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：

View Code

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

转载于:https://www.cnblogs.com/wangfeng520/p/5727971.html