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;
}