【移动应用开发技术】Android中怎么利用AsyncTask实现下载文件动态更新进度条功能

【移动应用开发技术】Android中怎么利用AsyncTask实现下载文件动态更新进度条功能

Android中怎么利用AsyncTask实现下载文件动态更新进度条功能，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。1.泛型AysncTask<Params,Progress,Result>Params：启动任务时传入的参数，通过调用asyncTask.execute(param)方法传入。Progress：后台任务执行的进度，若不用显示进度条，则不需要指定。Result：后台任务结束时返回的结果。2.重要方法doInBackground(Params...params)：必须重写的方法，后台任务就在这里执行，会开启一个新的线程。params为启动任务时传入的参数，参数个数不定。onPreExecute()：在主线程中调用，在后台任务开启前的操作在这里进行，例如显示一个进度条对话框。onPostExecute(Resultresult)：当后台任务结束后，在主线程中调用，处理doInBackground()方法返回的结果。onProgressUpdate(Progress...values)：当在doInBackground()中调用publishProgress(Progress...values)时，返回主线程中调用，这里的参数个数也是不定的。onCancelled()：取消任务。3.注意事项（1）execute()方法必须在主线程中调用；（2）AsyncTask实例必须在主线程中创建；（3）不要手动调用doInBackground()、onPreExecute()、onPostExecute()、onProgressUpdate()方法；（4）注意防止内存泄漏，在doInBackground()方法中若出现对Activity的强引用，可能会造成内存泄漏。4.下载文件动态更新进度条（未封装）布局：<?xml

version="1.0"

encoding="utf-8"?>

<LinearLayout

xmlns:android="/apk/res/android"

xmlns:tools="/tools"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:orientation="vertical"

android:padding="20dp"

tools:context="com.studying.asynctaskdemo.MainActivity">

<ProgressBar

android:id="@+id/progressBar"

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:progress="0"

/>

<Button

android:id="@+id/download"

android:layout_width="match_parent"

android:layout_height="50dp"

android:layout_marginTop="20dp"

android:text="@string/start_btn"

/>

<TextView

android:id="@+id/status"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:layout_marginTop="20dp"

android:text="@string/waiting"

/>

</LinearLayout>Activity：public

class

MainActivity

extends

Activity

{

private

static

final

String

FILE_NAME

=

"test.pdf";//下载文件的名称

private

static

final

String

PDF_URL

=

"/class/assist/118/1328281/AsyncTask.pdf";

private

ProgressBar

mProgressBar;

private

Button

mDownloadBtn;

private

TextView

mStatus;

@Override

protected

void

onCreate(Bundle

savedInstanceState)

{

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

initView();

setListener();

}

private

void

initView()

{

mProgressBar

=

(ProgressBar)

findViewById(R.gressBar);

mDownloadBtn

=

(Button)

findViewById(R.id.download);

mStatus

=

(TextView)

findViewById(R.id.status);

}

private

void

setListener()

{

mDownloadBtn.setOnClickListener(new

View.OnClickListener()

{

@Override

public

void

onClick(View

v)

{

//AsyncTask实例必须在主线程创建

DownloadAsyncTask

asyncTask

=

new

DownloadAsyncTask();

asyncTask.execute(PDF_URL);

}

});

}

/**

*

泛型：

*

String：传入参数为文件下载地址

*

Integer：下载过程中更新ProgressBar的进度

*

Boolean：是否下载成功

*/

private

class

DownloadAsyncTask

extends

AsyncTask<String,

Integer,

Boolean>

{

private

String

mFilePath;//下载文件的保存路径

@Override

protected

Boolean

doInBackground(String...

params)

{

if

(params

!=

null

&&

params.length

>

0)

{

String

pdfUrl

=

params[0];

try

{

URL

url

=

new

URL(pdfUrl);

URLConnection

urlConnection

=

url.openConnection();

InputStream

in

=

urlConnection.getInputStream();

int

contentLength

=

urlConnection.getContentLength();//获取内容总长度

mFilePath

=

Environment.getExternalStorageDirectory()

+

File.separator

+

FILE_NAME;

//若存在同名文件则删除

File

pdfFile

=

new

File(mFilePath);

if

(pdfFile.exists())

{

boolean

result

=

pdfFile.delete();

if

(!result)

{

return

false;

}

}

int

downloadSize

=

0;//已经下载的大小

byte[]

bytes

=

new

byte[1024];

int

length

=

0;

OutputStream

out

=

new

FileOutputStream(mFilePath);

while

((length

=

in.read(bytes))

!=

-1)

{

out.write(bytes,

0,

length);

downloadSize

+=

length;

publishProgress(downloadSize

/

contentLength

*

100);

}

in.close();

out.close();

}

catch

(IOException

e)

{

e.printStackTrace();

return

false;

}

}

else

{

return

false;

}

return

true;

}

@Override

protected

void

onPreExecute()

{

super.onPreExecute();

mDownloadBtn.setText("下载中");

mDownloadBtn.setEnabled(false);

mStatus.setText("下载中");

mProgressBar.setProgress(0);

}

@Override

protected

void

onPostExecute(Boolean

aBoolean)

{

super.onPostExecute(aBoolean);

mDownloadBtn.setText("下载完成");

mStatus.setText(aBoolean

?

