react-native-0.16.1 自定义Android组件部分的源码初探

从react-native的官方文档中我们已经知道facebook的react-native团队已经为我们实现了很多组件，例如 Image、Text、ViewPagerAndroid等，我们在index.android.js中可以直接使用这些组件，这些组件为什么能直接使用呢？

大家会很自然的想到已经封装好了呗。那在哪封装的？如何封装的？其实只要通过命令react-native init ProjectName创建过react-native工程的同学来说，在哪儿封装的一目了然。我们来看react-native工程的结构图：

react-native工程中，在node_modules下有一个很特别的react-native文件夹，android的工程中的build.gradle 文件多了一个依赖，不用想肯定在这两个地方封装的，这也是react-native的关键。

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dependencies {    
      compile 'com.facebook.react:react-native:0.16.+'   
 }

首先我们从入口MainActivity开始，看了我的前两篇文章，如何自定义react-native的android组件（一）和（二），要使用一个自定义组件，必须在MainActivity中加入【.addPackage(new AndroidSegmentedPackage()) 】才能使用。

...
  @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mReactRootView = new ReactRootView(this);

        mReactInstanceManager = ReactInstanceManager.builder()
                .setApplication(getApplication())
                .setBundleAssetName("index.android.bundle")
                .setJSMainModuleName("index.android")
                .addPackage(new MainReactPackage())
                .addPackage(new AndroidSegmentedPackage())  
                .setUseDeveloperSupport(BuildConfig.DEBUG)
                .setInitialLifecycleState(LifecycleState.RESUMED)
                .build();

        mReactRootView.startReactApplication(mReactInstanceManager, "ReactNativeSegmented", null);

        setContentView(mReactRootView);
    }
...

那么官方的Android组件是如何实现的呢，我们肯定注意到了【.addPackage(new MainReactPackage())】和自定义的是不是很像，格式也一样，我想肯定在这里面有实现，进入MainReactPackage类中，代码如下：

//react-native 源码
public class MainReactPackage implements ReactPackage {

  @Override
  public List<NativeModule> createNativeModules(ReactApplicationContext reactContext) {
    return Arrays.<NativeModule>asList(
      new AsyncStorageModule(reactContext),
      new FrescoModule(reactContext),
      new IntentModule(reactContext),
      new LocationModule(reactContext),
      new NetworkingModule(reactContext),
      new WebSocketModule(reactContext),
      new ToastModule(reactContext));
  }

  @Override
  public List<Class<? extends JavaScriptModule>> createJSModules() {
    return Collections.emptyList();
  }

  @Override
  public List<ViewManager> createViewManagers(ReactApplicationContext reactContext) {
    return Arrays.<ViewManager>asList(
      new ReactDrawerLayoutManager(),
      new ReactHorizontalScrollViewManager(),
      new ReactImageManager(),
      new ReactProgressBarViewManager(),
      new ReactRawTextManager(),
      new ReactScrollViewManager(),
      new ReactSwitchManager(),
      new ReactTextInputManager(),
      new ReactTextViewManager(),
      new ReactToolbarManager(),
      new ReactViewManager(),
      new ReactViewPagerManager(),
      new ReactTextInlineImageViewManager(),
      new ReactVirtualTextViewManager(),
      new SwipeRefreshLayoutManager());
  }
}

看了MainReactPackage中的代码，果不其然，首先我们看createViewManagers（）方法中的集合，看看集合子集的命名是不是很熟悉，
看看这里一共实现了多少原生控件：DrawerLayout、HorizontalScrollView、HorizontalScrollView、Image等等，还有SwipeRefreshLayout官网上还没有更新这个组件，其实这个版本已经可以使用了。

1.public class ReactDrawerLayoutManager extends ViewGroupManager<ReactDrawerLayout>
2.public class ReactImageManager extends SimpleViewManager<ReactImageView>
3.public class ReactProgressBarViewManager extends BaseViewManager<ProgressBarContainerView, ProgressBarShadowNode>
...

//贴上一个ReactDrawerLayoutManager源码，大家看看实现
public class ReactDrawerLayoutManager extends ViewGroupManager<ReactDrawerLayout> {

  private static final String REACT_CLASS = "AndroidDrawerLayout";

  public static final int OPEN_DRAWER = 1;
  public static final int CLOSE_DRAWER = 2;

  @Override
  public String getName() {
    return REACT_CLASS;
  }

  @Override
  protected void addEventEmitters(ThemedReactContext reactContext, ReactDrawerLayout view) {
    view.setDrawerListener(
        new DrawerEventEmitter(
            view,
            reactContext.getNativeModule(UIManagerModule.class).getEventDispatcher()));
  }

  @Override
  protected ReactDrawerLayout createViewInstance(ThemedReactContext context) {
    return new ReactDrawerLayout(context);
  }

