.setFactory2(layoutInflater, this);
        } else {
            if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImpl)) {
                Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"
                        + " so we can not install AppCompat's");
            }
        }
    }

最终进入到这个方法

    public void setFactory2(Factory2 factory) {
      	...
        mFactorySet = true;
        if (mFactory == null) {
            mFactory = mFactory2 = factory;
        } else {
            mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
        }
    }

总结

基于以上两个流程，我们只需要在super.onCreate(savedInstanceState);之前设置factory2即可实现对view 创建的过程进行修改，从而实现换肤的功能。

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        LayoutInflaterCompat.setFactory2(LayoutInflater.from(this), new LayoutInflater.Factory2() {
            @Nullable
            @Override
            public View onCreateView(@Nullable View parent, @NonNull String name, @NonNull Context context, @NonNull AttributeSet attrs) {
                Log.e(TAG, "name = " + name);
                int n = attrs.getAttributeCount();
                for (int i = 0; i < n; i++) {
                    Log.e(TAG, attrs.getAttributeName(i) + " , " + attrs.getAttributeValue(i));
                }

                return null;
            }

            @Nullable
            @Override
            public View onCreateView(@NonNull String name, @NonNull Context context, @NonNull AttributeSet attrs) {
                return null;
            }
        });
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}