(Android)在Bundle中保存和恢复自定义类的ArrayList

最新推荐文章于 2021-05-26 02:32:10 发布

FrankJenq

最新推荐文章于 2021-05-26 02:32:10 发布

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分类专栏： Android

本文链接：https://blog.youkuaiyun.com/qq_39861176/article/details/91632550

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这篇博客探讨了如何在Android应用中，不依赖数据库的情况下，保存和恢复包含自定义类的ArrayList。当应用因系统原因关闭时，如何避免因ArrayList清空导致的空指针异常。通过实现Parcelable接口，使得自定义的Recipe、Ingredient和Step类能够存储在Bundle中，然后在适当的时候进行恢复。

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在不使用数据库的前提下，如何保存并恢复一个自定义的ArrayList?

在线获取JSON格式的数据，并存储到一个ArrayList。由于应用被系统在后台关闭时，这个ArrayList会被清空，导致用户从“最近使用的应用”列表返回应用时，会抛出空指针异常。所以我们需要在onSavedInstanceState中保存这个ArrayList，并在用户返回时检查ArrayList是否为空，如果为空就应该恢复之前保存的数据。

自定义的Recipe类结构如下：

public class Recipe {

    @SerializedName("ingredients")
    @Expose
    private ArrayList<Ingredient> ingredients;

    @SerializedName("name")
    @Expose
    private String name;

    @SerializedName("steps")
    @Expose
    private ArrayList<Step> steps;
    ///...

注意其中的ingredients和steps是两个自定义的ArrayList，因此无法直接存储在Bundle中。Ingredient和Step两个类的结构如下：

public class Ingredient {
    @SerializedName("measure")
    @Expose
    private String measure;

    @SerializedName("ingredient")
    @Expose
    private String ingredient;

    @SerializedName("quantity")
    @Expose
    private String quantity;
    ///...
public class Step {

    @SerializedName("shortDescription")
    @Expose
    private String shortDescription;

    @SerializedName("description")
    @Expose
    private String description;

    @SerializedName("videoURL")
    @Expose
    private String videoURL;

    @SerializedName("thumbnailURL")
    @Expose
    private String thumbnailURL;
    ///...

Ingredient和Step是两个只包含String对象的类，可以实例化Parcelable接口，并在onSaveInstanceState中使用putParcelableArrayList方法来存储ArrayList。

以Ingredient类为例：

public class Ingredient implements Parcelable {
    @SerializedName("measure")
    @Expose
    private String measure;

    @SerializedName("ingredient")
    @Expose
    private String ingredient;

    @SerializedName("quantity")
    @Expose
    private String quantity;

    private Ingredient(Parcel in) {
        measure = in.readString();
        ingredient = in.readString();
        quantity = in.readString();
    }

    public static final Creator<Ingredient> CREATOR = new Creator<Ingredient>() {
        @Override
        public Ingredient createFromParcel(Parcel in) {
            return new Ingredient(in);
        }

        @Override
        public Ingredient[] newArray(int size) {
            return new Ingredient[size];
        }
    };

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(measure);
        dest.writeString(ingredient);
        dest.writeString(quantity);
    }
    ///...

实例化Parcelable接口后，可以在onSaveInstanceState中储存数据：

@Override
    protected void onSaveInstanceState(Bundle outState) {
        ///...
        //把ArrayList存进Bundle中
        outState.putInt(NUMBER_OF_RECIPES, RecipeList.recipes.size());
        for (int i = 0; i < RecipeList.recipes.size(); i++) {
            outState.putParcelableArrayList(RecipeList.sIngredientsLabel + i, 
                                            RecipeList.recipes.get(i).getIngredients());
            outState.putString(RecipeList.sNamesLabel + i, RecipeList.recipes.get(i).getName());
            outState.putParcelableArrayList(RecipeList.sStepsLabel + i,
                                            RecipeList.recipes.get(i).getSteps());
        }
        super.onSaveInstanceState(outState);
    }

随后，我们可以在onCreate或者onRestoreInstanceState方法中恢复数据：

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ///...
        //假如RecipeList.recipes为空，则尝试从Bundle中恢复数据
        if (savedInstanceState != null && savedInstanceState.containsKey(NUMBER_OF_RECIPES)) {
            if (RecipeList.recipes == null || RecipeList.recipes.size() == 0) {
                int numberOfRecipes = savedInstanceState.getInt(NUMBER_OF_RECIPES);
                for (int i = 0; i < numberOfRecipes; i++) {
                    ArrayList<Ingredient> ingredients = 
                                          savedInstanceState
                                          .getParcelableArrayList(RecipeList.sIngredientsLabel + i);
                    String name = savedInstanceState.getString(RecipeList.sNamesLabel + i);
                    ArrayList<Step> steps = 
                                    savedInstanceState
                                    .getParcelableArrayList(RecipeList.sStepsLabel + i);
                    Recipe recipe = new Recipe(ingredients, name, steps);
                    RecipeList.recipes.add(recipe);
                }
            }
        }