"下载完成"

+

mFilePath

:

"下载失败");

}

@Override

protected

void

onProgressUpdate(Integer...

values)

{

super.onProgressUpdate(values);

if

(values

!=

null

&&

values.length

>

0)

{

mProgressBar.setProgress(values[0]);

}

}

}

}5.下载文件动态更新进度条（封装）Activity：public

class

MainActivity

extends

Activity

{

private

static

final

String

FILE_NAME

=

"test.pdf";

private

static

final

String

PDF_URL

=

"/class/assist/118/1328281/AsyncTask.pdf";

private

ProgressBar

mProgressBar;

private

Button

mDownloadBtn;

private

TextView

mStatus;

@Override

protected

void

onCreate(Bundle

savedInstanceState)

{

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

initView();

setListener();

}

private

void

initView()

{

mProgressBar

=

(ProgressBar)

findViewById(R.gressBar);

mDownloadBtn

=

(Button)

findViewById(R.id.download);

mStatus

=

(TextView)

findViewById(R.id.status);

}

private

void

setListener()

{

mDownloadBtn.setOnClickListener(new

View.OnClickListener()

{

@Override

public

void

onClick(View

v)

{

String

localPath

=

Environment.getExternalStorageDirectory()

+

File.separator

+

FILE_NAME;

DownloadHelper.download(PDF_URL,

localPath,

new

DownloadHelper.OnDownloadListener()

{

@Override

public

void

onStart()

{

mDownloadBtn.setText("下载中");

mDownloadBtn.setEnabled(false);

mStatus.setText("下载中");

mProgressBar.setProgress(0);

}

@Override

public

void

onSuccess(File

file)

{

mDownloadBtn.setText("下载完成");

mStatus.setText(String.format("下载完成：%s",

file.getPath()));

}

@Override

public

void

onFail(File

file,

String

failInfo)

{

mDownloadBtn.setText("开始下载");

mDownloadBtn.setEnabled(true);

mStatus.setText(String.format("下载失败：%s",

failInfo));

}

@Override

public

void

onProgress(int

progress)

{

mProgressBar.setProgress(progress);

}

});

}

});

}

}DownloadHelper：class

DownloadHelper

{

static

void

download(String

url,

String

localPath,

OnDownloadListener

listener)

{

DownloadAsyncTask

task

=

new

DownloadAsyncTask(url,

localPath,

listener);

task.execute();

}

private

static

class

DownloadAsyncTask

extends

AsyncTask<String,

Integer,

Boolean>

{

private

String

mFailInfo;

private

String

mUrl;

private

String

mFilePath;

private

OnDownloadListener

mListener;

DownloadAsyncTask(String

mUrl,

String

mFilePath,

OnDownloadListener

mListener)

{

this.mUrl

=

mUrl;

this.mFilePath

=

mFilePath;

this.mListener

=

mListener;

}

@Override

protected

Boolean

doInBackground(String...

params)

{

String

pdfUrl

=

mUrl;

try

{

URL

url

=

new

URL(pdfUrl);

URLConnection

urlConnection

=

url.openConnection();

InputStream

in

=

urlConnection.getInputStream();

int

contentLength

=

urlConnection.getContentLength();

File

pdfFile

=

new

File(mFilePath);

if

(pdfFile.exists())

{

boolean

result

=

pdfFile.delete();

if

(!result)

{

mFailInfo

=

"存储路径下的同名文件删除失败！";

return

false;

}

}

int

downloadSize

=

0;

byte[]

bytes

=

new

byte[1024];

int

length;

OutputStream

out

=

new

FileOutputStream(mFilePath);

while

((length

=

in.read(bytes))

!=

-1)

{

out.write(bytes,

0,

length);

downloadSize

+=

length;

publishProgress(downloadSize

/

contentLength

*

100);

}

in.close();

out.close();

}

catch

(IOException

e)

{

e.printStackTrace();

mFailInfo

=

e.getMessage();

r