  @ReactProp(name = "drawerPosition", defaultInt = Gravity.START)
  public void setDrawerPosition(ReactDrawerLayout view, int drawerPosition) {
    if (Gravity.START == drawerPosition || Gravity.END == drawerPosition) {
      view.setDrawerPosition(drawerPosition);
    } else {
      throw new JSApplicationIllegalArgumentException("Unknown drawerPosition " + drawerPosition);
    }
  }

  @ReactProp(name = "drawerWidth", defaultFloat = Float.NaN)
  public void getDrawerWidth(ReactDrawerLayout view, float width) {
    int widthInPx = Float.isNaN(width) ?
        ReactDrawerLayout.DEFAULT_DRAWER_WIDTH : Math.round(PixelUtil.toPixelFromDIP(width));
    view.setDrawerWidth(widthInPx);
  }

  @Override
  public boolean needsCustomLayoutForChildren() {
    // Return true, since DrawerLayout will lay out it's own children.
    return true;
  }

  @Override
  public @Nullable Map<String, Integer> getCommandsMap() {
    return MapBuilder.of("openDrawer", OPEN_DRAWER, "closeDrawer", CLOSE_DRAWER);
  }

  @Override
  public void receiveCommand(
      ReactDrawerLayout root,
      int commandId,
      @Nullable ReadableArray args) {
    switch (commandId) {
      case OPEN_DRAWER:
        root.openDrawer();
        break;
      case CLOSE_DRAWER:
        root.closeDrawer();
        break;
    }
  }

  @Override
  public @Nullable Map getExportedViewConstants() {
    return MapBuilder.of(
        "DrawerPosition",
        MapBuilder.of("Left", Gravity.START, "Right", Gravity.END));
  }

  @Override
  public @Nullable Map getExportedCustomDirectEventTypeConstants() {
    return MapBuilder.of(
        DrawerSlideEvent.EVENT_NAME, MapBuilder.of("registrationName", "onDrawerSlide"),
        DrawerOpenedEvent.EVENT_NAME, MapBuilder.of("registrationName", "onDrawerOpen"),
        DrawerClosedEvent.EVENT_NAME, MapBuilder.of("registrationName", "onDrawerClose"),
        DrawerStateChangedEvent.EVENT_NAME, MapBuilder.of(
            "registrationName", "onDrawerStateChanged"));
  }

  /**
   * This method is overridden because of two reasons:
   * 1. A drawer must have exactly two children
   * 2. The second child that is added, is the navigationView, which gets panned from the side.
   */
  @Override
  public void addView(ReactDrawerLayout parent, View child, int index) {
    if (getChildCount(parent) >= 2) {
      throw new
          JSApplicationIllegalArgumentException("The Drawer cannot have more than two children");
    }
    if (index != 0 && index != 1) {
      throw new JSApplicationIllegalArgumentException(
          "The only valid indices for drawer's child are 0 or 1. Got " + index + " instead.");
    }
    parent.addView(child, index);
    parent.setDrawerProperties();
  }

  public static class DrawerEventEmitter implements DrawerLayout.DrawerListener {

    private final DrawerLayout mDrawerLayout;
    private final EventDispatcher mEventDispatcher;

    public DrawerEventEmitter(DrawerLayout drawerLayout, EventDispatcher eventDispatcher) {
      mDrawerLayout = drawerLayout;
      mEventDispatcher = eventDispatcher;
    }

    @Override
    public void onDrawerSlide(View view, float v) {
      mEventDispatcher.dispatchEvent(
          new DrawerSlideEvent(mDrawerLayout.getId(), SystemClock.uptimeMillis(), v));
    }

    @Override
    public void onDrawerOpened(View view) {
      mEventDispatcher.dispatchEvent(
        new DrawerOpenedEvent(mDrawerLayout.getId(), SystemClock.uptimeMillis()));
    }

    @Override
    public void onDrawerClosed(View view) {
      mEventDispatcher.dispatchEvent(
          new DrawerClosedEvent(mDrawerLayout.getId(), SystemClock.uptimeMillis()));
    }

    @Override
    public void onDrawerStateChanged(int i) {
      mEventDispatcher.dispatchEvent(
          new DrawerStateChangedEvent(mDrawerLayout.getId(), SystemClock.uptimeMillis(), i));
    }
  }
}

原生控件的实现步骤、方法、例子等其实在源码中都有了，想实现什么组件就照着源码开发，绝不会出错啦。

到此只是完成了android端的java代码，那么组件如何与js代码联系起来，并且供js代码调用呢，我们来看看工程中的react-navie文件夹吧，秘密都在它里面。
react-navie文件结构：

打开react-native文件夹，我一眼就注意到了ReactAndroid目录（因为做Android嘛，对含有Android的词比较敏感>_<），翻遍了其下所有的目录文件，终于找到一个有用点的文件package.json，在其中找到关键的一句话
【”main”: “Libraries/react-native/react-native.js”】图上用红框标注了。
下一步就该看看Libraries目录了，Libraries目录结构：