【设计模式】模板方法模式——模板方法模式在Android SDK源码中的应用

Android的View类是模板方法模式的集大成者，首先我们看draw()方法的部分代码：

public void draw(Canvas canvas) {
	/*
     * 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)
     *      7. If necessary, draw the default focus highlight
     */

	// Step 1, draw the background, if needed
    int saveCount;
	drawBackground(canvas);

    // ……代码省略……

	// Step 2, save the canvas' layers
   	// ……代码省略……
    
    // Step 3, draw the content
    onDraw(canvas);

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

    // Step 5, draw the fade effect and restore layers
    // ……代码省略……

    // Step 6, draw decorations (foreground, scrollbars)
    onDrawForeground(canvas);

    // Step 7, draw the default focus highlight
    drawDefaultFocusHighlight(canvas);

    // ……代码省略……
}

private void drawBackground(Canvas canvas) {
    // ……代码省略……
}

protected void onDraw(Canvas canvas) {
}

protected void dispatchDraw(Canvas canvas) {
}

public void onDrawForeground(Canvas canvas) {
    // ……代码省略……
}

以上drawBackground()方法是private的，不可以重写，但onDraw()和dispatchDraw()是被protected修饰的空实现，是典型的模板方法，在View的不同子类（TextView、ImageView等）里有不同的实现。