Drawable Mutations

下圖展示了不同View設置相同的圖片作爲背景時的實體關係圖，可以看出，兩個Drawable被創建，他們共享Constant State

code：

public class Main extends Activity implements OnClickListener{
    /** Called when the activity is first created. */

    Button button;
    Button button2;
    TextView textView1;
    TextView textView2;
    Drawable icon;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        button = (Button)findViewById(R.id.button1);
        textView1 = (TextView)findViewById(R.id.textView1);
        textView2 = (TextView)findViewById(R.id.textView2);
        icon = getResources().getDrawable(R.drawable.icon);

        button.setOnClickListener(this);

    }
    @Override
    public void onClick(View v) {
        if (v.getId() == R.id.button1) {
            icon = getResources().getDrawable(R.drawable.icon);
            icon.setAlpha(50);
            textView1.setBackgroundDrawable(icon);
            icon = getResources().getDrawable(R.drawable.icon);
            icon.setAlpha(0);
            textView2.setBackgroundDrawable(icon);
        } else if (v.getId() == R.id.button2) {
                icon = getResources().getDrawable(R.drawable.icon);
                //icon.setAlpha(0);
                textView1.setBackgroundDrawable(icon);
                //icon.setAlpha(255);
                textView2.setBackgroundDrawable(icon);
        }
    }
}

從運行效果可以看到，textView1和textView2的Alpha值都設置爲0，說明.setAlpha()方法改變了Drawable的Constant State

Android 1.5版本後，加入了mutate()方法，Drawable的Constant State不會發生變化，因爲mutate()方法返回Drawable的副本，修改屬性只修改Drawable副本的屬性。修改以上代碼：

@Override
    public void onClick(View v) {
        if (v.getId() == R.id.button1) {
             icon = getResources().getDrawable(R.drawable.icon);
            icon.mutate().setAlpha(50);
            textView1.setBackgroundDrawable(icon);
            icon = getResources().getDrawable(R.drawable.icon);
            icon.mutate().setAlpha(0);
            textView2.setBackgroundDrawable(icon);
        }
    }

從運行結果來看，textView1和textView2有不同的透明度