View中的Context是哪裏來的

時間 2019-11-07

標籤 view context 哪裏來的简体版

原文原文鏈接

前言

在自定義View中咱們常常經過getContext()方法來使用Context獲取資源、設置樣式等。那麼你們有沒有想過View中getContext()獲取的Context是來自哪裏的？接下來就分析View中Context的生成過程。app

View的Context

咱們都知道在View中有個getContext()方法供開發者使用。佈局

/** * Returns the context the view is running in, through which it can * access the current theme, resources, etc. * * @return The view's Context. */
    @ViewDebug.CapturedViewProperty public final Context getContext() {
        return mContext;
    }
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在上面的註釋中能夠看出，經過View中的Context是View正在運行的上下文環境中，而且能夠經過Context獲取資源和主題等。根據這些信息咱們能夠判斷View中的Context是一個ContextThemeWrapper類型的Context。post

View的Context初始化

在獲取到上面的信息後，咱們能夠知道View的Context是一個ContextThemeWrapper類型的Context，而且在View的構造方法中也能夠看到Context被賦值。根據這些信息咱們能夠判斷View在初始化的時候Context被賦值，因此接下來從Activity的setContentView()開始分析。this

在Activity的setContentView()方法中View會被建立以及繪製。spa

public void setContentView(@LayoutRes int layoutResID) {
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
}
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能夠看到在Activity的setContentView()方法中調用了Window的setContentView()方法。咱們知道這個Window就是PhoneWindow，接下來直接在PhoneWindow中分析setContentView()方法。code

public void setContentView(int layoutResID) {
    if (mContentParent == null) {
        installDecor(); //首先初始化DecorView
    } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        mContentParent.removeAllViews();
    }
    //.........
}
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能夠看到再說上面的代碼中首先初始化了DecorView也就是頂級View。接下來看下DecorView是如何初始化的。遞歸

private void installDecor() {
    mForceDecorInstall = false;
    if (mDecor == null) {
        mDecor = generateDecor(-1); //建立DecorView
        mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
        mDecor.setIsRootNamespace(true);
        if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
            mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
        }
    } else {
        mDecor.setWindow(this);
    }
    if (mContentParent == null) {
        mContentParent = generateLayout(mDecor); //生成佈局
     }
    //........
}

protected DecorView generateDecor(int featureId) {
    Context context;
    if (mUseDecorContext) {
        Context applicationContext = getContext().getApplicationContext();
        if (applicationContext == null) {
            context = getContext();
        } else {
            context = new DecorContext(applicationContext, getContext().getResources());
            if (mTheme != -1) {
                context.setTheme(mTheme);
            }
        }
    } else {
        context = getContext();
    }
    return new DecorView(context, featureId, this, getAttributes());
}
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從上面的代碼中能夠看到，在建立DecorView的過程當中，首先會建立一個DecorContext。這個類型的Context就是DecorView中要使用的Context，能夠看到這個Context是能夠操做主題的。若是Context是空，就使用Window的Context，而Window的Context來自於Activity。ci

在DecorView建立完成後，接下來就是生成佈局。這裏就開始了View的初始化過程。element

protected ViewGroup generateLayout(DecorView decor) {
      //......
    if (mIsFloating) {
        setLayout(WRAP_CONTENT, WRAP_CONTENT);
        setFlags(0, flagsToUpdate);
    } else {
        setFlags(FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR, flagsToUpdate);
    }
    //......
    mDecor.startChanging();
    mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);  //生成佈局
    //......
    mDecor.finishChanging();
	return contentParent;
}

void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
    mDecorCaptionView = createDecorCaptionView(inflater);
    final View root = inflater.inflate(layoutResource, null);  //初始化View
    if (mDecorCaptionView != null) {
        if (mDecorCaptionView.getParent() == null) {
            addView(mDecorCaptionView,
                 new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        }
        mDecorCaptionView.addView(root,
            new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
    } else {
        addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    }
    mContentRoot = (ViewGroup) root;
    initializeElevation();
}
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在上面的代碼中能夠看出，佈局是從LayoutInflater的inflate開始的。資源

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            //......
           // Inflate all children under temp against its context.
           rInflateChildren(parser, temp, attrs, true);
            //......
            return result;
        }
    }

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) {
            //......
            final String name = parser.getName();
            if (TAG_REQUEST_FOCUS.equals(name)) {
                pendingRequestFocus = true;
                consumeChildElements(parser);
            } else if (TAG_TAG.equals(name)) {
                //經過tag建立
                parseViewTag(parser, parent, attrs);
            } else if (TAG_INCLUDE.equals(name)) {
                if (parser.getDepth() == 0) {
                    throw new InflateException("<include /> cannot be the root element");
                }
                // include 標籤
                parseInclude(parser, context, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {
                throw new InflateException("<merge /> must be the root element");
            } else {
                //建立View
                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);
            }
        }
    }
複製代碼

一路跟代碼下去，咱們能夠看到在rInflate()方法中根據佈局XML文件開始遞歸的建立View樹。在這個過程當中每一個View都會被建立。能夠看到在createViewFromTag(parent, name, context, attrs)方法中使用了Context的參數，接下來繼續分析。

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) {
            //這裏爲View建立了一個ContextThemeWrapper類型的Context
            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;
        } 
    }
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