launcher修改--页面标记实现原理（代码追踪）

         大家看这篇文章的时候，可能已经看过在launcher 2.1上实现2.2的屏幕标记（http://gqdy365.iteye.com/blog/897636），使用一个imageView显示不同的图片，看一下android里面launcher2里面源代码的实现，在drawable文件夹里面，可以找到一个home_arrows_left.xml的文件，内容如下：

<?xml version="1.0" encoding="utf-8"?>
<level-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:maxLevel="0" android:drawable="@android:color/transparent" />
    <item android:maxLevel="1" android:drawable="@drawable/home_arrows_left_1" />
    <item android:maxLevel="2" android:drawable="@drawable/home_arrows_left_2" />
    <item android:maxLevel="3" android:drawable="@drawable/home_arrows_left_3" />
    <item android:maxLevel="4" android:drawable="@drawable/home_arrows_left_4" />
</level-list>

同时，在launcher.xml文件中，

<ImageView android:id="@+id/previous_screen"
		android:layout_width="93dip"                                           
		android:layout_height="@dimen/button_bar_height"
		android:layout_gravity="bottom|left" 
                android:layout_marginLeft="6dip"
		android:scaleType="center" 
                android:src="@drawable/home_arrows_left"
		android:onClick="previousScreen"  
                android:focusable="true"
		android:clickable="true" />

引用了这个文件，但是他真正在代码中是如何实现的哪，在launcher.java中的setupViews()方法中，有一下代码：

 mPreviousView = (ImageView) dragLayer.findViewById(R.id.previous_screen);
        mNextView = (ImageView) dragLayer.findViewById(R.id.next_screen);

        Drawable previous = mPreviousView.getDrawable();
        Drawable next = mNextView.getDrawable();
        mWorkspace.setIndicators(previous, next);

通过代码获取这个两旁的图片，然后我们再看一看在workspace.java里面的setIndicator()方法，

void setIndicators(Drawable previous, Drawable next) {
        mPreviousIndicator = previous;
        mNextIndicator = next;
        previous.setLevel(mCurrentScreen);
        next.setLevel(mCurrentScreen);
    }

让我们找到mCurrentScreen这个参数的定义（73行）：

 private int mCurrentScreen;

定义的一个整数，对应着home_arrows_left.xml里面的maxLevel，不同的参数ImageView上面显示不同的图片，用以标记页面。

在上面的launcher代码中，有android:onClick="previousScreen" 注册了imageView的点击相应事件，在launcher.java中，对应的代码如下：

@SuppressWarnings({"UnusedDeclaration"})
    public void previousScreen(View v) {
        if (!isAllAppsVisible()) {
            mWorkspace.scrollLeft();
        }
    }

    @SuppressWarnings({"UnusedDeclaration"})
    public void nextScreen(View v) {
        if (!isAllAppsVisible()) {
            mWorkspace.scrollRight();
        }
    }

我们继续追踪，在workspace.java代码中的scrollLeft和scrollRight()方法的代码：

public void scrollLeft() {
        clearVacantCache();
        if (mScroller.isFinished()) {
            if (mCurrentScreen > 0) snapToScreen(mCurrentScreen - 1);
        } else {
            if (mNextScreen > 0) snapToScreen(mNextScreen - 1);            
        }
    }

    public void scrollRight() {
        clearVacantCache();
        if (mScroller.isFinished()) {
            if (mCurrentScreen < getChildCount() -1) snapToScreen(mCurrentScreen + 1);
        } else {
            if (mNextScreen < getChildCount() -1) snapToScreen(mNextScreen + 1);            
        }
    }

先判断是否滑动到头，然后跳转到目标屏，snapToScreen（）方法代码也在workspace中，代码如下：

private void snapToScreen(int whichScreen, int velocity, boolean settle) {
        //if (!mScroller.isFinished()) return;

        whichScreen = Math.max(0, Math.min(whichScreen, getChildCount() - 1));
        
        clearVacantCache();
        enableChildrenCache(mCurrentScreen, whichScreen);

        mNextScreen = whichScreen;

        mPreviousIndicator.setLevel(mNextScreen);
        mNextIndicator.setLevel(mNextScreen);
		enableDragLayerChildrenCache();

        View focusedChild = getFocusedChild();
        if (focusedChild != null && whichScreen != mCurrentScreen &&
                focusedChild == getChildAt(mCurrentScreen)) {
            focusedChild.clearFocus();
        }
        
        final int screenDelta = Math.max(1, Math.abs(whichScreen - mCurrentScreen));
        final int newX = whichScreen * getWidth();
        final int delta = newX - mScrollX;
        int duration = (screenDelta + 1) * 100;

        if (!mScroller.isFinished()) {
            mScroller.abortAnimation();
        }
        
        if (settle) {
            mScrollInterpolator.setDistance(screenDelta);
        } else {
            mScrollInterpolator.disableSettle();
        }
        
        velocity = Math.abs(velocity);
        if (velocity > 0) {
            duration += (duration / (velocity / BASELINE_FLING_VELOCITY))
                    * FLING_VELOCITY_INFLUENCE;
        } else {
            duration += 100;
        }

        awakenScrollBars(duration);
        mScroller.startScroll(mScrollX, 0, delta, 0, duration*2/3);
        invalidate();
    }

这就是其实现的基本过程。