安卓开发日记--2017.10.17

继续积累吧。

RecyclerView

RecyclerView是什么？

我们已经使用过一些View来构建我们的App，例如ScrollView，ListView等。当时RecyclerView的应用场景是在于它能够对数据进行缓存，从而不需要直接将所有的数据加载到内存中。例如我们的应用中需要使用大量的图片列表，如果将所有的图片直接放到内存中，会导致应用的速度变慢，同时浪费内存。
所以我们使用RecyclerView来优化数据的展示。RecyclerView的主要模式是RecyclerView+Adapter+ViewHolder。

RecyclerView

1. RecyclerView需要放置在layout文件中，并分配相应的资源id：

    <android.support.v7.widget.RecyclerView        android:id="@+id/recyclerview_forecast"        android:layout_width="match_parent"        android:layout_height="match_parent"/>

1. RecyclerView需要使用不同的LayoutManager来完成不同的布局分布。RecyclerView通过setLayoutManager函数来设定指定的LayoutManager。RecyclerView可以使用setAdapter方法，设置指定的Adapter。

        /*         * LinearLayoutManager can support HORIZONTAL or VERTICAL orientations. The reverse layout         * parameter is useful mostly for HORIZONTAL layouts that should reverse for right to left         * languages.         */        LinearLayoutManager layoutManager                = new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false);        mRecyclerView.setLayoutManager(layoutManager);        /*         * Use this setting to improve performance if you know that changes in content do not         * change the child layout size in the RecyclerView         */        mRecyclerView.setHasFixedSize(true);        // TODO (11) Pass in 'this' as the ForecastAdapterOnClickHandler        /*         * The ForecastAdapter is responsible for linking our weather data with the Views that         * will end up displaying our weather data.         */        mForecastAdapter = new ForecastAdapter(this);        /* Setting the adapter attaches it to the RecyclerView in our layout. */        mRecyclerView.setAdapter(mForecastAdapter);

Adapter

1. 当RecyclerView需要加载一个数据项的时候，调用函数onCreateViewHolder，从Adapter中获取一个ViewHolder。每当RecyclerView需要获得更多的ViewHolder的时候，会不断从Adapter中获得。但是，ViewHolder并不是无限的创建，而是创建有限个ViewHolder之后，重复利用之前的ViewHolder，并不断更新旧的ViewHolder中的数据来刷新页面。
   在onCreateViewHolder方法中，需要将一个item项对应的layout资源进行填充到ViewHolder中。使用函数LayoutInflater.inflate(,,false)，最后一个参数需要注意，虽然不知道为什么需要用false，但是这个是必须的，否则就会报错。

    /**     * This gets called when each new ViewHolder is created. This happens when the RecyclerView     * is laid out. Enough ViewHolders will be created to fill the screen and allow for scrolling.     *     * @param viewGroup The ViewGroup that these ViewHolders are contained within.     * @param viewType  If your RecyclerView has more than one type of item (which ours doesn't) you     *                  can use this viewType integer to provide a different layout. See     *                  {@link android.support.v7.widget.RecyclerView.Adapter#getItemViewType(int)}     *                  for more details.     * @return A new ForecastAdapterViewHolder that holds the View for each list item     */    @Override    public ForecastAdapterViewHolder onCreateViewHolder(ViewGroup viewGroup, int viewType) {        Context context = viewGroup.getContext();        int layoutIdForListItem = R.layout.forecast_list_item;        LayoutInflater inflater = LayoutInflater.from(context);        boolean shouldAttachToParentImmediately = false;        View view = inflater.inflate(layoutIdForListItem, viewGroup, shouldAttachToParentImmediately);        return new ForecastAdapterViewHolder(view);    }

1. 当RecyclerView需要向ViewHolder中放置数据的时候，需要调用Adapter的函数onBindViewHolder，函数使用当前的position，获取对应的位置，并绑定数据。

    /**     * OnBindViewHolder is called by the RecyclerView to display the data at the specified     * position. In this method, we update the contents of the ViewHolder to display the weather     * details for this particular position, using the "position" argument that is conveniently     * passed into us.     *     * @param forecastAdapterViewHolder The ViewHolder which should be updated to represent the     *                                  contents of the item at the given position in the data set.     * @param position                  The position of the item within the adapter's data set.     */    @Override    public void onBindViewHolder(ForecastAdapterViewHolder forecastAdapterViewHolder, int position) {        String weatherForThisDay = mWeatherData[position];        forecastAdapterViewHolder.mWeatherTextView.setText(weatherForThisDay);    }

1. 这个方法是最简单的一个方法，直接返回Adapter中所有的数据量。

    /**     * This method simply returns the number of items to display. It is used behind the scenes     * to help layout our Views and for animations.     *     * @return The number of items available in our forecast     */    @Override    public int getItemCount() {        if (null == mWeatherData) return 0;        return mWeatherData.length;    }

ViewHolder

1. 真正实现数据的绑定，对一个RecyclerView中的一个数据项，需要使用ViewHolder来进行保存，更新和展示。
2. 同时ViewHolder也可以处理Click事件，需要实现RecyclerView.OnClickListener接口，并在构造函数内完成ClickListener的绑定（使用函数setOnClickListener）。
   在onClick函数（类似于回调函数）中，使用getAdapterPosition函数中，可以得到当前数据在Adapter中，对应的位置，使用这个位置信息，确定所要修改和更新的数据。

“`
/**
* Cache of the children views for a forecast list item.
*/
public class ForecastAdapterViewHolder extends RecyclerView.ViewHolder
implements RecyclerView.OnClickListener{
public final TextView mWeatherTextView;

    public ForecastAdapterViewHolder(View view) {        super(view);        mWeatherTextView = (TextView) view.findViewById(R.id.tv_weather_data);        // TODO (7) Call setOnClickListener on the view passed into the constructor (use 'this' as the OnClickListener)        view.setOnClickListener(this);    }    @Override    public void onClick(View view) {        int index = getAdapterPosition();        mClickHandler.onClick(mWeatherData[index]);    }    // TODO (6) Override onClick, passing the clicked day's data to mClickHandler via its onClick method}

“`1