Android 自定义View实现画背景和前景（ViewGroup篇）

        在定义ListView的Selector时候，有个drawSelectorOnTop的属性，如果drawSelectorOnTop为true的话，Selector的效果是画在List Item的上面（Selector是盖住了ListView的文字或者图片），即Foreground前景。如果drawSelectorOnTop为false的话，Selector的效果是画在List Item的下面，即Background背景。由于项目中恰好需要自定义View，需要实现此效果。

       本文借ListView的代码来剖析一下，

       ListView完成此部分功能在frameworks\base\core\java\android\widget\AbsListView.java文件中。

用mSelector即ListView要画的Selector(资源文件)，而mSelectorRect则是想要画的区域。

    /**
     * Indicates whether the list selector should be drawn on top of the children or behind
     */
    boolean mDrawSelectorOnTop = false; 决定画前景还是背景
    /**
     * The drawable used to draw the selector
     */
    Drawable mSelector; ListView用中来显示Selector的Drawable，即ListSelector对应的XML文件

    /**
     * The current position of the selector in the list.
     */
    int mSelectorPosition = INVALID_POSITION;

    /**
     * Defines the selector's location and dimension at drawing time
     */
    Rect mSelectorRect = new Rect(); 用来画Selector的区域，即Selector画的位置

AbsListView中构造方法中有获取selector

        Drawable d = a.getDrawable(com.android.internal.R.styleable.AbsListView_listSelector);
        if (d != null) {
            setSelector(d);
        }
        //默认为false,画的是背景
        mDrawSelectorOnTop = a.getBoolean(
                com.android.internal.R.styleable.AbsListView_drawSelectorOnTop, false);
         

下面看一下setSelector是如何实现的

    /**
     * Controls whether the selection highlight drawable should be drawn on top of the item or
     * behind it.
     *
     * @param onTop If true, the selector will be drawn on the item it is highlighting. The default
     *        is false.
     *
     * @attr ref android.R.styleable#AbsListView_drawSelectorOnTop
     */
    public void setDrawSelectorOnTop(boolean onTop) { //提供是否画前景或者背景的接口
        mDrawSelectorOnTop = onTop;
    }

    /**
     * Set a Drawable that should be used to highlight the currently selected item.
     *
     * @param resID A Drawable resource to use as the selection highlight.
     *
     * @attr ref android.R.styleable#AbsListView_listSelector
     */
    public void setSelector(int resID) {
        setSelector(getResources().getDrawable(resID)); 设置listSelector的XML文件
    }

    public void setSelector(Drawable sel) {
        if (mSelector != null) {
            mSelector.setCallback(null);
            unscheduleDrawable(mSelector);
        }
        mSelector = sel;
        Rect padding = new Rect();
        sel.getPadding(padding);
        mSelectionLeftPadding = padding.left;
        mSelectionTopPadding = padding.top;
        mSelectionRightPadding = padding.right;
        mSelectionBottomPadding = padding.bottom;
        sel.setCallback(this); //需要给Selector设置Callback
        updateSelectorState(); 
    }

    /**
     * Returns the selector {@link android.graphics.drawable.Drawable} that is used to draw the
     * selection in the list.
     *
     * @return the drawable used to display the selector
     */
    public Drawable getSelector() {
        return mSelector;
    }

    void updateSelectorState() {
        if (mSelector != null) {
            if (shouldShowSelector()) {
                mSelector.setState(getDrawableState());//更新Selector的状态
            } else {
                mSelector.setState(StateSet.NOTHING);
            }
        }
    }

