Android源码学习之模板方法模式应用

一、模板方法模式定义

模板方法模式定义：

defines the skeleton of an algorithm in a method, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.

定义一个方法操作算法的框架（骨架结构），而将一些步骤延迟到子类中。模板方法使子类可以在不改变一个算法的结构的情况下，就可以重定义该算法的某些特定步骤。

　　如上图所示（截取自《Head First Design Patterns》一书），具体各个部分有什么作用见上图，这里就不啰嗦了。

二、模板方法模式优势

封装不变部分，扩展可变部分。把不变部分的算法封装到父类实现，而可变部分的根据子类的具体需要，则可以通过继承来扩展。

提取公共部分，构成一个“模板”，模板的作用在于对算法或者流程的一个结构化、规范化，子类不能修改“模板方法”的整个算法骨架或者流程的顺序等，只能根据自身的不同，对模板方法中算法的某一步进行扩展。

行为由父类控制，子类实现。子类可以通过扩展的方法增加相应的功能，符合开闭原则。

三、模板方法模式在Android源码中的应用

　　在Android源码中，View中的Draw()方法就是一个“模板方法”。它定义了一系列“Draw”过程，主要包括这几个步骤（截取自源代码）：

/*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

具体函数代码如下所示：

其中第三步（ Step 3）Draw view's content函数：

    /**
     * Implement this to do your drawing.
     *
     * @param canvas the canvas on which the background will be drawn
     */
    protected void onDraw(Canvas canvas) {
    }

第四步（ Step 4） draw children

    /**
     * Called by draw to draw the child views. This may be overridden
     * by derived classes to gain control just before its children are drawn
     * (but after its own view has been drawn).
     * @param canvas the canvas on which to draw the view
     */
    protected void dispatchDraw(Canvas canvas) {
    }

　　从上面的Draw()“模板方法”可以看出，当继承View子类中，如果要重写或者扩展这个方法时，整个方法流程和基本内容不能够修改，子类只能通过扩展onDraw(Canvas canvas)和dispatchDraw(Canvas canvas)两个函数，使子类自己的View显示效果和别的具体子类的不同。现在来看看继承自View类的具体子类如何扩展Draw()模板方法显示自己的与众不同：

　　1、TextView类中重写了OnDraw函数

@Override
    protected void onDraw(Canvas canvas) {
        if (mPreDrawState == PREDRAW_DONE) {
            final ViewTreeObserver observer = getViewTreeObserver();
            observer.removeOnPreDrawListener(this);
            mPreDrawState = PREDRAW_NOT_REGISTERED;
        }

        if (mCurrentAlpha <= ViewConfiguration.ALPHA_THRESHOLD_INT) return;

        restartMarqueeIfNeeded();

        // Draw the background for this view
        super.onDraw(canvas);

        final int compoundPaddingLeft = getCompoundPaddingLeft();
        final int compoundPaddingTop = getCompoundPaddingTop();
        final int compoundPaddingRight = getCompoundPaddingRight();
        final int compoundPaddingBottom = getCompoundPaddingBottom();
        final int scrollX = mScrollX;
        final int scrollY = mScrollY;
        final int right = mRight;
        final int left = mLeft;
        final int bottom = mBottom;
        final int top = mTop;

        final Drawables dr = mDrawables;
        if (dr != null) {
            /*
             * Compound, not extended, because the icon is not clipped
             * if the text height is smaller.
。。。
}

　　2、SurfaceView重写了dispatchDraw()函数：

    @Override
    protected void dispatchDraw(Canvas canvas) {
        if (mWindowType != WindowManager.LayoutParams.TYPE_APPLICATION_PANEL) {
            // if SKIP_DRAW is cleared, draw() has already punched a hole
            if ((mPrivateFlags & SKIP_DRAW) == SKIP_DRAW) {
                // punch a whole in the view-hierarchy below us
                canvas.drawColor(0, PorterDuff.Mode.CLEAR);
            }
        }
        super.dispatchDraw(canvas);
    }

