Android后臺(tái)線程和UI線程通訊實(shí)例

2020-04-11 11:49:33

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供稿：網(wǎng)友

本節(jié)向你展示如何在任務(wù)中發(fā)送數(shù)據(jù)給UI線程里的對(duì)象，這個(gè)特性允許你在后臺(tái)線程工作，完了在UI線程展示結(jié)果。

在UI線程定義一個(gè)Handler

Handler是Android系統(tǒng)線程管理框架里的一部分。一個(gè)Handler對(duì)象接收消息，并且運(yùn)行代碼來處理消息。正常情況下，你為新線程創(chuàng)建Handler,但你也可以為已有的線程創(chuàng)建一個(gè)Handler.當(dāng)你連接Handler到UI線程時(shí)，處理消息的代碼會(huì)在UI線程上運(yùn)行.

在創(chuàng)建線程池的類的構(gòu)造器里實(shí)例化Handler對(duì)象，保存在全局變量里。用Handler(Looper)方法實(shí)例化，連接到UI線程，構(gòu)造方法使用Looper對(duì)象，也是Android系統(tǒng)線程管理框架里的一部分.Looper類有一個(gè)靜態(tài)方法getMainLooper()可以獲取UI線程的Looper對(duì)象。如:

復(fù)制代碼代碼如下:

private PhotoManager() {
...
    // Defines a Handler object that's attached to the UI thread
    mHandler = new Handler(Looper.getMainLooper()) {
    ...

在Handler里，覆蓋handleMessage()。Android系統(tǒng)會(huì)在Handler管理的線程收到新消息時(shí)，調(diào)用該方法。一個(gè)指定線程的所有Handler對(duì)象都會(huì)收到相同的消息。

復(fù)制代碼代碼如下:

        /*
         * handleMessage() defines the operations to perform when
         * the Handler receives a new Message to process.
         */
        @Override
        public void handleMessage(Message inputMessage) {
            // Gets the image task from the incoming Message object.
            PhotoTask photoTask = (PhotoTask) inputMessage.obj;
            ...
        }
    ...
    }
}

從任務(wù)里移動(dòng)數(shù)據(jù)到UI線程

要從后臺(tái)線程的任務(wù)里移動(dòng)數(shù)據(jù)到UI線程的對(duì)象，先保存引用到數(shù)據(jù)和任務(wù)對(duì)象的UI對(duì)象里，接下來把任務(wù)對(duì)象和狀態(tài)碼傳給Handler對(duì)象。在這個(gè)對(duì)象里，發(fā)送一個(gè)包含狀態(tài) 和任務(wù)對(duì)象的消息給Handler.因?yàn)镠andler在UI線程上運(yùn)行，它可以移動(dòng)數(shù)據(jù)給UI對(duì)象。

在任務(wù)對(duì)象里存儲(chǔ)數(shù)據(jù)

如，這是一個(gè)Runnable,運(yùn)行在后臺(tái)線程，它解析Bitmap,并保存到它的父對(duì)象。Runnable同時(shí)保存狀態(tài)碼DECODE_STATE_COMPLETED。

復(fù)制代碼代碼如下:

// A class that decodes photo files into Bitmaps
class PhotoDecodeRunnable implements Runnable {
    ...
    PhotoDecodeRunnable(PhotoTask downloadTask) {
        mPhotoTask = downloadTask;
    }
    ...
    // Gets the downloaded byte array
    byte[] imageBuffer = mPhotoTask.getByteBuffer();
    ...
    // Runs the code for this task
    public void run() {
        ...
        // Tries to decode the image buffer
        returnBitmap = BitmapFactory.decodeByteArray(
                imageBuffer,
                0,
                imageBuffer.length,
                bitmapOptions
        );
        ...
        // Sets the ImageView Bitmap
        mPhotoTask.setImage(returnBitmap);
        // Reports a status of "completed"
        mPhotoTask.handleDecodeState(DECODE_STATE_COMPLETED);
        ...
    }
    ...
}
...

PhotoTask還包含一個(gè)ImageView引用，用來顯示Bitmap.盡管引用Bitmap和ImageView是在同一個(gè)對(duì)象里，但因?yàn)椴皇窃赨I線程，你不能直接讓ImageView顯示Bitmap.

沿對(duì)象層次逐級(jí)發(fā)送狀態(tài)

PhotoTask持有解碼的數(shù)據(jù)和顯示數(shù)據(jù)的View對(duì)象的引用，它從PhotoDecodeRunnable接收到狀態(tài)碼，并且沿著線程池里引用的對(duì)象和Handler實(shí)例傳送。

復(fù)制代碼代碼如下:

public class PhotoTask {
    ...
    // Gets a handle to the object that creates the thread pools
    sPhotoManager = PhotoManager.getInstance();
    ...
    public void handleDecodeState(int state) {
        int outState;
        // Converts the decode state to the overall state.
        switch(state) {
            case PhotoDecodeRunnable.DECODE_STATE_COMPLETED:
                outState = PhotoManager.TASK_COMPLETE;
                break;
            ...
        }
        ...
        // Calls the generalized state method
        handleState(outState);
    }
    ...
    // Passes the state to PhotoManager
    void handleState(int state) {
        /*
         * Passes a handle to this task and the
         * current state to the class that created
         * the thread pools
         */
        sPhotoManager.handleState(this, state);
    }
    ...
}

移動(dòng)數(shù)據(jù)到UI

PhotoManager從PhotoTask對(duì)象接收到狀態(tài)碼和PhotoTask對(duì)象的句柄。因?yàn)闋顟B(tài)是TASK_COMPLETE，創(chuàng)建一個(gè)包含狀態(tài)和任務(wù)對(duì)象的Message，發(fā)送給Handler。

復(fù)制代碼代碼如下:

public class PhotoManager {
    ...
    // Handle status messages from tasks
    public void handleState(PhotoTask photoTask, int state) {
        switch (state) {
            ...
            // The task finished downloading and decoding the image
            case TASK_COMPLETE:
                /*
                 * Creates a message for the Handler
                 * with the state and the task object
                 */
                Message completeMessage =
                        mHandler.obtainMessage(state, photoTask);
                completeMessage.sendToTarget();
                break;
            ...
        }
        ...
    }

最終，Handler.handleMessage()為每個(gè)進(jìn)來的Message檢查狀態(tài)碼。如果狀態(tài)碼是TASK_COMPLETE，任務(wù)就是完成了，Message里的PhotoTask對(duì)象包含Bitmap和ImageView.因?yàn)镠andler.handleMessage()運(yùn)行在UI線程，它可以安全地為ImageView設(shè)置Bitmap.

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