这样就将Selector设置给ListView了，并且更新了drawable的状态。

接下来我们再看一下Android是如何将drawable画到ListView的Item上的。

在AbsListView中有个onTouchEvent的方法用来处理Touch事件，其中有一段代码就是确定Selector要画的区域。

if (mTouchMode == TOUCH_MODE_DOWN || mTouchMode == TOUCH_MODE_TAP) {
                        final Handler handler = getHandler();
                        if (handler != null) {
                            handler.removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ?
                                    mPendingCheckForTap : mPendingCheckForLongPress);
                        }
                        mLayoutMode = LAYOUT_NORMAL;
                        if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
                            mTouchMode = TOUCH_MODE_TAP;
                            setSelectedPositionInt(mMotionPosition);
                            layoutChildren();
                            child.setPressed(true);//设置List Item状态为 pressed
                            positionSelector(mMotionPosition, child);//确定画Selector的区域
                            setPressed(true); //设置ListView 的状态为pressed
                            if (mSelector != null) {
                                Drawable d = mSelector.getCurrent();
                                if (d != null && d instanceof TransitionDrawable) {
                                    ((TransitionDrawable) d).resetTransition();
                                }
                            }
                            if (mTouchModeReset != null) {
                                removeCallbacks(mTouchModeReset);
                            }
                            mTouchModeReset = new Runnable() {
                                @Override
                                public void run() {
                                    mTouchMode = TOUCH_MODE_REST;
                                    child.setPressed(false);
                                    setPressed(false);
                                    if (!mDataChanged) {
                                        performClick.run();
                                    }
                                }
                            };
                            postDelayed(mTouchModeReset,
                                    ViewConfiguration.getPressedStateDuration());
                        } else {
                            mTouchMode = TOUCH_MODE_REST;
                            updateSelectorState();
                        }
                        return true;
                    } else if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
                        performClick.run();
                    }
                }

接下来看看positionSelector的实现，

   void positionSelector(int position, View sel) {
        if (position != INVALID_POSITION) {
            mSelectorPosition = position;
        }
        //设置Selector的区域为List Item View的边界
        final Rect selectorRect = mSelectorRect;   selectorRect.set(sel.getLeft(), sel.getTop(), sel.getRight(), sel.getBottom());
        if (sel instanceof SelectionBoundsAdjuster) {
            ((SelectionBoundsAdjuster)sel).adjustListItemSelectionBounds(selectorRect);
        }
        positionSelector(selectorRect.left, selectorRect.top, selectorRect.right,
                selectorRect.bottom);

        final boolean isChildViewEnabled = mIsChildViewEnabled;
        if (sel.isEnabled() != isChildViewEnabled) {
            mIsChildViewEnabled = !isChildViewEnabled;
            if (getSelectedItemPosition() != INVALID_POSITION) {
                refreshDrawableState();//根据View状态更新drawable的状态
            }
        }
    }

    private void positionSelector(int l, int t, int r, int b) {
        mSelectorRect.set(l - mSelectionLeftPadding, t - mSelectionTopPadding, r
                + mSelectionRightPadding, b + mSelectionBottomPadding);
    }

好了现在已经决定了将selector画在哪里，Selector的状态也已经更新OK。

还差一步没有做，那就是到底是将其怎么画上面的呢？

答案就在AbsListView.java里的dispatchDraw方法里面。

   @Override
    protected void dispatchDraw(Canvas canvas) {
        int saveCount = 0;
        final boolean clipToPadding = (mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
        if (clipToPadding) {
            saveCount = canvas.save();
            final int scrollX = mScrollX;
            final int scrollY = mScrollY;
            canvas.clipRect(scrollX + mPaddingLeft, scrollY + mPaddingTop,
                    scrollX + mRight - mLeft - mPaddingRight,
                    scrollY + mBottom - mTop - mPaddingBottom);
            mGroupFlags &= ~CLIP_TO_PADDING_MASK;
        }

        final boolean drawSelectorOnTop = mDrawSelectorOnTop;
        if (!drawSelectorOnTop) { //将Selector画为背景
            drawSelector(canvas);
        }

        super.dispatchDraw(canvas);// 用Canvas画ListView

        if (drawSelectorOnTop) { //将Selector画为前景
            drawSelector(canvas);
        }

        if (clipToPadding) {
            canvas.restoreToCount(saveCount);
            mGroupFlags |= CLIP_TO_PADDING_MASK;
        }
    }

    private void drawSelector(Canvas canvas) {
        if (!mSelectorRect.isEmpty()) {
            final Drawable selector = mSelector;
            selector.setBounds(mSelectorRect);//设置drawable画的区域
            selector.draw(canvas); //使用canvas将drawable画上去
        }
    }

看到这里，想必大家都已经明白如何画前景和背景了吧。在dispatchDraw之前调用就是画前景，在dispatchDraw之后调用就是画背景。

另外补充一下，本文并没有介绍动画部分，有兴趣的可以自己研究下。

总结一下，实现这个功能需要有三个步骤：

1.设置Selector，并更新状态（初始化时候）

2.确定Selector画的区域，设置View的状态，根据View状态，更新Selector的状态(一般是对Event的处理方法中)

3.使用Canvas在dispatchDraw中，将Selector画上去，画Drawable的时候需要先设置区域，再调用drawable的draw方法。

后面我再将View如何画背景和前景补上，今天就先到这里吧。