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"<span style="white-space: pre;"> </span>android:layout_height="@dimen/button_bar_height" android:layout_gravity="bottom|left" <span style="white-space: pre;"> </span>android:layout_marginLeft="6dip" android:scaleType="center" <span style="white-space: pre;"> </span>android:src="@drawable/home_arrows_left" android:onClick="previousScreen" <span style="white-space: pre;"> </span>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。