android Parcelable類

android Parcelable類表示該類可以用來序列化，打包爲數據流對象Parcel，通常用於進程間通信傳遞自定義數據類型。

Parcel也相應的提供了一系列write/get方法方便打包和解包Parcel類。

需要實現的方法主要是如何打包和解包Parcel類，具體如下：

    /**
     * Flatten this object in to a Parcel.
     * 
     * @param dest The Parcel in which the object should be written.
     * @param flags Additional flags about how the object should be written.
     * May be 0 or {@link #PARCELABLE_WRITE_RETURN_VALUE}.
     */
    public void writeToParcel(Parcel dest, int flags);

在此方法中，需要將當前類需要保存的數據用Parcel.write*方法寫入dest中，打包爲Parcel對象。

同時在當前類中也需要定義一個全局static final常量CREATOR，同時實現其中的解包Parcel的方法

    /**
     * Interface that must be implemented and provided as a public CREATOR
     * field that generates instances of your Parcelable class from a Parcel.
     */
    public interface Creator<T> {
        /**
         * Create a new instance of the Parcelable class, instantiating it
         * from the given Parcel whose data had previously been written by
         * {@link Parcelable#writeToParcel Parcelable.writeToParcel()}.
         * 
         * @param source The Parcel to read the object's data from.
         * @return Returns a new instance of the Parcelable class.
         */
        public T createFromParcel(Parcel source);
        
        /**
         * Create a new array of the Parcelable class.
         * 
         * @param size Size of the array.
         * @return Returns an array of the Parcelable class, with every entry
         * initialized to null.
         */
        public T[] newArray(int size);
    }

如下爲實現了Parcelable接口的Book類，其中有兩個基本類型的變量，在進程間傳遞數據時我們希望可以傳遞這些信息，則可以將這些變量都寫入Parcel對象中。代碼如下：

import android.os.Parcel;
import android.os.Parcelable;

public class Book implements Parcelable{
    private String name;
    private int count;

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

    // write all parameters to parcel
    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(name);
        dest.writeInt(count);
    }

    public static final Parcelable.Creator<Book> CREATOR = new Parcelable.Creator<Book>() {

        @Override
        public Book createFromParcel(Parcel source) {
            Book book = new Book();
            book.name = source.readString();
            book.count = source.readInt();
            return book;
        }

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

需要注意的是打包/解包時數據應該儘量一致，即wirteToParcel和createFromParcel中讀寫的順序應該一致。