　　3、ViewGroup类重写了dispatchDraw()函数：

 @Override
    protected void dispatchDraw(Canvas canvas) {
        final int count = mChildrenCount;
        final View[] children = mChildren;
        int flags = mGroupFlags;

        if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {
            final boolean cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE;

            final boolean buildCache = !isHardwareAccelerated();
            for (int i = 0; i < count; i++) {
                final View child = children[i];
                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
                    final LayoutParams params = child.getLayoutParams();
                    attachLayoutAnimationParameters(child, params, i, count);
                    bindLayoutAnimation(child);
                    if (cache) {
                        child.setDrawingCacheEnabled(true);
                        if (buildCache) {
                            child.buildDrawingCache(true);
                        }
                    }
                }
            }

            final LayoutAnimationController controller = mLayoutAnimationController;
            if (controller.willOverlap()) {
                mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;
            }

            controller.start();

            mGroupFlags &= ~FLAG_RUN_ANIMATION;
            mGroupFlags &= ~FLAG_ANIMATION_DONE;

            if (cache) {
                mGroupFlags |= FLAG_CHILDREN_DRAWN_WITH_CACHE;
            }

            if (mAnimationListener != null) {
                mAnimationListener.onAnimationStart(controller.getAnimation());
            }
        }

        int saveCount = 0;
        final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
        if (clipToPadding) {
            saveCount = canvas.save();
            canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
                    mScrollX + mRight - mLeft - mPaddingRight,
                    mScrollY + mBottom - mTop - mPaddingBottom);

        }

        // We will draw our child's animation, let's reset the flag
        mPrivateFlags &= ~DRAW_ANIMATION;
        mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;

        boolean more = false;
        final long drawingTime = getDrawingTime();

        if ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) {
            for (int i = 0; i < count; i++) {
                final View child = children[i];
                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                    more |= drawChild(canvas, child, drawingTime);
                }
            }
        } else {
            for (int i = 0; i < count; i++) {
                final View child = children[getChildDrawingOrder(count, i)];
                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                    more |= drawChild(canvas, child, drawingTime);}}}// Draw any disappearing views that have animationsif(mDisappearingChildren !=null){finalArrayList<View> disappearingChildren = mDisappearingChildren;finalint disappearingCount = disappearingChildren.size()-1;// Go backwards -- we may delete as animations finishfor(int i = disappearingCount; i >=0; i--){finalView child = disappearingChildren.get(i);
                more |= drawChild(canvas, child, drawingTime);}}if(clipToPadding){
            canvas.restoreToCount(saveCount);}// mGroupFlags might have been updated by drawChild()
        flags = mGroupFlags;if((flags & FLAG_INVALIDATE_REQUIRED)== FLAG_INVALIDATE_REQUIRED){
            invalidate(true);}if((flags & FLAG_ANIMATION_DONE)==0&&(flags & FLAG_NOTIFY_ANIMATION_LISTENER)==0&&
                mLayoutAnimationController.isDone()&&!more){// We want to erase the drawing cache and notify the listener after the// next frame is drawn because one extra invalidate() is caused by// drawChild() after the animation is over
            mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;finalRunnableend=newRunnable(){publicvoid run(){
                   notifyAnimationListener();}};
            post(end);}}

　　总之，首先在View.Draw()“模板方法”函数将设计界面分成6个步骤，合并不变部分，然后将可变部分提取、独立出来，让子类TextView等扩展修改成自己“独特”的界面效果。在我们自己的实际开发中，如果要设计自己的界面效果大部分的时候，我们都是覆盖重写onDraw(Canvas canvas)函数。

　　此外：根据模版方法中的方法，可以分为两大类：模版方法（Template Method）和基本方法（Primitive Method)。其中我们这里的例子Draw()函数就是一个“模板方法”。

而基本方法又可以分为三种：抽象方法（Abstract Method）、具体方法（Concrete Method）和钩子方法（Hook Method）：

抽象方法：一个抽象方法由抽象类声明，由具体子类实现。

具体方法：一个具体方法由抽象类声明并实现，而子类并不实现或置换。

钩子方法：一个钩子方法由抽象类声明并实现，而子类会加以扩展。我们这里的onDraw()函数就是一个钩子方法。

　　最后记住：

　　本人能力和时间有限（缺少“模式使用”内容，以后会添加），写的很粗糙，恭候大家的批评指正，谢谢~~~