Android源码学习之模板方法模式应用

主要内容：

模板方法模式定义

模板方法模式优势

模板方法模式在Android源码中的应用

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。

View Code

/**
* Manually render this view (and all of its children) to the given Canvas.
* The view must have already done a full layout before this function is
* called.  When implementing a view, implement
* {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
* If you do need to override this method, call the superclass version.
*
* @param canvas The Canvas to which the View is rendered.
*/
public void draw(Canvas canvas) {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW);
}

final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & DIRTY_MASK) == DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~DIRTY_MASK) | DRAWN;

/*
* 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)
*/

// Step 1, draw the background, if needed
int saveCount;

if (!dirtyOpaque) {
final Drawable background = mBGDrawable;
if (background != null) {
final int scrollX = mScrollX;
final int scrollY = mScrollY;

if (mBackgroundSizeChanged) {
background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false;
}

if ((scrollX | scrollY) == 0) {
background.draw(canvas);
} else {
canvas.translate(scrollX, scrollY);
background.draw(canvas);
canvas.translate(-scrollX, -scrollY);
}
}
}

// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);

// Step 4, draw the children
dispatchDraw(canvas);

// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);

// we're done...
return;
}

/*
* Here we do the full fledged routine...
* (this is an uncommon case where speed matters less,
* this is why we repeat some of the tests that have been
* done above)
*/

boolean drawTop = false;
boolean drawBottom = false;
boolean drawLeft = false;
boolean drawRight = false;

float topFadeStrength = 0.0f;
float bottomFadeStrength = 0.0f;
float leftFadeStrength = 0.0f;
float rightFadeStrength = 0.0f;

// Step 2, save the canvas' layers
int paddingLeft = mPaddingLeft;

final boolean offsetRequired = isPaddingOffsetRequired();
if (offsetRequired) {
paddingLeft += getLeftPaddingOffset();
}

int left = mScrollX + paddingLeft;
int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY + getFadeTop(offsetRequired);
int bottom = top + getFadeHeight(offsetRequired);

if (offsetRequired) {
right += getRightPaddingOffset();
bottom += getBottomPaddingOffset();
}

final ScrollabilityCache scrollabilityCache = mScrollCache;
final float fadeHeight = scrollabilityCache.fadingEdgeLength;
int length = (int) fadeHeight;

// clip the fade length if top and bottom fades overlap
// overlapping fades produce odd-looking artifacts
if (verticalEdges && (top + length > bottom - length)) {
length = (bottom - top) / 2;
}

// also clip horizontal fades if necessary
if (horizontalEdges && (left + length > right - length)) {
length = (right - left) / 2;
}

if (verticalEdges) {
topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
drawTop = topFadeStrength * fadeHeight > 1.0f;
bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
}

if (horizontalEdges) {
leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
drawLeft = leftFadeStrength * fadeHeight > 1.0f;
rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
drawRight = rightFadeStrength * fadeHeight > 1.0f;
}

saveCount = canvas.getSaveCount();

int solidColor = getSolidColor();
if (solidColor == 0) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

if (drawTop) {
canvas.saveLayer(left, top, right, top + length, null, flags);
}

if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}

if (drawLeft) {
canvas.saveLayer(left, top, left + length, bottom, null, flags);
}

if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}

// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);

// Step 4, draw the children
dispatchDraw(canvas);

// Step 5, draw the fade effect and restore layers
final Paint p = scrollabilityCache.paint;
final Matrix matrix = scrollabilityCache.matrix;
final Shader fade = scrollabilityCache.shader;

if (drawTop) {
matrix.setScale(1, fadeHeight * topFadeStrength);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
canvas.drawRect(left, top, right, top + length, p);
}

if (drawBottom) {
matrix.setScale(1, fadeHeight * bottomFadeStrength);
matrix.postRotate(180);
matrix.postTranslate(left, bottom);
fade.setLocalMatrix(matrix);
canvas.drawRect(left, bottom - length, right, bottom, p);
}

if (drawLeft) {
matrix.setScale(1, fadeHeight * leftFadeStrength);
matrix.postRotate(-90);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
canvas.drawRect(left, top, left + length, bottom, p);
}

if (drawRight) {
matrix.setScale(1, fadeHeight * rightFadeStrength);
matrix.postRotate(90);
matrix.postTranslate(right, top);
fade.setLocalMatrix(matrix);
canvas.drawRect(right - length, top, right, bottom, p);
}

canvas.restoreToCount(saveCount);

// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
}

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 animations
if (mDisappearingChildren != null) {
final ArrayList<View> disappearingChildren = mDisappearingChildren;
final int disappearingCount = disappearingChildren.size() - 1;
// Go backwards -- we may delete as animations finish
for (int i = disappearingCount; i >= 0; i--) {
final View 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;
final Runnable end = new Runnable() {
public void 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~~~